Book

of

Abstracts

XIII-FLTPD&

I-FLTPS

RUHR-UNIVERSITÄT BOCHUM

13th Frontiers in Low-Temperature

Plasma Diagnostics

and

1st Frontiers in Low-Temperature

Plasma Simulations

Boo

k of

Abs

trac

ts –

13

thFL

TPD

& 1

stFL

TPS,

Bad

Hon

nef

20

19

Sun

day M

ay 12th

Wed

nesd

ay May 15

th

FLTPD DiagnosticsFLTPD Diagnostics

FLTPS Simulations

FLTPD Diagnostics &FLTPS Sim

ulationsFLTPD

DiagnosticsFLTPS Sim

ulations

07:30-

08:1507:30

-08:15

08:15-

08:3008:15

-08:30

08:30-

09:15R

. Brandenburg

M. Turner

T. Gans

O. G

uiatellaM

. Kushner08:30

-09:15

09:15-

10:00A

. SobotaT. Charoy

S. StarikovskaiaJ.-P. van H

eldenJ. van D

ijk09:15

-10:00

10:00-

10:30Coffee break

L. Rutkow

skiU

. Ebert10:00

-10:30

10:30-

10:55Y. Inada

D. Erem

inJ. H

eld10:30

-10:55

10:55-

11:20M

. Dam

enP. H

artmann

Y. Liu10:55

-11:20

11:20-

11:45L. Tahri

A. Tavant

L. M. M

artini11:20

-11:45

11:45-

12:10N

. SadeghiA

. Alvarez Laguna

C. Wang

11:45-

12:10

12:15-

13:15Lunch

12:15-

13:15

13:15-

15:05Inform

al discussions

13:15-

15:05

15:05-

15:50J.-P. B

ooth15:05

-15:50

15:50-

16:15L. Xu

15:50-

16:15

16:15-

16:4516:15

-16:45

16:45-

17:30Q

. XiongL. A

lves16:45

-17:30

17:30-

17:55V

. Gonzalez-

FernandezF. Jardali

17:30-

17:55

17:55-

18:20V

. S. S. K. KondetiE. Kem

aneci17:55

-18:20

18:20-

20:00D

inner18:20

-20:00

20:00-

22:00Conference D

inner20:00

-22:00

Generic item

Track FLTPD: M

ain Hall

Track FLTPS: Small Lecture H

all 1st Floor

Track FLTPD &

FLTPS: Main H

all

Break or lunch

Time\D

ateTim

e\Date

Thu

rsday M

ay 16th

FLTPD Diagnostics &FLTPS Sim

ulations

Ro

om

distrib

utio

n

Invited oral presentation

Informal discussions

Informal discussions

Oral presentation

Legend

Coffee break

Closure(com

mon discussion)

Poster Session FTLPD

Tuesd

ay May 14

th

Coffee break

Lunch

Coffee break

Dinner

Poster SessionFLTPD

& FLTPS

Breakfast

Coffee break

I. V. A

damovich

S. Reuter

T. Hoder

Dinner

Program

13th Frontiers in Low

-Temperature Plasm

a Diagnostics &

1st Frontiers in Low

-Temperature Plasm

a Simulations

Registration

Excursion

Mo

nd

ay May 13

th

Opening

L. de Pocque

G. R

itchie

Coffee break

M. Sim

ek

A. von Keudell

T. L. Chng

Lunch

F. X. Bronold

Breakfast

Breakfast

Breakfast

Dear Participants,

Welcome to the 13th Workshop on Frontiers in Low-Temperature Plasma Diagnostics (FLTPD)

and the 1st Workshop on Frontiers in Low-Temperature Plasma Simulations (FLTPS) and

welcome also to the Physikzentrum Bad Honnef. After 22 successful years of FLTPD his-

tory, today the meeting is returning to the venue of its second gathering in 1997. During

this time, the field has seen many important developments in plasma science, diagnostics,

and instrumentation. The FLTPD has always been at the leading edge of its time and has

provided an open forum for lively exchange between experts in the field. The success of the

FLTPD has now inspired colleagues from the simulation community to found a similar mee-

ting for their own field. This year the first meeting of its kind, the FLTPS, will be held along

with the FLTPD at the very same location here at the Physikzentrum.

When the suggestion for the parallel meetings was made by Pascal Chabert and Miles Tur-

ner about two years ago, it was enthusiastically welcomed by the scientific committee of

the FLTPD. Back in 1993, when the idea of the FLTPD workshop was born by Hans-Frida

Dobele, Bill Graham, and Nader Sadeghi, simulations were still in their infant years and had

just started to make significant contributions to the understanding of the low-temperature

plasmas. However, nowadays simulations are no longer exclusive to experts in theory and

numerical computation but have become a versatile tool for experimentalists as well. In fact,

an increasing and very fruitful exchange between simulation and diagnostics has established

itself over recent years. This development was a strong motivation for holding both meetings

together, with a common session and ample of opportunities for exchange between the dia-

gnostics and simulation communities during breaks, excursions or meals.

It has been an honor and a great pleasure at the same time to organize these two work-

shops. We wish all participants a pleasant stay, exciting talks, lively discussions, lots of new

science, and in particular a stimulating exchange with strong cross-fertilization between the

two communities.

Uwe Czarnetzki, Marina Prenzel, Susanne Hentrich

Cover photo: Inductively coupled array discharge in argon [PSST 27, 105010 (2018)]

I

II

We would like to thank the sponsorsfor their great support!

III

IV

The 13th Frontiers in Low-Temperature Plasma Diagnostics &the 1st Frontiers in Low-Temperature Plasma Simulations

is organised by the Research Department Plasmas with ComplexInteractions, Ruhr University Bochum, Germany

International scientific committee FLTPD:Nader Sadeghi (Chair), Universite Grenoble AlpesUwe Czarnetzki (Vice-Chair), Ruhr University BochumGilles Cartry, Aix-Marseille UniversiteFranta Krcma, Brno University of TechnologyNicholas St.J. Braithwaite, The Open UniversityRichard A.H. Engeln, Technical University EindhovenGiorgio Dilecce, Italian National Research Council

International scientific committee FLTPS:Pascal Chabert, Ecole Polytechnique de PalaiseauMiles Turner, National Centre for Plasma Science Technology

Local organization committee FLTPD & FLTPS:Uwe CzarnetzkiSusanne HentrichMarina Prenzel

We would like to give special thanks to Achim von Keudell forsetting up the homepage and supporting the registration pro-cess!

V

VI

General information

The Phyikzentrum Bad Honnef

Built in the 19th century, the monastery building now boasts an extensive renovation with

state-of-the-art infrastructure in a historic atmosphere and a guest house with an innovative

energy concept opened in 2015. Equipped in this way, the Physikzentrum Bad Honnef has

long been one of the world’s most important conference venues in the field of physics. The

programme consists primarily of scientific seminars lasting several days, which attract more

than 5,000 young and established scientists from all over the world to the Rhine every year,

including Nobel Prize winners in physics on a regular basis.

The Physikzentrum Bad Honnef (PBH) is a scientific

conference centre with accommodation and its own kit-

chen. The PBH is managed by the German Physical

Society with the support of the University of Bonn and

the state of NRW. Since its establishment in 1976, the

Physikzentrum Bad Honnef has been a central mee-

ting point for physics in Germany. It is managed by the

German Physical Society (DPG), which operates the

Physikzentrum in partnership with the University of Bonn (Elly Holterhoff Bocking Foundati-

on) and with funding from the state of North Rhine-Westphalia. The Physikzentrum is also

the headquarter of the DPG e.V. with its office. The Physikzentrum is housed in the beautiful

building of the Elly Holterhoff Bocking Foundation, part of the University of Bonn, a castle-

like building situated between the Rhine and the Siebengebirge mountains.

The door of the Physikzentrum can be opened with a door code. This is 1301 for the whole

week. Furthermore, the car parking gate can be opened with the same code.

Food is served by the PBH during the week and it is included within your registration fee.

Breakfast and lunch will be served in the dining room on the ground floor. You are welcome

to have you dinner in the Georg Christoph Lichtenberg-celler in the basement of the Physik-

zentrum. The drinks during the week needs to be paid by the participants themselves, when

leaving the PBH. However, coffee, tea, and water is free at all times. At Wednesday evening,

we plan to have our conference dinner at the Physikzentrum Bad Honnef. At this evening, all

drinks are free.

Furthermore, we also prepared an announcement poster board for the whole week. Plea-

se feel free to announce upcoming conferences, jobs announcements etc. at this board (of

course in a small format of DIN A3 or A4).

VII

Oral and poster presentations

For participants with an oral presentation, please prepare presentations of 20 minutes (+ 5

minutes discussion) if you have a regular presentation. Invited presentations have a length of

40 minutes (+ 5 minutes discussion). Please bring a USB stick with your presentation before

your session starts to the front of the auditorium to copy it to the computer of the auditorium.

Of course, your presentation will be deleted directly after the end of your session, so that

your data are handled safety.

Poster presentations for the diagnostics workshop are planned for Monday (13/05/2019)

and Tuesday (14/05/2019). The simulations workshop will have their poster presentation on

Tuesday (14/05/2019). More details can be found on the homepage (frontiers2019.rub.de).

Conference excursion

The excursion of the FLTPD and FLTPS is planned for Wednesday, May 15th. We will walk to

Schloss Drachenburg, which is a historical castle. Baron Stephan von Sarter had already laid

the foundation stone for an imposing residence, namely, Schloss Drachenburg a mixture of

villa, mansion and castle in 1882. Two Dusseldorf-

based architects, Leo von Abbema and Bernhard

Tushaus, drew up the original plans which were sub-

sequently revised by Wilhelm Hoffmann, an architect

resident in Paris and a former pupil of Ernst Friedrich

Zwirner, a Cologne Cathedral architect. The historical

architecture and splendid furnishings of Schloss Dra-

chenburg were to find much admiration amongst contemporaries. Yet Sarter was never to

live there. His chosen place of domicile was Paris where he died in 1902, still a bachelor,

without having regulated his inheritance. Jakob Biesenbach, one of his nephews, bought the

castle from the state.

Within the years, Schloss Drachenburg was used as a summer resort and as a christian

boys’ boarding school. The Adolf-Hitler-Schule – a Nazi elite school – moved into Schloss

Drachenburg in 1942. At the end of the war, US soldiers occupied the castle and afterwards

it was commandeered as a refugee camp. It was a stroke of luck for Schloss Drachenburg

that in 1947 the German Rail Regional Office based in Wuppertal realised that the property

could be used as a training centre and took out rent. But by 1960, the German Rail autho-

rities had transferred the training premises and left Schloss Drachenburg. For the next ten

years, from 1960 to 1970, Schloss Drachenburg stood empty, visibly disintegrating. In 1973,

Schloss Drachenburg was opened to the general public.

VIII

Schloss Drachenburg was eventually listed as a monument in 1986. In 1989, urgent mea-

sures for full restoration were initiated by the North Rhine-Westphalia Foundation of Nature,

Heritage & Culture. Since 1995, in close collaboration with the City of Konigswinter, this

NRW foundation has supervised the careful restoration of the castle complex. From 2003 to

2009, a dedicated exhibition entitled ”Open Due to Restoration – a Look at the Building Site

known as Schloss Drachenburg“ has provided a wealth of information about the restoration

process. And in early 2010, the rehabilitation work inside the castle was completed and all

the restored and refurnished rooms again made accessible to visitors. The restoration of the

landscape park was finished in 2011.

We are going to start walking to Schloss Drachenburg at around 13:15 on Wednesday. The

meeting point is in front of the Physikzentrum builduing’s main entrance. However, wi-

thin the castle there will be no official guide, but you can download an app in advance, which

works as an audio guide. Note that you might not have internet connection with your mobile

phone at the castle site. Please load the app here:

https://www.schloss-drachenburg.de/index.php/de/info/allgemeines/aktuelles/288-lauschtour.

At the nearby cafe, Winzerhauschen (Drachenfelsstraße 100, 53639 Konigswinter), we are

going to have coffee and cakes. If you are not able to walk the complete way, we will offer a

shuttle bus, which will bring you directly to the cafe or to the castle.

IX

X

Contents

Program XIX

Monday May 13th 1

Track: FLTPD Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Session: Spectroscopy of particles . . . . . . . . . . . . . . . . . . 1Time resolved LIF studies on sputtered atoms velocity distribution function

L. de Poucques, M. Desecures, A. El Farsy, J. Bougdira . . . . . . . . 3Cavity-enhanced laser spectroscopy of radicals in atmospheric pressure plas-

masG. Ritchie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Session: Spectroscopy of plasma processes . . . . . . . . . . . . 5Infrared spectroscopy of wall charges in plasma-facing dielectrics

F. X. Bronold, K. Rasek, H. Fehske . . . . . . . . . . . . . . . . . . . 5Imaging techniques reveal initial phases of a nanosecond discharge in liquid

water with sub-ns temporal resolutionM. Simek, P. Hoffer, V. Prukner, J. Schmidt . . . . . . . . . . . . . . . 6

Nanosecond plasmas in water: ignition, cavitation, temperaturesK. Grosse, M. Kai, J. Held, A. von Keudell . . . . . . . . . . . . . . . . 7

Towards Ultrashort Pulse Generation from Nanosecond Laser Sources forElectric Field MeasurementsT. L. Chng, M. Naphade, B. Goldberg, I. Adamovich, S. M. Starikovs-kaia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Session: Plasma kinetics and electric fields . . . . . . . . . . . . . 9Laser diagnostics for electric field measurements in air plasmas, plasma-

enhanced flames, and atmospheric pressure plasma jetsI. Adamovich, M. Simeni Simeni, Y. Tang, K. Orr . . . . . . . . . . . . 9

Ultrafast Laser Diagnostics of Electric Fields and Turbulent Reactive Flows inPlasma JetsS. Reuter, B. Goldberg, A. Dogariu, Y. Zhang, R. Miles . . . . . . . . 10

Time dependent optical methods for electric field determination with picose-cond resolution in discharges in gases and liquidsT. Hoder, M. Becker, P. Hoffer, M. Simek, D. Loffhagen . . . . . . . . 11

XI

Session: Plasma Diagnostic Poster I . . . . . . . . . . . . . . . . . 12Spectroscopic diagnostics on a pulsed CO2 microwave discharge

E. Carbone, F. D’Isa, A. Hecimovic, U. Fantz . . . . . . . . . . . . . . 12Static and RF electric field direct measurement based on Lyman-alpha emis-

sion from a hydrogen probe beamL. Cherigier-Kovacic, F. Doveil . . . . . . . . . . . . . . . . . . . . . . 13

Gas temperature characterization of annular shape RF APPJ for biomedicalapplicationI. Sremauocki, A. Jurov, M. Modic, U. Cvelbar, C. Leys, A. Nikiforov . 14

Spectroscopic study of the neutral gas temperature of silicon based DC MH-CD in various gases close to atmospheric pressureS. Iseni, R. Michaud, P. Lefaucheux, V. Schulz-von der Gathen, G. Sre-tenovic, R. Dussart . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Temperature distribution in DBD-driven helium atmospheric pressure plasmajet by schlieren techniqueO. Stepanova, M. Pinchuk, A. Astafiev . . . . . . . . . . . . . . . . . 16

Radio frequency emission of Transient Spark DischargeA. Hennecke, M. Janda . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Glow discharge formation of high molecular products in prebiotic atmosphe-res: PTR-TOF analyzisF. Krcma, S. Chudjak . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Microwave Cutoff Probe Modeling ResearchS. You, S. J. Kim, D. Kim, H. Chang . . . . . . . . . . . . . . . . . . . 19

Electric field measurements on plasma bullets in nitrogen with nanosecondElectric Field Induced Second-Harmonic GenerationA. Limburg, M. van de Schans, S. Nijdam . . . . . . . . . . . . . . . . 20

Optimisation of a ns-pulsed micro-hollow cathode discharge array in Ar/N2 foratomic nitrogen productionG. Lombardi, S. Kasri, K. Gazeli, J. Santos Sousa, G. Bauville, M.Fleury, S. Pasquiers, X. Aubert, J. Achard, A. Tallaire, C. Lazzaroni. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Collisional radiative model for low temperature laser produced Zn plasma withfine structure resolved electron impact excitation cross sectionsS. Gupta, R. Gangwar, R. Srivastava . . . . . . . . . . . . . . . . . . 22

Oxygen 3P atom density and temperature determined by Cavity Ringdownspectroscopy of the forbidden 1D2 → 3P2 transitionJ.-P. Booth, A. Chatterjee, O. Guaitella, C. Drag, K. Manfred, G.A.D.Ritchie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Floating harmonic probe for diagnostics of pulsed dischargesM. Zanaska, Z. Turek, Z. Hubicka, M. Cada, P. Kudrna, M. Tichy . . . 24

Gas heating in the ignition phase of pure molecular plasmas assessed byThomson and Raman scatteringA. van de Steeg, T. Butterworth, G. van Rooij . . . . . . . . . . . . . 25

Precautions for using krypton as a calibration species for Two-Photon Absorp-tion Laser Induced Fluorescence of hydrogen and nitrogen atomsC. Y. Duluard, X. Aubert . . . . . . . . . . . . . . . . . . . . . . . . . 26

XII

Electric field measurements in the INCA dischargeC. Lutke Stetzkamp, P. Ahr, T. V. Tsankov, U. Czarnetzki . . . . . . . 27

Ro-vibrational distribution measurements in transient atmospheric pressureplasmas by coherent anti-Stokes Raman scatteringJ. Kuhfeld, D. Luggenholscher, U. Czarnetzki . . . . . . . . . . . . . . 28

Sub-ns electric field measurements in a nanosecond pulsed atmospheric plas-ma jetN. Lepikhin, D. Luggenholscher, U. Czarnetzki . . . . . . . . . . . . . 29

Ultra-fast dynamics of a pulsed microwave surface wave discharge in argonE. Carbone, E. van Veldhuizen, G. Kroesen, N. Sadeghi . . . . . . . 30

Laser-Induced Fluorescence Measurements of Ion Implantation in InductivelyCoupled Plasma PIII DeviceJ. Moreno, L. Couedel, M. Bradley . . . . . . . . . . . . . . . . . . . . 31

Broadband afterglow emission as a diagnostic tool in CO2 microwave plasmaF. J. J. Peeters, H. J. L. Hendrickx, A. W. van de Steeg, T. W. H. Rig-hart, A. J. Wolf, G. J. van Rooij, W. A. Bongers, M. C. M. van de Sanden 32

Diagnostics of hollow cathode plasma jet plasma for deposition of iron oxidethin filmsK. Tuharin, M. Zanka, P. Kudrna, M. Tichy . . . . . . . . . . . . . . . 33

Mesenchymal Stem Cells Behavior After Nanosecond Capillary Cold Atmos-pheric Plasma Helium Jet TreatmentI. Orel, S. Celik, S. Audonnet, S. M. Starikovskaia, H. Kerdjoudj . . . 34

Ideal Multipole Resonance Probe: a Spectral Kinetic ApproachJ. Gong, M. Friedrichs, S. Wilczek, D. Eremin, J. Oberrath, R. P. Brink-mann . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Experimental study of a cold plasma column by spectro-tomographyV. Gonzalez-Fernandez, A. Escarguel, Y. Camenen, A. Poye, R. Bau-de, P. David . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Formation pathways of HO2 in a cold atmospheric pressure plasma jet investi-gated by cavity ring-down and two-photon laser induced fluorescencespectroscopyS.-J. Klose, A. Schmidt-Bleker, K. Manfred, H. Norman, M. Gianella,S. Press, F. Riedel, T. Gans, D. O’Connell, G. Richtig, J. H. van Helden 37

Laser-induced breakdown spectroscopy (LIBS) in open air applied to the ele-mental analysis, specifically Si, in pig iron samples. Comparison froma handheld instrument and a bench-top apparatusG.S. Senesi, G. Dilecce, A. Bove, O. De Pascale . . . . . . . . . . . . 38

Tuesday May 14th 39

Track: FLTPD Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Session: Electric field measurements . . . . . . . . . . . . . . . . . 39Time-correlated single photon counting on transient plasmas at atmospheric

pressureR. Brandenburg, S. Jahanbakhsh . . . . . . . . . . . . . . . . . . . . 41

XIII

Electric field measurements in atmospheric pressure non-thermal plasmas:Pockels-based Mueller polarimetryA. Sobota, E. Slikboer, O. Guaitella, E. Garcia-Caurel . . . . . . . . . 42

Session: Species determination . . . . . . . . . . . . . . . . . . . . 43Two-Dimensional Electron Density Measurement over Streamer Discharge in

Atmospheric-Pressure Air Using Laser Wavefront SensorY. Inada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

CO detection by two-photon absorption laser-induced fluorescence (TALIF) ina CO2 glow dischargeM. Damen, D. Hage, L. M. Martini, R. Engeln . . . . . . . . . . . . . . 44

Surface production of negative ion in low pressure H2/D2 plasmas: measure-ment of the absolute negative ion fluxL. Tahri, D. Kogut, A. Anesland, D. Rafalskyi, A. Simonin, J. M. Layet,G. Cartry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Densities of He∗(3S1)

and He2∗ (a3Σu

+) metastable species in an atmosphe-ric helium RF discharge measured by Broad-Brand Absorption Spec-troscopG. Nayak, N. Sadeghi, P. Bruggemann . . . . . . . . . . . . . . . . . 46

Session: Discharge characterization . . . . . . . . . . . . . . . . . 47Water-contacting Micro-discharge: Diagnostics of Gaseous Thermal Field and

Reactive SpeciesQ. Xiong, L. Xiong, Q. H. Huang, Z. Shu . . . . . . . . . . . . . . . . 47

Two-photon spectroscopy applied to study of the cathode fall characteristics ina hollow cathode glow discharge operated in hydrogen and deuteriumV. Gonzalez-Fernandez, K. Grutzmacher, C. Perez, M. I. de la Rosa . 48

Absolute H density measurement in an RF driven Ar + 0.27% H2O plasmaV. S. S. K. Kondeti, P. J. Bruggeman . . . . . . . . . . . . . . . . . . . 49

Session: Plasma Diagnostic Poster II . . . . . . . . . . . . . . . . . 50Separated effects of ions, metastables and photons on the properties of bar-

rier layers on polymersM. Boke, B. Biskup, M. Brochhagen, J. Benedikt . . . . . . . . . . . . 50

Laser Absorption Spectroscopy for H (D) (n = 2) Density MeasurementsF. Merk, R. Friedl, C. Frohler, S. Briefi, U. Fantz . . . . . . . . . . . . 51

Plasma chemical studies of nitrocarburizing with an active screen made ofcarbon in laboratory and industrial scale reactorsA. D. F. Puth, S. Hamann, L. Kusyn, I. Burlacov, A. Dalke, H.-J. Spies,H. Biermann, J. Rpcke, J.-P. H. van Helden . . . . . . . . . . . . . . . 52

Tracking the evolution of rotating electrode gliding arc discharge channelsusing fast-framing camera and electrical diagnosticsJ. Cech, L. Dostal, M. Zemnek, Z. Navratil, J. Valenta, P. Sahel . . . . 53

Rate Coefficients of OH(A,v=0,1) quenching and vibrational relaxation for non-thermal rotational distributionsM. Ceppelli, L. M. Martini, M. Scotoni, G. Dilecce, P. Tosi . . . . . . . 54

XIV

Formation of Nanoparticles of Copper and Zinc in a Magnetic FieldV. F. Myshkin, V. A. Khan, M. Tichy, A. Kapran . . . . . . . . . . . . . 55

Automated determination of pressure profile generated by discharges in con-tact with liquid phase by interferometric techniqueL. Kusyn, P. Hoffer, Z. Bonaventura, T. Hoder . . . . . . . . . . . . . . 56

Some Problems in Todays Diagnostics of Low Temperature PlasmaV. Godyak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Dissociation in nanosecond capillary pulsed discharge in CO2G. V. Pokrovskiy, S. M. Starikovskaia . . . . . . . . . . . . . . . . . . 58

Oxygen effects on streamer-to-filament transition in nanosecond surface diel-ectric barrier dischargeC. Ding, S. A. Shcherbanev, S. M. Starikovskaia . . . . . . . . . . . . 59

Analysis of homogenous nanosecond discharge at moderate pressure: disso-ciation of oxygen for plasma assisted detonation.A. Ali Cherif, S. M. Starikovskaia . . . . . . . . . . . . . . . . . . . . . 60

Experimental characterization of CO2/Ar glow discharges and model validati-onA. Silva, A. Morillo-Candas, A. Tejero-del-Caz, O. Guaitella, V. Guerra 61

CO2 splitting in low temperature atmospheric plasma sustained with nanosec-ond microwave pulsesS. Soldatov, A. Navarrete, R. Dittmeyer, J. Jelonnek, G. Link,C. Schmedt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Study of particle transport above the target in high power impulse magnetronsputtering plasmas using a marker techniqueS. Thiemann-Monje, M. Sackers, A. von Keudell . . . . . . . . . . . . 63

Track: FLTPS Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Session: Plasma Simulation Slot 1 . . . . . . . . . . . . . . . . . . . 66Benchmarks for two-dimensional electrostatic particle-in-cell simulations

M. Turner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Challenges in the modeling of low temperature partially magnetized plasmas

J.P. Bouef, T. Charoy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Session: Plasma Simulation Slot 2 . . . . . . . . . . . . . . . . . . . 683D simulations of magnetron discharges with an energy-conserving implicit

particle-in-cell codeD. Eremin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Cylindrical (2D) PIC/MCC simulations: Acceleration techniques and accuracycompromiseP. Hartmann, Z. Donko, Z. Juhasz . . . . . . . . . . . . . . . . . . . . 70

Re-injection schemes for ExB Particles in Cells simulationsA. Tavant, R. Lucken, T. Charoy, A. Bourdon, P. Chabert . . . . . . . 71

An asymptotic-preserving well-balanced scheme for the fluid plasma equati-ons in low-temperature plasma applicationsA. Alvarez Laguna, T. Magin, P. Chabert, M. Massot, A. Bourdon . . . 72

XV

Session: Plasma Simulation Slot 3 . . . . . . . . . . . . . . . . . . . 72A comprehensive diagnostic study of a DC positive column in O2: a test-bed

for models of plasmas in a diatomic gasJ.-P. Booth, A. Chatterjee, O. Guaitella, C. Drag, N. De Oliviera, L.Nahon, D. Lopaev, S. Zyryanov, D. Voloshin, T. Rakhimova . . . . . . 73

Kinetic interpretation of a magnetically enhanced hollow cathode arc dischar-geL. Xu, J.-P. Heinß, I. Stefanovic, D. Eremin, P. Awakowicz, R. P. Brink-mann . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Session: Plasma Simulation Slot 4 . . . . . . . . . . . . . . . . . . . 74Boltzmann-Chemistry global models: status and future challenges

L. L. Alves, A. Tejero-del-Caz . . . . . . . . . . . . . . . . . . . . . . 75Modeling for NOx production by gliding arc plasma technology

F. Jardali, A. Bogaerts, M. B. Jensen, R. Ingels . . . . . . . . . . . . . 76Zero-dimensional volume-averaged modelling formalism: implementations from

intermediate- to atmospheric-pressureE. Kemaneci, Y. He . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Session: Simulation Poster . . . . . . . . . . . . . . . . . . . . . . . 77The collisionally modified Bohm criterion: Insight or artifact?

R. P. Brinkmann . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78Kinetic modeling of the electric double layer at the plasma-wall interface

K. Rasek, F. X. Bronold, H. Fehske . . . . . . . . . . . . . . . . . . . 79Chemical kinetic modeling of Transient Spark discharge

M. Janda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Self-organized pattern formation and kinetic simulation in dielectric barrier

dischargeW. Fan, L. Dong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Fluid modelling of a capacitively-coupled radio-frequency discharge in hydro-gen excited by tailored voltage waveformsJ. M. Orlac’h, T. Zhang, T. Novikova, V. Giovangigli, E. Johnson, P. Ro-ca i Cabarrocas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Theoretical Backgrounds of the Apokamp-type Atmospheric Plasma Jet in theElectro-negative Gas MediumV. Kozhevnikov, A. Kozyrev, A. Kokovin, A. Sitnikov . . . . . . . . . . 83

Modelling for NH3 synthesis in dielectric barrier discharges with catalystsK. van ’t Veer, F. Reniers, A. Bogaerts . . . . . . . . . . . . . . . . . . 84

2D (axial-azimuthal) Particle-In-Cell benchmark for ExB dischargesT. Charoy, A. Tavant, A. Bourdon, P. Chabert . . . . . . . . . . . . . . 85

Electron kinetics in fast-pulsed dischargesA. Tejero-del-Caz, V. Guerra, D. Goncalves, M. Lino da Silva, L. Mar-ques, N. Pinhao, C. D. Pintassilgo, L. L. Alves . . . . . . . . . . . . . 86

DC magnetron discharge used for nanoparticle growth: comparison of particle-in-cell simulations with experimental measurementsL. Couedel, A. Chami, C. Arnas . . . . . . . . . . . . . . . . . . . . . 87

XVI

Towards a benchmark for two-dimensional particle-in-cell simulationR. Lucken, A. Tavant, A. Bourdon, P. Chabert . . . . . . . . . . . . . 88

Control of the plasma jet dynamics by external electric fieldsN. Babaeva, G. Naidis . . . . . . . . . . . . . . . . . . . . . . . . . . 89

A high performance, two-dimensional electrostatic particle-in-cell simulationcode with verificationM. M. Turner, H. J. Leggate . . . . . . . . . . . . . . . . . . . . . . . . 90

Wednesday May 15th 92

Track: FLTPD Diagnostics & FLTPS Simulations . . . . . . . . . . . 92

Session: Common Session Plasma Diagnostic and Plasma Simu-lation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Advanced optical diagnostics and validation of computational approaches forthe chemical kinetics in atmospheric pressure plasmasT. Gans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Diagnostic and modeling of fast pulsed dischargesT. L. Chng, I. Orel, C. Ding, G. Pokrovskiy, M. Alicherif, S. Shcherba-nev, I. Adamovich, N. Popov, S. M. Starikovskaia . . . . . . . . . . . . 94

Velocity distribution function of atoms and ions in HiPIMS by Doppler broade-ning of optical emission linesJ. Held, A. Hecimovic, A. von Keudell, V. Schulz-von der Gathen . . . 95

Numerical and experimental study of radio frequency micro atmospheric pres-sure plasma jets driven by voltage waveform tailoring: effects on elec-tron heating and generated speciesY. Liu, I. Korolov, J. Schulze, T. Hemke, T. Mussenbrock . . . . . . . . 96

Collision Energy Transfer LIF: a molecular probe for CO2 dissociation in ananosecond pulsed dischargeL. M. Martini, M. Ceppelli, M. Scotoni, G. Dilecce, P. Tosi . . . . . . . 97

Kinetic Modeling and Simulation of the Planar Multipole Resonance ProbeC. Wang, M. Friedrichs, J. Oberrath, R. P. Brinkmann . . . . . . . . . 98

Thursday May 16th 100

Track: FLTPD Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Session: Time resolved measurements . . . . . . . . . . . . . . . . 100Precise and sensitive absorption spectroscopy using Fourier transform and

Vernier spectrometers based on optical frequency combsL. Rutkowski, A. C. Johansson, P. Maslowski, A. Foltynowicz . . . . . 101

Mid-infrared direct frequency comb spectroscopy of plasma processesJ.-P. van Helden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Investigating CO2 plasma kinetics with in situ infrared absorption Fourier trans-formed spectroscopy and complementary diagnosticsO. Guaitella, A. Morillo-Candas, M. Grofulovic, R. Engeln, T. Silva, V.Guerra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

XVII

Track: FLTPS Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . 106

Session: Plasma Simulation Slot 5 . . . . . . . . . . . . . . . . . . . 106Time-Slicing in Multi-Physics Modeling: Using Hybrid Methods in Low Tempe-

rature Plasma Simulations to Address Disparate Time ScalesM. Kushner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Passing XSAMS: Towards a More Efficient Handling of Plasma Input DataJ. van Dijk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

Modeling streamers and related transient dischargesU. Ebert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

XVIII

Program

Break or lunchOral presentationInvited presentation

Sunday May 12th

Track: FLTPD Diagnostics Track: FLTPS Simulations17:30 Registration Frontiers in Low-Temperature Plasma Diagnostics18:20 Dinner

Monday May 13th

Track: FLTPD Diagnostics Track: FLTPS Simulations07:30 Breakfast08:15 OpeningPlenary Session: Spectroscopy of particles(Main Hall)08:30 Time resolved LIF studies on sputtered atoms velocity distribution function (invited)

L. de PoucquesInstitut Jean Lamour, France

09:15 Cavity-enhanced laser spectroscopy of radicals in atmospheric pressure plasmas (in-vited)G. RitchieUniversity of Oxford, United Kingdom

10:00 Coffee breakPlenary Session: Spectroscopy of plasma processes(Main Hall)10:30 Infrared spectroscopy of wall charges in plasma-facing dielectrics

F. X. BronoldInstitut fur Physik, Universitat Greifswald, Germany

10:55 Imaging techniques reveal initial phases of a nanosecond discharge in liquid water withsub-ns temporal resolutionM. SimekInstitute of Plasma Physics of the Czech Academy of Sciences, Department of PulsePlasma Systems, Czech Republic

11:20 Nanosecond plasmas in water: ignition, cavitation, temperaturesA. von KeudellRuhr University Bochum, Germany

11:45 Towards Ultrashort Pulse Generation from Nanosecond Laser Sources for Electric FieldMeasurementsT. L. ChngLaboratory of Plasma Physics, Ecole Polytechnique, France

XIX

12:15 Lunch13:15 Informal Discussions16:15 Coffee breakPlenary Session: Plasma kinetics and electric fields(Main Hall)16:45 Laser diagnostics for electric field measurements in air plasmas, plasma-enhanced

flames, and atmospheric pressure plasma jets (invited)I. V. AdamovichOhio State University, USA & Tsinghua University, China

17:30 Ultrafast Laser Diagnostics of Electric Fields and Turbulent Reactive Flows in PlasmaJetsS. ReuterPrinceton University, USA

17:55 Time dependent optical methods for electric field determination with picosecond resolu-tion in discharges in gases and liquidsT. HoderMasaryk University, Czech Republic

17:30 Registration Frontiers in Low-Temperature Plasma Simulations18:20 Dinner20:00 Poster: Plasma Diagnostic Poster I

(Main Hall)

Tuesday May 14th

Track: FLTPD Diagnostics Track: FLTPS Simulations07:30 BreakfastSession: Electric field measurements(Main Hall)

Session: Plasma Simulation Slot 1(Small Lecture Hall 1st Floor)

08:30 Time-correlated single photon count-ing on transient plasmas at atmo-spheric pressure (invited)R. BrandenburgLeibniz Institute for Plasma Scienceand Technology (INP Greifswald),Germany

08:30 Benchmarks for two-dimensionalelectrostatic particle-in-cell simula-tions (invited)M. TurnerDublin City University, Ireland

09:15 Electric field measurements in atmo-spheric pressure non-thermal plas-mas: Pockels-based Mueller po-larimetry (invited)A. SobotaEindhoven University of Technology,The Netherlands

09:15 Challenges in the modeling of lowtemperature partially magnetizedplasmas (invited)T. CharoyLAPLACE, CNRS, University ofToulouse, France

10:00 Coffee break

XX

Session: Species determination(Main Hall)

Session: Plasma Simulation Slot 2(Small Lecture Hall 1st Floor)

10:30 Two-Dimensional Electron DensityMeasurement over Streamer Dis-charge in Atmospheric-Pressure AirUsing Laser Wavefront SensorY. InadaSaitama University, Japan

10:30 3D simulations of magnetron dis-charges with an energy-conservingimplicit particle-in-cell codeD. EreminRuhr-Universitat Bochum, Germany

10:55 CO detection by two-photon ab-sorption laser-induced fluorescence(TALIF) in a CO2 glow dischargeM. DamenDepartment of Applied Physics, Eind-hoven University of Technology, TheNetherlands

10:55 Cylindrical (2D) PIC/MCC simula-tions: Acceleration techniques andaccuracy compromiseP. HartmannWigner Research Centre for Physicsof the Hungarian Academy of Sci-ences, Hungary

11:20 Surface production of negative ion inlow pressure H2/D2 plasmas: mea-surement of the absolute negative ionfluxL. TahriPIIM, France

11:20 Re-injection schemes for ExB Parti-cles in Cells simulationsA. TavantLPP, CNRS, Ecole polytechnique,France

11:45 Densities of He∗(3S1)

andHe2

∗ (a3Σu

+) metastable speciesin an atmospheric helium RF dis-charge measured by Broad-BrandAbsorption SpectroscopyN. SadeghiUniv. Minnesota, USA and Univ.Grenoble & CNRS, France

11:45 An asymptotic-preserving well-balanced scheme for the fluid plasmaequations in low-temperature plasmaapplicationsA. Alvarez LagunaLaboratoire de Physique des Plas-mas, Ecole Polytechnique, France

12:15 Lunch13:15 Informal Discussions

Session: Plasma Simulation Slot 3(Small Lecture Hall 1st Floor)15:05 A comprehensive diagnostic study

of a DC positive column in O2: atest-bed for models of plasmas in adiatomic gas (invited)J.-P. BoothLPP-CNRS, Ecole Polytechnique,Palaiseau, France

15:50 Kinetic interpretation of a magneti-cally enhanced hollow cathode arcdischargeL. XuInstitute for Theoretical Electrical En-gineering, Ruhr-University Bochum,Germany

16:15 Coffee break

XXI

Session: Discharge characterization(Main Hall)

Session: Plasma Simulation Slot 4(Small Lecture Hall 1st Floor)

16:45 Water-contacting Micro-discharge:Diagnostics of Gaseous ThermalField and Reactive Species (invited)Q. XiongState Key Laboratory of PowerTransmission Equipment & Sys-tem Security and New Technology,Chongqing University, China

16:45 Boltzmann-Chemistry global models:status and future challenges (invited)L. L. AlvesInstituto de Plasmas e Fusao Nuclear,Instituto Superior Tecnico, Portugal

17:30 Two-photon spectroscopy applied tostudy of the cathode fall character-istics in a hollow cathode glow dis-charge operated in hydrogen anddeuteriumV. Gonzalez-FernandezDpto. de Fisica Teorica, Atomicay Optica, Universidad de Valladolid,Spain

17:30 Modeling for NOx production by glid-ing arc plasma technologyF. JardaliResearch group PLASMANT, De-partment of Chemistry, University ofAntwerp, Belgium

17:55 Absolute H density measurement inan RF driven Ar + 0.27% H2O plasmaV. S. S. K. KondetiUniversity of Minnesota, Minneapolis,USA

17:55 Zero-dimensional volume-averagedmodelling formalism: implemen-tations from intermediate- toatmospheric-pressureE. KemaneciTheoretical Electrical Engineering,Ruhr University Bochum, Germany

18:20 Dinner20:00 Poster: Plasma Diagnostic Poster II

(Main Hall)20:00 Poster: Simulation Poster

(Main Hall)

Wednesday May 15th

Track: FLTPD Diagnostics Track: FLTPS Simulations07:30 BreakfastPlenary Session: Common Session Plasma Diagnostic and Plasma Simulation(Main Hall)08:30 Advanced optical diagnostics and validation of computational approaches for the chemi-

cal kinetics in atmospheric pressure plasmas (invited)T. GansUniversity of York, United Kingdom

09:15 Diagnostic and modeling of fast pulsed discharges (invited)S. M. StarikovskaiaLPP Ecole Polytechnique, France

10:00 Coffee break

XXII

Plenary Session: Common Session Plasma Diagnostic and Plasma Simulation II(Main Hall)10:30 Velocity distribution function of atoms and ions in HiPIMS by Doppler broadening of

optical emission linesJ. HeldExperimental Physics II, Ruhr University Bochum, Germany

10:55 Numerical and experimental study of radio frequency micro atmospheric pressureplasma jets driven by voltage waveform tailoring: effects on electron heating and gener-ated speciesY. LiuElectrodynamics and Physical Electronics Group, Brandenburg University of TechnologyCottbus-Senftenberg, Germany

11:20 Collision Energy Transfer LIF: a molecular probe for CO2 dissociation in a nanosecondpulsed dischargeL. M. MartiniDepartment of Applied Physics – Eindhoven University of Technology,The Netherlands

11:45 Kinetic Modeling and Simulation of the Planar Multipole Resonance ProbeC. WangTheoretical Electrical Engineering, Ruhr-University Bochum & Institute of Product- andProcess Innovation, Leuphana University Luneburg, Germany

12:15 Lunch13:15 Excursion20:00 Conference Dinner

Thursday May 16th

Track: FLTPD Diagnostics Track: FLTPS Simulations07:30 BreakfastSession: Time resolved measurements(Main Hall)

Session: Plasma Simulation Slot 5(Small Lecture Hall 1st Floor)

08:15 Precise and sensitive absorptionspectroscopy using Fourier transformand Vernier spectrometers based onoptical frequency combs (invited)L. RutkowskiUniv Rennes, CNRS, IPR (Institut dePhysique de Rennes), France

08:15 Time-Slicing in Multi-Physics Model-ing: Using Hybrid Methods in LowTemperature Plasma Simulations toAddress Disparate Time Scales (in-vited)M. KushnerUniversity of Michigan, Dept. Elec-trical Engineering and Computer Sci-ence, Ann Arbor, USA

09:00 Mid-infrared direct frequency combspectroscopy of plasma processesJ.-P. van HeldenLeibniz Institute for Plasma Scienceand Technology (INP), Greifswald,Germany

09:00 Passing XSAMS: Towards a More Ef-ficient Handling of Plasma Input Data(invited)J. van DijkEindhoven University of Technology,The Netherlands

XXIII

09:25 Investigating CO2 plasma kineticswith in situ infrared absorption Fouriertransformed spectroscopy and com-plementary diagnostics (invited)O. GuaitellaLaboratoire de Physique des Plas-mas, Ecole Polytechnique-CNRS-Univ, France

09:45 Modeling streamers and related tran-sient discharges (invited)U. EbertCWI Amsterdam and TU Eindhoven,The Netherlands

10:30 Coffee break11:00 Closure

XXIV

Monday May 13th

Diagnostics

Diagnostics invited

Time resolved LIF studies on sputtered atoms velocity distribution function

L. de Poucques1, M. Desecures1, A. El Farsy1, J. Bougdira1

1Institut Jean Lamour UMR 7198, CNRS - Université de Lorraine, Campus ARTEM, F-54011 Nancy, France

1. Introduction Magnetron sputter deposition is an established and

widely used method for thin films growth. Nevertheless, the high level of expectations regarding new applications requires a better understanding, controlling and mastering the basic processes governing atoms transport for process optimization purpose. The spatio-temporal knowledge of the properties of incoming film-forming atomic species in HiPIMS (high power impulse magnetron sputtering) may be exploited to adjust the film physical properties as de-sired for applications, and modelling both plasma and particle-depositing processes. In HiPIMS process, the degree of ionization of the sputtered vapour is usually greater than 50 %.However, a significant fraction of sput-tered neutral atoms remains and may influence thin film properties. Ion energy can be fixed by polarizing the sub-strate, while neutrals are difficult to control. Experimental measurements are then required for a better understanding of the transport properties of neutral atoms in view of a potential optimization, in terms of their number or energy when they reach the substrate. Thin films are deposited in order to compare and corroborate the TR-TDLIF (time resolved-tunable diode laser induced fluorescence) meas-urements and deposition rate, estimated by scanning elec-tron microscopy. The main goal is to present and discuss reliable TR-TDLIF measurements made on the sputtered atoms.

2. TR-TDLIF measurements

In our previous papers [1, 2], we reported on measure-ments of atoms velocity distribution functions (AVDFs) of the metastable low-energy level (0.36 eV) of neutral tungsten (W) obtained by means of TR-TDLIF (λla-

ser(W)=407.435 nm) in HiPIMS discharges. New meas-urements on neutral W sputtered atoms will be shown. This method has been drastically improved to be able to measure the very low intensity of the ground state titani-um (Ti) fluorescence signal (λlaser(Ti)=398.170 nm).

3. Results

From the absolute AVDFs measurements (time re-solved-tunable diode laser absorption spectroscopy was used to calibrate the TR-TDLIF signal), we can determine the spatio-temporal variations of densities (AVDF), ener-gies and fluxes (FVDF : see Fig.1 ; z corresponding to the distance from the sputtered target).

We will present the influence of He in the Ar/He gas mixture on the improvement of W atoms transport and the evolution of Ti AVDFs. This shows the different stages of

the atoms transport and highlights an intermediate regime between ballistic (energetic atoms : EN) and diffusive (thermalized atoms : TH) ones, named quasi-diffusive or quasi-thermalized regime of transport (QTH atoms). The different regimes of neutral Ti atoms transport are char-acterized by varying the pressure of the Ar gas (the TR-TDLIF measurements were carried out at fixed dis-charge power (350 W.cm-2) and discharge time (Ton=10 µs and Toff=1000 µs)). These studies may be of a significant step in the ability to control thin film properties.

4. Conclusion

The insight into the spatio-temporal behavior of the film-forming species may be useful to tune the film phys-ical properties as desired and as input data in modeling plasma deposition processes. This technique may be ex-tended with appropriate diode laser to probe any species with rapidly changing AVDF and FVDF in pulsed and strongly oscillating plasmas.

Fig.1 2D-image of total flux-velocity distribution function

FVDFTOT=FVDFEN+FVDFTH of metastable W atoms. Pure Ar ; z = 3 cm ; p=4 Pa; HiPIMS discharge time Ton=7.5 µs & Toff=1500 µs (peak power=1200 W. cm-2). References [1] M. Desecures, L. de Poucques, T. Easwarakhanthan, J.

Bougdira, Applied Physics Letters, 105(18), 181120 (2014).

[2] M. Desecures, L. de Poucques, J. Bougdira, Plasma Sources Science and Technology, 26(2), 025003 (2017).

3

invited Diagnostics

Cavity-enhanced laser spectroscopy of radicals in atmospheric pressure plasmas

Grant A.D. Ritchie

Department of Chemistry, Physical and Theoretical Chemistry Laboratory,

University of Oxford, South Parks Road, Oxford, OX1 3QZ

Radicals are ubiquitous in both the ambient atmosphere and atmospheric pressure plasmas and intimately linked to the oxidizing capacity of these environments. As such, there is widespread interest in the quantitative detection of radical species. In this presentation I will describe a selec-tion of laser based cavity enhanced spectroscopies which allow small levels of these transient species to be probed; these include optical feedback cavity enhanced absorption spectroscopy [1] and Faraday rotation spectroscopy [2],[3].

Faraday rotation spectroscopy (FRS) selectively detects paramagnetic species by measuring the rotation of the plane of polarized light upon interaction with a paramag-netic substance in the presence of a longitudinal magnetic field. FRS quantifies this rotation with the Faraday rotation angle, , which is dependent on the concentration of the paramagnetic substance, the pressure, and the field strength and offers an inherently sensitive means of detecting para-magnetic molecules with limited interference from dia-magnetic species. Pioneered by Liftin et al. [4] in 1980 on the OH radical, the technique has since been developed for the detection of a range of paramagnetic molecules, includ-ing HO2 [5] with a noise equivalent (bandwidth-normal-ized) rotation angle of 8.9 nrad Hz1/2 corresponding to a sensitivity of 0.35  ppmv Hz−1/2 of HO2. In FRS the rotation angle is cumulative, so the more passes the light makes through the sample in the magnetic field, the stronger the rotation signal. Thus, coupling a high finesse cavity-en-hanced technique with FRS can improve sensitivity through a significant increase in the interaction length. Engeln et al. [6] reported the first experimental demonstra-tion of cavity enhanced polarization spectroscopy to probe the b X transition of molecular oxygen. More recently, quantum cascade laser optical feedback cavity enhanced spectroscopy was been paired with Faraday modulation to detect NO [2]. With very competitive detection limits, this technique has now been extended to HO2 with an equiva-lent CRDS device [7]. Figure 1 shows exemplar FRS-CRDS spectra of HO2 radicals sampled from an atmos-pheric pressure plasma as a function of magnetic field strength.

Other example data on the detection of peroxy radical (RO2) species will be presented: HO2 in particular has been highlighted as an important intermediate [8], implicated in the production of reactive oxygen species in cold atmos-

pheric plasma sources, and is integral to the complex chem-ical network which generates hydrogen peroxide as one of the by-products. These data will be supplemented by meas-urements of the absolute number densities of O(3P) atoms produced in both atmospheric and reduced pressure plas-mas.

Fig.1 FRS-CRDS as a function of magnetic field strength.

References [1] M. Gianella, S. Reuter, A. Lawry Aguila, J.H. van Hel-

den, G.A.D. Ritchie, New Journal of Physics 18, 113027, (2016).

[2] M. Gianella, T.H. Pinto, X. Wu, G.A.D. Ritchie, J. Chem. Phys. 147, 054201, (2017).

[3] T.H. Pinto, M. Gianella, G.A.D. Ritchie, J. Chem, Phys. 149, 174202, (2018).

[4] G. Litfin, C.R. Pollock, R.F. Curl, F.K. Tittel, J. Chem, Phys. 72 6602 (1980).

[5] B. Brumfield, W.T. Sun, Y. Wang, Y.G. Ju, G. Wysocki, Optics Letters 39 1783 (2014).

[6] R. Engeln, G. Berden, E. van den Berg, G. Meijer, Jour-nal of Chemical Physics 107, 4458 (1997).

[7] S.A. Press, M.Chem thesis, University of Oxford (2018).

[8] M. Gianella, S. Reuter, S.A. Press, A. Schmidt-Bleker, J.H. van Helden, G.A.D. Ritchie, Plasma Sources Sci. Technol. 27, 095013, (2018).

4

Diagnostics oral

Infrared spectroscopy of wall charges in plasma-facing dielectrics

F. X. Bronold, K. Rasek, H. Fehske

Institut für Physik, Universität Greifswald, 17493 Greifswald, Germany

The most fundamental manifestation of the interactionof a plasma with a solid surface is the formation of anelectric double layer consisting, respectively, of anelectron-depleted and electron-rich space charge regionon the plasma and solid side of the interface. It arisesbecause electrons are deposited more efficiently onto orinto the surface, depending on the electronic structure,than they are extracted from it by neutralization/de-excitation of ions/radicals. Since the beginning of gaseous electronics it is knownthat a double layer is formed at plasma-solid interfaces.Yet a microscopic understanding of the solid-based part ofthe double layer is still missing, mostly because of thelimitations of the available diagnostics for surface chargesand because it was perhaps not essential for mainstreamplasma applications. Continuing progress in theminiaturization of integrated micro-discharges [1],however, driven by the desire to combine solid-state andgaseous electronics [2], makes the embracing solidstructure an integral part of the plasma device. In thesestructures the charge dynamics in the plasma and the solidare intimately linked. A complete understanding of thedischarge requires then experimental techniques whichprovide a view on the charges inside the solid. With this goal in mind we recently proposed infraredattenuated reflection (IR-ATR) spectroscopy as a tool forgaining access to the plasma-induced surplus chargesburied in an dielectric solid facing a low temperatureplasma [3]. The proposal relies on the infrared reflectivityof a layered inset into the wall of the discharge asillustrated in Fig.1. It should be understood as a judicious(local) restructuring of the wall with the aim of measuringits charge. The width of the remaining film of wallmaterial in contact with the plasma is such that itsupports a Berreman mode. This can be easily fulfilled inpractice. An electro-negative dielectric prevents surpluselectrons accumulated from the plasma from entering thegold layer which together with the prism constitutes aKretschmann configuration for exciting surface plasmonpolariton (SPP) resonances at the gold-substrate interface.The hybridization of the SPP resonances with theBerreman mode of the film leads to charge-dependentshifts in the reflectivity dips of the structure. They are themessengers carrying the charge information. In Ref. [3] we gave a (theoretical) proof of principle forthe device, focusing on measuring the total surface chargedensity. Using the device for that purpose, the width ofthe plasma-facing film can be tuned to increase thesensitivity. Due to the electro-negativity of the substratethe film simply collects any electron coming from the

plasma. The spatial distribution of the chargeperpendicular to the interface does not matter in thismode. Our contribution to the workshop addresses thefeasibility of the device for mapping out also the profileρ(z) of the wall charge. The width of the film has then tobe large enough to host the whole space charge regionimplying in practice a few hundred nm. To account for thecharge inhomogeneity at the plasma-solid interface wereplace the Fresnel boundary conditions for electro-magnetic fields at this interface by generalized boundaryconditions as used in the ellipsometry of metallic over-layers [4]. The generalized boundary conditions enableus also to feed into the calculation of the reflectivity ofthe charge-measuring device the charge density profilesobtained from the self-consistent kinetic modeling of thedouble layer and hence to work out their fingerprint inthe charge measuring IR-ATR spectroscopy we suggest.

Fig. 1. Geometry of the suggested IR-ATR wall chargemeasuring device. Our contribution to the workshoppresents the theoretical analysis required for extractingfrom the reflectivity dip, recorded in such an experimentas a function of wavenumber and angle of incidence,parameters of the charge density profile ρ(z) inside theplasma-facing dielectric film which in the present designconsists of the wall material.

References

[1] J. G. Eden et al., IEEE Trans. Plasma Sci. 41, 661(2013). [2] M. Tabib-Azar and P. Pai, Micromachines 8, 117(2017). [3] K. Rasek, F. X. Bronold, M. Bauer, H. Fehske, Eur.Phys. Lett. 124, 25001 (2018).[4] K. Kempa and R. R. Gerhardts, Surface Sci. 150, 157(1985).

5

oral Diagnostics

Imaging techniques reveal initial phases of a nanosecond discharge in liquid water with sub-ns temporal resolution

M.Šimek, P. Hoffer, V. Prukner and J.Schmidt

Institute of Plasma Physics of the Czech Academy of Sciences,

Department of Pulse Plasma Systems, Za Slovankou 3, 18000 Prague, Czech Republic

1. Introduction Initiation and evolution of micro-discharges in liquid

water due to highly divergent electric field of (sub)nano-second duration is a very complex phenomenon involving generation and multiplication of charged species in highly collisional environment. Numerous studies have concluded that discharges in liquid could be formed due to the pres-ence of voids with minimal thermal effects and without any liquid-vapour phase change. Unambiguous experimental evidence of underlying physical mechanisms is yet missing mainly because of short time- and small space-scales asso-ciated with the initial stages of such discharge events.

2. Experiment

Typically, experimental configurations based on needle-to-plane electrode geometry (typical curvature of the tip of the needle of tens of micrometers) stressed by HV pulses (amplitude of 50-150 kV) have been used and such config-urations lead to sub-millimetre corona-like structures evolving with velocities of up to about 200 km/s [1-3].

In this work, we have explored the morphology, dynam-ics, and emission characteristics of the micro-discharges produced in deionized liquid water by applying fast-rising HV pulses in a point-to-plane geometry.

We have employed the shadowgraphic and interferomet-ric techniques of to reveal dynamics of the initial non-lu-minous pre-discharge phase with very high spatio-tem poral resolution (<50 ps and <1 µm) using Katana 05 Onefive laser (532 nm/30 ps/4 nJ) as the probe/reference beam. We have also employed the techniques of time-re-solved ICCD microscopy (Questar QM-1 long-distance microscope) and spectroscopy (JY iHR-320 spectrometer) to register both morphologic fingerprints and emission spectra with high temporal (from 0.2 to 2 ns) resolution of secondary luminous discharge phase. 3. Results

We succeeded in capturing footprints of the initial dark phase and shockwaves developing around the streamer head (Fig.1). It comes out that initial non-luminous bush-like phase is followed by an inception of a few isolated lu-minous micro-discharge filaments developing in tree-like structure (Fig.2). After initial expansion and branching of luminous filaments, the length of luminous filaments col-lapses. Furthermore, we observed that plasma-induced emission produced in liquid water is given by the broad-band continua occurring in UV-vis-NIR spectral range.

Fig.1 Snapshot of the discharge chamber (left) and inter-ferometric image of non-luminous initial phase taken simultaneously with subsequent luminous phase (right).

Fig.2 ICCD images of micro-discharges developing in deionized water during nanosecond HV pulse.

4. Summary Imaging techniques applied with (sub)nanosecond tem-

poral resolution reveal characteristics of non-luminous bush-like and luminous tree-like structures in DI water in response to positive ns HV pulses in needle-plane geometry.

Supported by the Czech Science Foundation (18-04676S).

References [1] M. Šimek et al., Plasma Sources Sci. Technol. 26,

07LT01, (2017). [2] B. Pongrác et al., J. Phys.D: Appl. Phys. 51, 124001,

(2018). [3] B. Pongrác et al., Plasma Sources Sci. Technol. 28,

000000 , (2019).

6

Diagnostics oral

Nanosecond plasmas in water: ignition, cavitation, temperatures

K.Grosse, M. Kai, J. Held, A. von Keudell

Chair for Experimental Physics II, Ruhr-University Bochum, Bochum, Germany

1. General

Nanosecond plasmas in liquids play an important role in the field of decontamination, electrolysis or plasma medi-cine. The understanding of these very dynamic plasmas re-quires information about the temporal variation of species densities and temperatures. This is analyzed by monitoring nanosecond plasmas that are generated by high voltages (HV) between 15 and 25 kV and pulse lengths of 10 ns ap-plied to a tungsten tip with 50 µm diameter immersed in water. Ignition of the plasma causes the formation of a cav-itation bubble that is monitored by shadowgraphy to meas-ure the dynamic of the created bubble and the sound speed of the emitted acoustic waves surrounding this tungsten tip, as indicated in Fig. 1.

The temporal evolution of the bubble size is compared

with cavitation theory yielding good agreement for an ini-tial bubble radius of 25 µm with an initial pressure of 5 x 108 Pa at a temperature of 1200 K for a high voltage of 20 kV. This yields an initial energy in the range of a few 10-5 J that varies with the applied high voltage. The dissipated

energy by the plasma drives the adiabatic expansion of wa-ter vapor inside the bubble from its initial super critical state to a low pressure, low temperature state at maximum bubble expansion reaching values of 103 Pa and 100 K, re-spectively.

These predictions from cavitation theory are corrobo-

rated by optical emission spectroscopy (OES) to determine both the electron density and the electron temperature from the Stark broadening and shift of the spectral lines of H-line (656 nm) and an OI-line (777 nm) after the initial plasma pulse. After the initial HV pulse igniting the nano-second plasma, the electrical power is oscillating in the feed line between HV pulser and plasma chamber leading to a ring down of the electrical power in the system with decay times of the order of 80 ns. These reflected pulses re-ignite a plasma inside the expanding bubble periodically. During the initial plasma, but also during the re-ignition, broad band emission due to recombination and Bremstrah-lung becomes visible within the first 100 ns and line emis-sion at later times after the pulse.

Fig. 1 Shadowgraph images of a ns plasma ignited in water at different voltages and times

14 kV 18 kV 22 kV 26 kV

6 ns

10.5 µs

1.5 µs

30 µs

351 µs

330 ns

1 µs

Fig. 2: Temporal development of the radius of the

cavitation bubbles (symbols) and comparison to mod-elling (solid lines)

10-6 10-5 10-4 10-30

500

1000

bubb

le ra

dius

[µm

]

time (s)

UkV= 14 kV UkV= 18 kV UkV= 22 kV UkV= 26 kV

7

oral Diagnostics

Towards Ultrashort Pulse Generation from Nanosecond Laser Sources for Elec-tric Field Measurements

Tat Loon Chng1, Maya Naphade2, Benjamin Goldberg2, Igor Adamovich3, Svetlana M. Starikovskaia1

1Laboratory of Plasma Physics (CNRS, Ecole Polytechnique, Sorbonne Universities, University of Pierre and Marie

Curie - Paris 6, University Paris-Sud), Ecole Polytechnique, route de Saclay, 91128 Palaiseau, France 2Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA

3Nonequilibrium Thermodynamics Laboratories, Department of Mechanical and Aerospace Engineering, Ohio State

University, Columbus, OH 43210, USA

1. Introduction

A laser diagnostic based upon the phenomenon of second

harmonic generation has been re-developed recently for the

purpose of electric field measurements in gas plasmas [1-

2]. More commonly known as electric field induced sec-

ond harmonic generation (E-FISH) [3], the relatively

straightforward implementation of this technique has be-

gun to find application in real-life plasma actuators [4-5].

Since the signal generation is intrinsically intensity-driven,

this naturally favours short laser pulses on the order of hun-

dreds of picoseconds (ps) or less. The use of high-energy,

nanosecond pulses (ns) is in principle possible, but increas-

ing the energy indefinitely is constrained by the occurrence

of laser-induced breakdown. Furthermore, ns pulses are

limited in time-resolution, which may prove important

when diagnosing fields associated with ns type discharges.

Unfortunately, ultrashort pulsed laser systems are not eas-

ily accessible and costs can often be prohibitive.

In this work, we attempt to use a Pockels cell to ‘slice’ a

ubiquitous 1064 nm YAG laser beam into a pulse of shorter

duration, with a view to using this shorter pulse for useful

E-FISH measurements in ns discharges.

2. Experimental setup

Fig. 1: E-FISH setup

The experimental setup consists of 3 main parts. The

high voltage pulses and electrode system used for demon-

strating the E-FISH measurements, the optical layout for

the pulse-slicing, and that for the E-FISH measurements.

An FID model number FPG 12-1NM high-voltage (HV)

generator delivers positive polarity, 9.4 kV amplitude volt-

age waveforms with a rise time of ~8 ns and a full width at

half maximum (FWHM) of ~30 ns at a repetition rate of 10

Hz. This voltage is applied across two rectangular, flat

plate electrodes separated by a gap of 4 mm as shown in

Fig. 1.

The Pockels cell used in these experiments was supplied

by Leysop Ltd and consists of a UV BBO crystal housed in

a custom-made casing with electrical leads. Applying a HV

pulse to the Pockels cell alters the birefringence of the BBO

crystal for the duration of this pulse, effectively rendering

the cell as an ‘electronic waveplate’. By placing the

Pockels cell between two crossed polarizers and timing it

such that it is triggered in coincidence with a laser pulse, a

shorter laser pulse may be obtained (see Fig. 2) [6].

Fig. 2 Pulse slicing optical schematic. Top: Single-pass, Bot-

tom: double-pass.

The energy of the sliced pulse is strongly dependent on

the polarization rotation that can be produced by the

Pockels cell. As seen in the top schematic of Fig. 2, for a

fully transmitting cell, performance is best when half-wave

rotation can be effected. Unfortunately, the present cell was

designed for 205 nm light and limits the amount of polari-

zation rotation that can be obtained at 1064 nm. Further-

more, the possibility of crystal damage at this off-optimum

wavelength also restricts the energy input into the cell. To

mitigate this problem, a double-pass arrangement is imple-

mented, results of which are shown in the following section.

The E-FISH setup given in Fig. 1 is essentially similar

in principle to that described in [2]. A Quanta-Ray Lab-230

YAG laser system is used as a source of 1064 nm, 50 mJ,

~15 ns pulses at repetition rate of 10 Hz. These pulses are

reduced to about 2.8 ns and 2 mJ after passing through the

double-pass pulse slicing layout given in the bottom panel

of Fig. 2. This beam is focused with a 30 cm lens into the

8

Diagnostics invited

Laser diagnostics for electric field measurements in air plasmas, plasma-enhanced flames, and atmospheric pressure plasma jets

Igor V. Adamovich1, M. Simeni Simeni1, Y. Tang2, and K. Orr1

1Department of Mechanical and Aerospace Engineering, Ohio State University, Columbus, OH, USA

2Tsinghua University, Beijing, China

Non-intrusive laser diagnostic measurements of tem-poral and spatial distributions of electric field in atmos-pheric pressure plasmas are essential for development of engineering applications such as plasma flow control, plasma-assisted combustion, and plasma medicine. This work presents an overview of electric field measurements in atmospheric pressure plasmas by ps four-wave mixing [1] and, more recently, by ps and fs electric-field induced second harmonic (E-FISH) generation [2,3]. In both cases, absolute calibration is obtained from measurements of a known Laplacian field.

Picosecond four-wave mixing measurements have been done in ns pulse discharges in ambient air for several elec-trode geometries. For short voltage rise times of several ns, peak electric field considerably exceed DC breakdown threshold. Sub-nanosecond time resolution is obtained by monitoring the timing of the individual laser shots relative to the voltage pulse, and post-processing four-wave mixing signals saved for each laser shot, placing them in the ap-propriate “time bins” [1].

The main advantages of second harmonic generation over four-wave mixing are that it is considerably more sen-sitive and species independent, such that is can be used in any high-pressure plasma. Ps second harmonic generation has been used to measure electric field in dielectric barrier discharge plasma flow actuators, flames enhanced by tran-sient plasmas, and atmospheric pressure plasma jets. Simi-lar to four-wave mixing, individual electric field vector components are isolated by measuring second harmonic signals with different polarizations.

In ns pulse plasma actuators, the data show that surface

charge accumulation strongly affects the electric field in the plasma [4] (see Fig. 1). In flames, combining ns pulses and ms time scale waveforms results in a strong effect of the plasma on the flame, due to the ionization produced by the ns pulses and the ion wind generated on the long time scale. In some cases, for relatively simple electrode geom-etries and in the absence of surface charge accumulation on dielectric surfaces, Laplacian electric field measured be-fore breakdown may be used for absolute calibration [5] (e.g. see Fig. 2), such that the technique becomes “self-cal-ibrating”. This is particularly important at the conditions when the mixture composition is not known, such as in plasma-enhanced flames and atmospheric pressure plasma jets. If such self-calibration does not apply, due to surface charge effects or ionization wave propagation, inference of the electric field in flames and plasma jets may be compli-cated considerably by chemical reactions and mixing with ambient air species.

References [1] M. Simeni Simeni, B. Goldberg, I. Gulko, K. Freder-

ickson, and I.V. Adamovich, J. Phys. D: Appl. Phys. 51 (2018) 01LT01

[2] A. Dogariu, B.M. Goldberg, S. O’Byrne, and R.B. Miles, Phys. Rev. Applied 7 (2017) 024024

[3] B.M. Goldberg, T.L. Chng, A. Dogariu, and R.B. Miles, Appl. Phys. Lett. 112 (2018) 064102

[4] M. Simeni Simeni, Y. Tang, K. Frederickson, and I.V. Adamovich, Plasma Sources Sci. Technol. 27 (2018) 104001

[5] M. Simeni Simeni, Y. Tang, Y.-C. Hung, Z. Eckert, K. Frederickson, and I.V. Adamovich, Combust. Flame 197 (2018) 254

Fig. 1 Electric field vector components in a surface plasma actuator, during a negative polarity ns pulse [4].

Fig. 2 Electric field in a positive polarity ns pulse discharge in a hydrogen flow below a hydrogen diffusion flame [5].

9

oral Diagnostics

*) affiliation during time of research

Ultrafast Laser Diagnostics of Electric Fields and Turbulent Reactive Flows in Plasma Jets

S. Reuter1*,2, B. Goldberg1, A. Dogariu1, Y. Zhang1*, R.B. Miles1,3

stephan.reuter@fn.de 1Applied Physics Group, Princeton University, Princeton, USA

2Electrical Engineering Department, Technical University Lublin, Lublin, Poland 3 Department of Aerospace Engineering, Texas A&M University, College Station, TX 77843

1. Introduction

Diagnostics of cold plasma jets interacting with liquids are limited by high gradients, multiphase transport pro-cesses and interfaces of plasma gas and liquid phases [1]. Most conventional plasma diagnostics fail in cases of non-equilibrium processes. Ultrafast laser spectroscopy, how-ever, permits the diagnostic of fundamental plasma proper-ties such as reduced electric field or flow properties and gas composition at timescales much shorter than collisional processes. The present work shows 1D space resolved elec-tric field measurements in a plasma jet effluent. Further-more, time resolved 1D-flow field analysis in turbulent flow of an atmospheric pressure plasma jet is presented. The techniques applied are femtosecond electric field in-duced second harmonic generation (E-FISH) [2] and Femtosecond Laser Electronic Excitation Tagging (FLEET) [3, 4]. Single shot capabilities and smart averag-ing methods are presented.

2. Experimental setup The plasma jet studied in this work consists of a dielec-

tric tube with a pin electrode in its center and a grounded ring electrode around the tube [5]. The jet is operated with a gas flow of 500 sccm argon feed gas and a 500 sccm air coflow surrounding the jet effluent. The discharge is oper-ated by a nanosecond pulser (FPG 20- 10 NM, FID GmbH, Germany). The plasma jet’s electric field can be measured by nonlinear mixing process that leads to second harmonic generation of the probing laser light. The polarization P at the second harmonic (2ω) is described by a third order non-linear process [2]:

E(F) is the electric field, measured, Ek,l are the electric fields of the incident laser radiation, which are of identical wave-length. N is the number density of the medium and χ is the third order susceptibility. χ depends on the dipole moments of the medium and the orientations of the electric fields. For the experiments, a femtosecond (fs) infrared laser is used to induce the second harmonic radiation [5, 6]. The Spectra Physics Solstice Ace laser operates at 780 nm with 6 mJ pulse energy and a FWHM of 50 fs and spectral band-width of 20 nm at a repetition rate of 1 kHz. A polarizer in the detection setup separates the vectorial field components of the electric field.

The same femtosecond laser is used to determine a 1D-

flow field in the turbulent flow of the plasma jet: the strongly focused laser excites and ionizes nitrogen, which subsequently dissociates via electron ion recombination. The first positive emission system of nitrogen is populated by nitrogen recombination, radiating for tens and hundreds of microseconds after laser excitation, allowing for an ob-servation of the flow with gated cameras. For the measure-ments of the flow profile, 4 mJ pulse energy and 300 mm spherical focus lenses was used. The advected FLEET line image was taken 100 µs after the laser pulse.

3. Results

Fig. 1: Vectorial electric field component in the axial di-rection along the jet flow with radial space resolution

Fig. 1 shows the axial component of the electric field of the plasma jet effluent with a cylindrical laser focus line in radial direction. Each electric field signal originates from a several µm sized volume element determined by the focal beam waist, the Rayleigh range, and the camera pixel size.

Flow field measurements show the laminar to turbulent transition with increasing gas flow and the effect of plasma on the flow regime. Acknowledgement The authors thank N. Tkach for his technical assistance. SR acknowledges funding by Princeton University and the Alexander von Humboldt Foundation, YZ acknowledges funding through the Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. References [1] S. Reuter et al. J. Phys. D 51 233001 (2018) [2] A. Dogariu et al. Phys. Rev. Appl. 7 024024 (2017). [3] J. B. Michael et al. Appl. Opt, 50 5158 (2011) [4] Y. Zhang et al. Optics Lett. 43, 551 [5] S. Reuter et al. Proc. 22nd Int. Conf. GD2018, NoviSad [6] B. Goldberg et al. APS Bulletin (2018)

4 kV at 1 kHz repetition rate. The signal is measuredwith a back current shunt (BCS) [3]. The incomingpulse ends shortly after 50 ns, the following reflectedpulse detected by the BCS exhibits a different shapewhich results from energy deposition in the discharge[4].

3. ELECTRIC FIELD MEASUREMENTS BYSECOND HARMONIC LIGHT GENERA-TION

A plasma jet’s electric field can be measured by a wavemixing process that leads to second harmonic genera-tion of the probing laser light. Generally, three wavenon-linear mixing processes are forbidden in symmet-ric and homogeneous media. The electric field – gen-erated by the charges of the plasma jet – acts, however,as additional wave of zero frequency and breaks thesymmetry. This leads to a wave mixing process whichgenerates light of double frequency with a square de-pendency on the electric field strength. The polariza-tion P at the second harmonic (2!) is described by athird order non-linear process [5]:

P 2!i =

3

2N�

(3)i,j,k,l(�2!, 0,!,!)E

(F )j E

(!)k E

(!)l (1)

E(F ) is the electric field, measured, Ek,l are the elec-tric fields of the incident laser radiation, which are ofidentical wavelength. N is the number density of themedium and � is the third order susceptibility. � de-pends on the dipole moments of the medium and theorientations of the electric fields. Fig. 3 shows the ex-perimental setup: A femtosecond infrared laser is usedto generate the second harmonic radiation. The Spec-tra Physics Solstice Ace laser operates at 780 nm with6 mJ pulse energy and a FWHM of 50 fs and spectralbandwidth of 20 nm at a repetition rate of 1 kHz.

Fig. 3. Experimental setup for electric field measurements in aplasma jet by second harmonic light generation from an infrared fs-laser pulse.

The laser beam is focused by a cylindrical lens of 20cm focal length into the plasma region. Immediatelybefore the plasma, a long pass filter filters second har-monic light generated on optics closer to the laser. Asecond cylindrical lens collimates the beam to guideit through a dispersive prism that separates the secondharmonic from the fundamental radiation. The pulse

intensity is monitored by a fast photo diode (DET10A).A mirror guides the second harmonic signal to a cylin-drical lens which focuses the signal onto a gated inten-sified CCD camera (PCO DiMax with an SCO inten-sifier). A short pass filter filters unwanted backgroundlight directly before the ICCD camera. The cylindricalfocus allows the use of the full laser energy in contrastto using a spherical focus, which leads to white lightgeneration in the optics and the beam focus already atlow pulse energies of around 100 µJ.

Fig. 4. Transmitted laser power for different z-positions across thelaser focus of a knife edge shadowing the laser beam.

The beam focus was characterized by moving a knifeedge across the focus width to determine the beamwaist at the focus [9]. Figure 4 shows the intensityof the transmitted laser radiation normalized to the fulllaser intensity for different knife edge positions. Thetransmitted laser power follows the function:

Ptrans(z) =1

2

"1 � erf

p2z

!0

!#(2)

the term erf denotes the error function which allowsfor the derivation of the properties of a Gaussian laserbeam. From the fit following equation 2, the beamwaist !0 at the laser focus was determined to be 5.3 µm.The Rayleigh range zr =

⇡·!20

� is 113 µm. The secondharmonic light can be expected to originate from a vol-ume element of the beam width, the height of the laserbeam of ⇠1 cm, and – due to the I2 dependence of thesecond harmonic generation – one or several Rayleighranges of ⇠100µm. The second harmonic light is im-aged onto the ICCD camera yielding a spatial reso-lution along one dimension according to the imagingresolution and an axial resolution of a few Rayleighranges. The electric field induced second harmonicgeneration is proportional to the number of moleculesthat contribute. This is valid throughout the length ofcoherent second harmonic generation, which is about1 cm at the given conditions. Since the plasma di-mensions in radial direction are much smaller than thecoherence length, it can be concluded that the secondharmonic light signal is not influenced by effects of sig-nal generation outside the coherence length. With thissetup, for the first time, a 1D spatially-resolved mea-surement of the electric field in a plasma jet ionizationfront has become possible.

10

Diagnostics oral

Time dependent optical methods for electric field determination with picosecond resolution in discharges in gases and liquids

T. Hoder1, M.M. Becker2, P. Hoffer3, M.Šimek3, D. Loffhagen2

1Masaryk University, Brno, Czech Republic

2Leibniz Institute for Plasma Science and Technology, INP Greifswald, Germany 3Institute of Plasma Physics, Czech Academy of Sciences, Prague, Czech Republic

1. Introduction

The determination of the electric field with high tem-poral resolution in discharges in gaseous or liquid environ-ment is a challenging task. In this contribution we will con-sider the methods of plasma investigation by optical emis-sion spectroscopy and by laser aided method using the non-linear electro-optical Kerr effect.

We will present our very recent results of application of non-steady-state collision-radiative models for electric field determination in atmospheric pressure argon. We will also introduce the method for direct electric field measure-ment in liquid water media as implemented for nanosecond discharges in our experimental arrangement.

2. Non-steady-state collision-radiative model

The collision-radiative models are well established method for diagnostics of plasmas in many different gases. Typically however, they are applied in their limited time-independent form, whether to investigate the density of metastable states or electrons in argon plasmas [1-3]. This limitation simplifies significantly the theoretical expres-sions of the model and also does not require an advanced high-time resolved instrumentations for spectra recording. As a consequence, the diagnostics by means of these mod-els is limited to steady-state modes of selected discharges or its accuracy is significantly reduced. The time-depend-ent processes in plasmas (pulsed discharges, streamers, ionizing waves, fast decay phases etc.) therefore remains hidden for these models even though some qualitative tendencies can be inspected [4].

In this contribution we will present a new methodology and non-steady-state radiative model for electric field de-termination in transient argon plasmas suitable for picosec-ond spectroscopy as utilized by the authors using time-cor-related single photon counting method. Based on the multi-term Boltzmann equation and fluid model of barrier dis-charge in local mean energy approximation we obtained an access to the full kinetic chemistry taken place in the sys-tem. The sensitive reactions were selected and based on them the electric field was reconstructed for the investi-gated streamer in barrier discharge.

The comparison of the original electric field waveform (as resulted from the fluid model) and the theoretically de-termined (or measurable) electric field as resulted from the developed non-steady state model is shown in figure 1. A reasonable agreement was achieved and understanding of

Fig.1 Comparison of the original electric field waveform (red) and the theoretically determined one (black) using the non-

steady state collision-radiative model.

the complexity of the theoretically obtained electric field waveform improves further the experimental diagnostics.

3. Kerr effect approach in liquids

The determination of the spatially resolved local electric field development in liquid media prior to the breakdown is an important task for verification of numerical methods applied to understand the crucial first picoseconds of the pulsed discharge. Using picosecond lasers and advanced optical techniques the generation of fast electrical and pres-sure changes in the liquid media can be measured. In this contribution, first results of this method will be presented.

This contribution was supported by the Czech Science

Foundation grant project number 18-04676S. References [1] D. Mariotti et al. Appl. Phys. Lett. 89, 201502, (2006). [2] X.-M. Zhu, Y.-K. Pu J. Phys. D: Appl. Phys. 43,

403001, (2010) [3] E. Desjardins et al. Plasma Sources Sci. Technol. 27,

015015 (2018) [4] M. Šimek, P. Ambrico, T. Hoder et al. Plasma Sources

Sci. Technol. 27, 055019 (2018)

11

poster Diagnostics

Spectroscopic diagnostics on a pulsed CO2 microwave discharge

E. Carbone, F. D’Isa, A. Hecimovic, U. Fantz

Max Planck Institute for Plasma Physics, Boltzmannstr. 2, D-85748 Garching, Germany

1. Introduction The large amount of fossil fuels burned in the last cen-

tury and half led to a large increase of CO2 concentration in the atmosphere due to the imbalance between anthro-pogenic emission and fixation by natural processes. The conversion of CO2 into value-added chemicals has been proposed in order to valorize economically “carbon cap-ture storage” processes. Also, no viable replacement for liquid hydrocarbons exists yet in sectors such as air transportation. Many groups concentrate therefore their efforts on the fabrication of synthetic fuels based on re-newable energies for the development of a sustainable energy economy. The first step for the valorization of CO2 is its conversion into CO and O2. Non-equilibrium plas-mas are a promising technology for the activation and conversion of CO2 into added-value chemicals like CO. Microwave plasmas have the potential to treat large gas flows and to operate at high pressure and high energies efficiencies for CO2 conversion into CO [1]. In the pre-sent contribution we present an initial experimental inves-tigation of a 2.45 GHz pure CO2 microwave plasma.

2. Experimental setup

The plasma torch (see Fig. 1 for a schematic) is based on a setup developed at the Uni. of Stuttgart which can ignite an atmospheric pressure plasma torch using only microwave power without the use of additional igniters [2]. A modified version of this device has been built at the Max-Planck Institute for Plasma Physics (IPP) so that it can be operated from low (~10 mbar) up to atmospheric pressure in molecular gases such as N2 and CO2 in a closed system. Stable operation of the plasma is obtained both in continuous and pulsed regime. The plasma is gen-erated inside a quartz tube of 26 mm inner diameter with 2.45 GHz microwave radiation with 900 W up to 3kW power input. The gas flow (10-100 L/min) is injected into the tube using a 4-gas inlet tangential injection system to stabilize the plasma in the center of the tube. 3. Results

Spatial and temporal resolved measurements of the plasma emission are done inside the microwave resonator and in the effluent. The spectra are absolute calibrated using an Ulbricht sphere and a deuterium lamp. The main emission features of the plasma at high pressure are the C2 (d3Πg-a3Πu) Swan band system in addition to a few typical O and C atoms lines. A spectroscopic database was con-structed using the recent compilation and calculations by Brooke et al. [3] of the line strengths and molecular con-

stants for the C2 (d3Πg-a3Πu) Swan band system and im-plemented into the open source software MassiveOES [4]. State by state fitting is employed to check the correctness of assuming a Boltzmann distribution for both the rota-tional and vibrational distribution functions. Uncertainties related to fitting parameters and experimental ones due for instance to noise or baseline correction are analyzed. Measurements are performed both in continuous and pulsed mode (1-18 kHz range) in the pressure range 200-900 mbar.

In order to quantify the energy efficiency and conver-sion rate of CO2 into CO, the gas composition of the ef-fluent is measured using a mass spectrometer with a cus-tom sampling system. For obtaining a more fundamental understanding of the plasma kinetics, it is however nec-essary to measure in situ the spatial distribution of reac-tive species. A nanosecond pulsed tunable dye laser sys-tem is implemented for detecting reactive species inside the reactor. First measurements of the O atoms spatial distribution will be reported inside of the resonator as a function of plasma parameters.

References [1] W. Bongers, et al. “Plasma-driven dissociation of

CO2 for fuel synthesis.” Plasma Processes and Polymers, 14 (2017), 6:1600126.

[2] M. Leins, S. Gaiser, A. Schulz, M. Walker, U. Schumacher, T. Hirth. “How to Ignite an Atmospheric Pressure Microwave Plasma Torch without Any Addition-al Igniters.” J. Vis. Exp. 98 (2015), e52816.

[3] J. S. A. Brooke, P. F. Bernath, T. W. Schmidt, and G. B. Bacskay. “Line strengths and updated molecular con-stants for the C2 Swan system.” Journal of Quantitative Spectroscopy and Radiative Transfer, 124 (2013), 11 – 20.

[4] J. Voráč, P. Synek, L. Potočňáková, J. Hnilica, and V. Kudrle. “Batch processing of overlapping molecular spectra as a tool for spatio-temporal diagnostics of power modulated microwave plasma jet.” Plasma Sources Sci-ence and Technology, 26 (2017), 025010.

Fig. 1: Schematic of the plasma torch. The wave-guide height is 4 cm and the tube length 20 cm.

12

Diagnostics poster

Static and RF electric field direct measurement based on Lyman- emission from a hydrogen probe beam

L. Chérigier-Kovacic, F. Doveil

Aix Marseille University, CNRS, PIIM, case 321, campus Saint-Jérôme, FR-13013 Marseille, France

Precisely measuring electric fields in plasmas, where

they play a fundamental role, has been a longstanding chal-lenge. We propose a new technique which allows to di-rectly deduce local electric field amplitude from the meas-ured physical quantity. We show in particular the perfor-mances of this diagnostic when measuring an oscillating electric field at frequencies around Lamb-shift frequency (~1 GHz), where Lyman- emission is enhanced by a fac-tor of almost 400. 1. Experimental principle

The diagnostic is based on the emission of the Lyman- line by a hydrogen probe-beam in the 2s state when the beam passes through a region where an electric field is pre-sent (Electric Field Induced Lyman- Emission). The elec-tric field couples the Lamb-shift spaced 2s and 2p atomic hydrogen levels. The 2s lifetime is shortened and this level decays via 2p to the ground state [1]. By measuring the in-tensity of the subsequent Lyman- radiation, it is possible to determine the magnitude of the field in a defined region.

In our experiment, the beam is exposed to an externally controlled electric field generated between two plates: a static voltage or radiofrequency power supply is connected to the lower plate while the upper one is grounded. The Lyman- emission is detected by a VUV-photomultiplier, perpendicularly to the beam. Either the beam or the electric field is pulsed and a lock-in amplifier is used to improve the signal/noise ratio. The spatial resolution of about 1 cm can be improved by inserting a diaphragm in the beam path.

A saturation of the signal is observed in both static and oscillating cases. It is due to the decay of metastable atoms along the beam path before the diagnosed volume, when the field is higher than a threshold value depending on the frequency [2].

2. Results

The static electric field profile between the plates in vacuum is in good agreement with the result of a numerical resolution of the problem. In a plasma, the same measure-ment shows that the electric field is null except in the sheath near the polarized plate. The width of the sheath var-ies accordingly with the plasma density.

Measurements of Lyman- signal in vacuum as a func-tion of electric field frequency in the range [0.8;1.4] GHz exhibit very thin peaks. These peaks are also observed on the signal given by a 2 cm-bare wire antenna. A numerical simulation confirms that they correspond to resonant modes of the vessel.

Since we know the transition rate as a function of the frequency of the field, we can deduce the amplitude of the

oscillating field from the ratio of signals of static and oscil-lating cases. However, the number of emitting atoms pre-sent in the diagnosed volume depends on the field along the beam path, which varies with the frequency. Thus the calibration is based on the measurement of the Lyman-a signal resulting from the superposition of static and an os-cillating electric field at a given frequency. The result is displayed on the graph of Fig. 1 for 1.25 GHz. Values as small as 1V/cm can be detected. As long as the signal is not saturated, the electric field amplitude varies like the square root of the power. Otherwise, the field is underestimated.

Measurements with an oscillating field in a plasma have been performed but could not be interpreted yet. This work is still in progress. Acknowledgements This work has been carried out within the framework of the French Federation for Magnetic Fusion Studies (FR-FCM) and of the Eurofusion consortium. References [1] W. E. Lamb, Jr. and R. C. Retherford, Phys. Rev. 79

549 (1950) [2] Chérigier-Kovacic L., Ström P., Lejeune A. and Doveil

F., Rev. Sci. Instrum. 86 063504 (2015).

13

poster Diagnostics

Gas temperature characterization of annular shape RF APPJ for biomedical application

Ivana Sremačkia, *, Andrea Jurovb, Martina Modicb, Uroš Cvelbarb, Christophe Leysa, Anton Nikiforova

aDepartment of Applied Physics, Ghent University, Sint-Pietersnieuwstraat 41, Gent, 9000, Belgium

bJozef Stefan Institute, Jamova cesta 39, Ljubljana, 1000, Slovenia

Abstract

The RF (13.56 MHz) argon atmospheric pressure plasma jet (APPJ) has been developed for biomedical application, in the aim of skin treatments such as wound healing and topical drug introduction. In order to validate applicability of novel APPJ for heat-sensitive surface treatments several plasma temperature diagnostics methods were used. Gas temperature in plasma jet was determined by the meaning of optical emission spectroscopy (OES), while Rayleigh and Raman laser scattering techniques were used for evaluation of OES. The complimentary of the methods based on different gas temperature interpretation allows comparing and validating their applicability for diagnostics of atmospheric pressure non-equilibrium discharges. OES temperature measurements were based on approach that RF APPJ is non-equilibrium plasma working at high frequency of collisions in between particles, so rotational temperature of molecules is equivalent to gas temperature. Rotational temperatures of N2 excited states and OH radicals were measured based on detection of emission from transition C3Пu→B3Пg (0,2) and A2Σ+→X2Пi (0,0), respectively. Approximation of Boltzmann population of rotational levels in the same vibrational band was applied here, following [1]. OH rotational temperature in the effluent of Ar plasma was estimated in a range of 310 - 350 K at a distance of 1.5 mm from the nozzle. The increase of the gas flow leads to decrease of Tg due to gas cooling effect. Interestingly, the RF power increase has negligible impact on rotational temperature of OH(A) radicals. However, rotational temperature estimated from the linear slop of the Boltzmann plot of N2 emission indicates the temperature in the range of 450-480 K. This result is strongly contradictory to data obtained from OH (A-X) emission. The discrepancy of the results is related to the drawback of the method as proper approximations need to be made on rotational population of excited states of molecules. On the other hand laser scattering spectroscopy (Rayleigh

and Raman scattering) for gas temperature measurement is using approach that intensity of scattered laser light is related to density or population of ground state particles. Both methods have high spatial and temporal resolution determined by size of the beam and laser pulse duration as presented in Fig. 1.

Fig.1. Rayleigh scattering in APPJ effluent obtained during 10 ns pulse at 532 nm at 2 mJ of the laser energy.

Rayleigh scattering reveals that RF APPJ is characterized by uniform gas temperature distribution in a core of the discharge with maximum value below 340±5 K. This result is in good agreement with estimations of gas temperature by OH(A) emission and obviously indicating strong overestimation of gas temperature by N2 emission. Furthermore, it is found that results of Rayleigh scattering spectroscopy and Raman scattering are in agreement indicating safe mode of the source operation for biomedical applications at Ar flow above 2 slm with forward RF power between 10 and 30 W. Based on analysis of OES results and laser scattering it is assumed that temperature overestimation based on N2 emission is due to resonant energy transfer from Ar (3P2) and Ar(3P0) metastables to N2 (C) molecules and overpopulation of N2(C) level leading to non-Boltzman behavior. References

1. Bruggeman, P.J., et al., Gas temperature determination from rotational lines in non-equilibrium plasmas: a review. Plasma Sources Science & Technology, 2014. 23(2): p. 32.

14

Diagnostics poster

Spectroscopic study of the neutral gas temperature of silicon based DC MHCD in various gases close to atmospheric pressure

S. Iseni 1, R. Michaud1, P. Lefaucheux1, V. Schulz-von der Gathen2, G. B. Sretenovic3, R. Dussart1

1GREMI, UMR7344 CNRS/Univ. Orléans, 45067 Orléans, France2Ruhr-Univ. Bochum, Experimental Physics II, 44801 Bochum, Germany

3Faculty of Physics, Univ. of Belgrade, P.O. Box 44, 11001 Belgrade, Serbia

1. IntroductionMicro hollow gas discharges (MHCD) have been on

high interest to produce highly ionized gas while keep-ing the gas temperature close to room temperature[1]. The first concept of MHCD reactors was introduced in the late 90’s and has been developed over the past decade to sup-port a large amount of applications[1]. For instance one can mention the production of (V)-UV radiation, lighten-ing, gas purification, surface activation, material and vol-ume decontamination via the production of biologically active reactive chemical species (O, O3, O2(a1Δg)). Al-though several designs and geometries have been devel-oped to produce micro-cavities, MEMS fabrication tech-nologies offer several advantages. For example, a silicon (Si) based MHCD allows for reducing significantly the electrode gap (8 µm SiO2 layer) and the cavity size (typi-cally from 50 to 200µm diameter, 30 µm depth)[2,3]. Op-erated in DC at pressure ranges from 2.104 to 105 Pa, the plasma ignites in the cavity and operates in the so-called quasi-normal regime. Transition to the abnormal regime leads to an expansion of the plasma out of the cavity. Al-though Si-based MHCD operating in DC used to suffer from their short lifetime[4], recent advances on the MHCD design allow for extending their lifetime to sev-eral days of operation[5].

2. MotivationThis study focuses on the accurate measurement of the

gas temperature inside and outside the micro-cavity in or-der to support other diagnostic techniques which require values of the gas temperature but also to better understand the plasma chemistry. Knowing the gas temperature will help to understand the material modifications and the de-velopment of new micro-reactors.

3. Approach and ResultsThe study of the gas temperature is carried out by

means of space resolved optical emission spectroscopy. Two approaches are used depending on the gas mixture (He, Ar and N2).The profile of resonant atomic lines taking in to account the Van der Waals broadening is thoroughly analyzed to infer the gas temperature. Figure 2 presents an example of a spectral line dominated by the resonant broadening compared to a transition with a line-shape resulting mainly from the Van der Waals interactions and the in-strumental function. The reevaluation of the value of a dedicated constant will be discussed.

A complementary approach based on the determination of the N2(C-B) rotational temperature is considered. Discrep-ancies regarding the rotational temperatures of different vibrational band are reported. Limitations of the latter ap-proach will be discussed specifically.

This research was partly funded by CNRS PEPS “In-génerie Verte” MiCaDEAU. SI acknowledges INSIS and SvdG the SFB1316 for the financial support.

References[1] K. H. Schoenbach, and K. Becker, Eur. Phys. J. D 70, 29–29 (2016).[2] L. Schwaederlé, M. K. Kulsreshath, L. J. Overzet, P. Lefaucheux, T. Tillocher, and R. Dussart, J. Phys. Appl. Phys. 45, 065201 (2012).[3] C. H. Sillerud, P. D. D. Schwindt, M. Moorman, B. T. Yee, J. Anderson, N. B. Pfeifer, E. L. Hedberg, and R. P. Manginell, Phys. Plasmas 24, 033502 (2017).[4] V. Felix, P. Lefaucheux, O. Aubry, J. Golda, V. Schulz-von der Gathen, L. J. Overzet, and R. Dussart, Plasma Sources Sci. Technol. 25, 025021–025021 (2016).[5] R. Michaud, V. Felix, A. Stolz, O. Aubry, P. Lefaucheux, S. Dzikowski, V. Schulz-von der Gathen, L. J. Overzet, and R. Dussart, Plasma Sources Sci. Technol. 27, 025005 (2018).

Fig. 1: Comparison of a dominated resonance broadening transition with a resonance-free lineshape in Ar spectrum.

15

poster Diagnostics

Temperature distribution in DBD-driven helium atmospheric pressureplasma jet measured by schlieren technique

O.M. Stepanova1,2, M.E. Pinchuk1,2∗, A.M. Astafiev1,2

1Saint Petersburg State University, St. Petersburg, Russia2Institute for Electrophysics and Electric Power of Russian Academy of Sciences (IEE RAS), St. Petersburg,

Russia∗Contact e-mail: pinchme@mail.ru

One of the prospective cold plasma scources intechnologies for biology and medicine is an atmo-spheric pressure plasma jet (APPJ) based on dielec-tric barrier discharge (DBD) [1].

The thermal action can be key factor for an APPJapplication in the treatment of living tissues. In mosttherapy applications, the target temperature must notexceed 50 ◦C by the end of treatment [2].

In the present report, a gas temperature deter-mination of the DBD-driven helium APPJ directedalong upward vertical axis. It has been obtained fromthe schielen images of APPJ [3]. High-voltage sinu-soidal power supplies with the peak-to-peak magni-tude of 4 kV and frequency of 22 kHz were used. Weconsidered the gas jet at laminar and turbulent flowregimes. As a dielectric barrier a quartz tube with theinner diameter of 5.58 mm and the thickness of a wallof 1 mm was used. The inner electrode which was acopper wire of 1.5 mm in diameter was placed insidethe tube along its centre line at the distance of 7.5 mmfrom the edge of the tube. The outer electrode was acopper foil strip of 5 mm wide wrapping around thetube at the distance of 5 mm from its edge. High volt-age was applied to the inner electrode. Some addi-tional details about the set-up can be found in [4].

(a) (b)Fig. 1. Schlieren images of helium APPJ for gas flow rate

of 3 (a) and 30 l/min (b).

10

20

30

40

50

0.00 0.02 0.04 0.06 0.08 0.10

Tem

per

ature

(oC

)

Length (m)

30 l/min

10

20

30

40

50

0.00 0.02 0.04 0.06 0.08 0.10

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ature

(oC

)

Length (m)

30 l/min

10

20

30

40

50

0.00 0.02 0.04 0.06 0.08 0.10

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)

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20

30

40

50

0.00 0.02 0.04 0.06 0.08 0.10

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3 l/min

Fig. 2. Temperature distribution along helium APPJ axis.

Schlieren images of the helium APPJ were com-pared with the reference images of the helium gas jetheated to a known temperature. According to the bestfitting the gas heating inside in the discharge cell andthe temperature distribution outside it were obtained.

Schlieren images of the helium APPJ for gas flowrate of 3 and 30 l/min are shown in Fig. 1. The tem-perature profiles along the jet axis are presented inFig. 2. Control points of temperature measurementby the thermocouple are marked in the graph.

Gas temperature does not exceed 45 ◦C, even nearthe discharge tube nozzle.

The study was financially supported by the Rus-sian Science Foundation (project 18-79-10048).

References[1] S. Bekeschus et al. Plasma Process Polym.,

e1800033, doi:10.1002/ppap.201800033(2018).

[2] Physics of Thermal Therapy: Fundamentals andClinical Applications. Edited by Eduardo G.Moros, CRC Press:Taylor & Francis Group,(2013).

[3] A.M. Astafiev et al. 42th IEEE InternationalConference on Plasma Sciences (ICOPS-2015),doi:10.1109/PLASMA.2015.7285025 (2015).

[4] M. Pinchuk et al. Journal of Physics: Confer-ence Series, 830, 012060, doi:10.1088/1742-6596/830/1/012060 (2017).

16

Diagnostics poster

Radio frequency emission of Transient Spark discharge

A. Hennecke1, M. Janda1

1Division of environmental physics, Faculty of mathematics, physics and informatics, Comenius

University in Bratislava, Mlynská dolina F2, 842 48 Bratislava, Slovakia

Fig. 1 Measured HF power spectrum of transient spark

discharge.

1. Introduction

Atmospheric pressure non-thermal plasmas generated

by electrical discharges in air are interesting for many ap-

plications like pollution control, or bio-decontamination

of water or surfaces. One type of discharge that produces

such a plasma is the transient spark (TS) [1]. The TS is a

dc-driven self-pulsing discharge based on repetitive

charging and discharging of internal capacity of used

electric circuit. Due to its transient nature the TS produces

electromagnetic noise, though it was not studied in detail

yet. Aim of this work is to investigate the emissions be-

low one GHz to improve electromagnetic compatibility

and to investigate the possibility of contact-less diagnosis

using standard equipment.

2. Radio frequency emissions of TS

For the first experiments a combination of a low gain

broadband scanner antenna and a Rigol DSA815 1.5 GHz

spectrum analyzer was used as a receiver. The distance

between the discharge and the receiver was about 4 m. In

a first step the electromagnetic background of the labora-

tory was measured and then the spectrum with discharge.

Figure 1 shows an example of background and spectrum

of a 1 kHz TS discharge. In the background man made

signals at 95 MHz in the lower end and 950 MHz at the

higher end are visible.

3. Conclusions

The signals generated by TS discharge are stronger than

background noise and there is no need for a shielded

chamber for measurements. Experiments with different

discharge conditions show spectra comparable to the

spectrum shown in Figure 1 and it seems to be character-

istic for TS discharge. Measurements with smaller fre-

quency ranges and higher resolution indicate that details

of the spectrum depend on discharge conditions e.g. repe-

tition frequency and other details might depend on funda-

mental processes.

References

[1] M. Janda, V. Martišovitš and Z. Machala, Plasma

Sources Science and Technology 20, 035015

17

poster Diagnostics

Glow discharge formation of high molecular products in prebiotic atmospheres: PTR-TOF analyzis

F. Krcma, S. Chudjak

Faculty of Chemistry, Brno University of Technology, Brno, Czech Republic

1. Introduction The reaction part-ways leading to life precursor com-

pounds synthesis in prebiotic atmospheres became a hot topic during the last decade because of extensive discov-ering of exo-planets [1]. The biggest atmospheric data collection is about Saturn’s moon Titan atmosphere that is composed mainly from nitrogen and methane at low tem-perature of about 94 K and pressure about 1.5 atmos-pheres [2]. One of possible initiators of these processes is electrical discharge. Lightning was confirmed in more planetary atmospheres recently [3, 4]. The presented con-tribution extends previous works [5, 6] completed by GC-MS at the laboratory temperature.

2. Experimental

The experiment was carried out in the small glass reac-tor equipped by a pair of tungsten rod electrodes (diame-ter of 1 mm) in distance of 0.7 mm. The glow discharge was operating at the currents of 10, 20, and 30 mA (cor-responding powers of 3.5, 7.0, and 10.5 W). Reactor was placed into Dewar vessel fill able by liquid nitrogen up to level of 2 cm above the electrode system. Various nitro-gen (99.999%) methane (99.95%) mixtures were used as processing gas in the flowing regime at pressure of 1.2 atmospheres. The discharge exhaust gas was continuously analyzed by in situ proton transfer reaction time of flight mass spectrometry. The experimental procedure is briefly described in the Fig. 1 caption.

3. Results

More than 40 different species were identified in the PTR-TOF spectra during the experiment, about the same number of peaks were not correctly identifies up to know. The examples of experimental results shown in Fig. 1 clearly demonstrate that formation of various molecules is dependent not only on the applied power but also on the temperature. The more complex molecules are more ef-fectively formed at higher applied powers (because of higher dissociation degree). The multipeak character visi-ble for the HCN at the highest applied power during the reactor heating is due to the desorption of heavy mole-cules and their consequent fragmentation. Similar effects were observed also for the other light molecules like ace-tonitrile or acetylene.

Fig. 1 Concentration of hydrogen cyanide (top) and formamide (bottom) during the experiment. The vertical lines indicate fol-lowing setting points: 3-discharge on; 4-beginning of LN2 cool-ing; 5-discharge off; 6-end of LN2 cooling; 7-start of hair drier heating; 8-start of pure nitrogen purge of reactor. 4. Conclusion

The contribution showed that PTR-TOF technique can be a powerful tool for the studies of dynamic chemical discharge initiated systems. The experimental data demonstrated the important role of temperature for the formation of different molecules that can be the life pre-cursors (like formamide) will be done to verify the role of discharge operation power and duration as well as role of reaction gas mixture compositions. It will be also neces-sary to develop complex kinetic model to be able to un-derstand the complex chemistry initiated by electrical discharges in this prebiotic atmosphere. References [1] N. Madhusudhan, et al Space Sci. Rev. 205, 285, 2016 [2] J.A. Kammer, et al. Planet. Space Sci. 88, 92, 2013 [3] A. Ardaseva, et al. Month. Not. Roy. Astonom. Soc.

470, 187, 2017 [4] R. D. Lorenz Progress Earth Planet. Sci. 5, 34 2018 [5] L. Torokova et al. Eur. Phys. J. 71, 20806, 2015 [6] L. Torokova et al. Contr. Plasma Phys. 55, 470 2015

18

Diagnostics poster

Microwave Cutoff Probe Modeling

ShinJae You*1, S. J. Kim1, D. W. Kim2, H. Y. Chang3

1Department of Physics, Chungnam National University, South Korea

2 KIMM , South Korea

3 Dept. Physics, KAIST, South Korea *Contact e-mail: sjyou@cnu.ac.kr

1. General (Times 11, bold) In this paper, we present our cutoff probe research which has been performed for last decade. This paper focus on the whole progress for the cutoff probe including how to start to develop the cutoff probe in the initial period, what idea has been included during the development, how to evolve the probe during ten years.

The cutoff probe is most simple diagnostics among the plasma diagnostics tools which was made by simple intuition for the cutoff phenomenon of the plasma wave. However, the cutoff probe has been used for a long time without test of validation of probe itself. Later, EM waver simulation supported the validation for the cutoff frequency determination. Recently, by supposing the circuit modeling, the physics behind for the cut off probe spectrum (S21) was revealed and the accuracy and the application window of the probe were established. Recently, as an extended version of the circuit model, we makes transmission line modeling to explain the cutoff spectrum in high density plasma as well as low density plasma.

Based on recent developments we also introduce a novel methodology to interpret the probe spectrum that eliminates the sheath and collisional effects and enables the use of this precise diagnostic technique in a broad range of practical processing conditions.

Fig. 1: Cutoff probe system composed of transmission line,

probe, and network analyizer.

Fig. 2: cutoff probe (blue line), Oscillation probe

spectrum (red line), Absorption spectrum (green line)

References [1] J.-H. Kim, et al. Metrologia 48, pp. 306, 2011. [2] D. W. Kim, et al. Appl. Phys. Lett. 99, pp.

131502, 2011. [3] J. H. Kwon et al. J. Appl. Phys. 110, p. 023304,

2011. [4] J. H. Kwon et al. Appl. Phys. Lett. 96, p.

081502, 2010.

19

poster Diagnostics

Electric field measurements on plasma bullets in nitrogen with nanosecond Electric Field Induced Second-Harmonic Generation

A. Limburg1, M. van der Schans1, S. Nijdam1

1EPG Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands

1. Background

Plasma bullets are guided ionization wave discharges generated by non-thermal pulsed plasma jets flowing into atmospheric pressure air. They propagate along the axis of the jet and are highly reproducible and periodic, which makes it possible to perform phase resolved measurements. Information on the electric field of these bullets is essential extend the theory on streamers and in order to develop ap-plications and. However, direct electric field measurements on these transient discharges are often invasive or limited by the photon emission and assumptions [1].

The plasma source investigated in this project is shown in figure 1. Nitrogen is used as a feed gas with a typical gas flow of 0.25-4 slm. The applied voltage varies in the range of 6-10 kV with a repetition rate of 2-6 kHz [2].

2. Experiment and current results In this work, a non-invasive method for measuring elec-

tric field distributions at atmospheric pressure with electric field induced second-harmonic generation (EFISHG) is presented. A nanosecond pulsed Nd:YAG laser emitting at 1064 nm is used as a source. The laser beam non-linearly interacts with the background gas and the electric field, as shown in figure 2, which results in frequency doubled light of which the intensity scales quadratically with the electric-field [3].

First, electric field measurements on a two parallel plate electrodes, separated by 3 mm, are performed to test the method and calibrate the set-up. An example is presented in figure 3. Secondly, the same method is applied on the plasma bullets in nitrogen, propagating approximately 140 ns over a distance of several millimeters. In this way, infor-mation about the spatial and the temporal development of the electric field strength and polarization is obtained. This method is not bound to the dimensions of the discharge.

3. Ongoing research In ongoing research, the resolution of these measure-ments, the influence of the laser on the plasma bullet and the quality of the calibration will be determined. Further-more, a pico- and femtosecond laser will be used to per-form the similar measurements and obtain a higher tem-poral resolution. References [1] K. Kozlov, H. Wagner, Progress in Spectroscopic Di-

agnostics of Barrier Discharges, Contrib. to Plasma Phys, 34, 3164-3176, (2007).

[2] M. van der Schans, P. Böhm et al., Electric field meas-urements on plasma bullets in N2 using four-wave mixing, Plasma Sources Science and Technology 26, 115006, (2017).

[3] A. Dogariu, B. Goldberg et al., Species-Independent Femtosecond Localized Electric Field Measurement, Phys. Rev. Applied 7, 024024, (2017).

Fig 2. EFISHG. IR beam is indicated in red. The dashed line indicates a virtual state.

Fig 1. Plasma source generating a plasma bullet edited from [2].

Fig 3. Example of a calibration measurement. The calibra-tion constant is obtained by the linear fit (red). The blue area indicates the region that is below the measurement threshold.

20

Diagnostics poster

Optimisation of a ns-pulsed micro-hollow cathode discharge array in Ar/N2 for atomic nitrogen production

G. Lombardi1, S. Kasri1, K. Gazeli2, J. Santos Sousa2, G. Bauville2, M. Fleury2, S. Pasquiers2,

X. Aubert1, J. Achard1, A. Tallaire1, C. Lazzaroni1

1LSPM, CNRS, Univ. Paris 13, Sorbonne Paris Cité, 93430 Villetaneuse, France 2LPGP, CNRS, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay, France

1. Context Micro-hollow cathode discharges (MHCDs) are a type

of microplasmas in which a several hundred micrometer diameter hole is drilled through an elec-trode-dielectric-electrode sandwich structure [1].

MHCDs generate high electron densities (up to 1016 cm-3) [2], and, therefore, a high dissociation degree of nitrogen is expected to be reached. This is particularly suited for nitride deposition, which is the targeted appli-cation of this study. In this work, we present the experi-mental study of an array of MHCDs operated in an Ar/N2 mixture, carried out to optimize the dissociation of N2, by using electrical diagnostics, optical emission spectroscopy and fast imaging. 2. Experimental setup

The reactor is composed of two chambers and the microplasma array is located at the junction of these two chambers. The 5 cm (2”) in diameter circular array con-sists of an anode-dielectric-cathode sandwich, through which seven 400 micrometer diameter holes were drilled. The 100 µm thick electrodes are made of molybdenum and separated by a 750 µm thick alumina dielectric foil. The upstream “high pressure” (50 mbar) chamber favors the production of high density plasma, and, consequently, a high nitrogen dissociation, while the downstream “low pressure” (1 mbar) chamber limits the nitrogen recombi-nation. Microdischarge arrays are subject to intrinsic in-stability. In order to overcome this problem, we pulse the discharge at high repetition frequency [3]. The discharge is ignited by applying nanosecond high voltage pulses to the cathode, while the anode is connected to the ground. The duration of the pulses is 500 ns and the repetition frequency is adjusted by a function generator. The fre-quency range studied in this paper is between 10 and 40 kHz. A 20 kV voltage probe connected to the cathode and a current probe connected to the anode allow the measurement of the discharge voltage and current, re-spectively. For optical emission spectroscopy measure-ments, a fiber connected to a spectrometer (750 mm focal length, 1200 g/mm grating, with a spectral resolution of 0.02 nm) is placed in front of the low pressure chamber The overall spectroscopic system is adequately calibrated in terms of relative emission intensity. To study the spatio-temporal evolution of the discharge in the low pressure chamber, an ICCD camera (Princeton Instru-ments PIMAX III), synchronized to the voltage generator, is positioned perpendicularly to the reactor.

3. Results

An example of the time evolution of the discharge voltage and current is presented in Fig. 1, for a MHCD operation in pure nitrogen at a repetition frequency of 10 kHz.

Fig. 1: Discharge voltage (dot black line), discharge current (solid black line) and energy deposited in the plasma (dash blue line) wave-forms obtained in pure nitrogen for a repetition frequency of 10 kHz

The peak of the voltage is - 870 V and is reached 150 ns

after the beginning of the voltage rise. The peak of the discharge current of 0.34 A is reached 34 ns after that of the discharge voltage. The time-integrated energy per pulse Ef deposited in the plasma is also plotted in Fig. 1. (37 µJ). The rotational temperature (Trot) is deduced from the population distribution in the rotational level of N2(C

3Πu) [4]. Its variation with the Ar dilution in the gas mixture and the pulse frequency will be presented in the poster. We observe a significant effect of the Ar concen-tration on the rotational temperature, while the effect of the frequency is weaker. The vibrational temperature (Tvib) has also been measured, and we observe a similar trend as a function of the Ar dilution in the gas mixture and the pulse frequency. The characterization of the different dis-charge regimes using fast imaging has also been carried out. This study of a 7-MHCDs array will be compared to that performed with only one hole.

4. References [1] K. H. Schoenbach et al, Appl. Phys. Lett. 68 (1996)

13. [2] C. Lazzaroni et al, Eur. J. Phys D. 60 (2010)

555-563. [3] S. Kasri et al, Acceptd in Plasma Sources Sci. Tech-

nol. [4] K. Gazeli et al., J. Appl. Phys. 117 (2015) 093302.

21

poster Diagnostics

Collisional radiative model for low temperature laser produced Zn plasma with fine structure resolved electron impact excitation cross sections

Shivam Gupta1, Reetesh Kumar Gangwar2 and Rajesh Srivastava1

1Department of Physics, Indian Institute of Technology (IIT) Roorkee, Roorkee, India

2Department of Physics, Indian Institute of Technology (IIT) Tirupati, Tirpati, India

It is important to study various plasmas generated by different experimental techniques as well as their charac-terization by developing a suitable plasma model to obtain the plasma parameters viz. electron temperature (Te) and density (ne) [1]. However, such a study has not focused on laser produced plasma in a detailed manner. From the literature we find that in case of laser produced plasmas many comprehensive experimental studies have been done by considering the effect of laser parameters (viz. wavelength, energy, focusing spot size), the ambient pressure and plume temperature [2]. To explain such measurements mostly local thermodynamic equilibrium (LTE) model has been used for the diagnostic purposes [3]. LTE model is very basic model with limited kinetic processes. Thus, there is a need of detailed plasma model for laser produced plasmas.

In the present work, we consider the laser produced zinc plasma (LPZP) which has many leading applications in EUV generation, high-order harmonic generation, at-tosecond pulse generation, wake-field acceleration, nano-particle and nanocluster generation etc.. The interaction of laser pulses with zinc target vaporizes the target from its surface leads to the plasma formation, known as laser ablation. Intense laser excitation leaves the ablated mate-rial in a highly excited plasma state, wherein a rapid for-mation of fast ions, atoms, and nanoparticles take place. The recorded spectra of such species are utilized to de-velop the quantitative and qualitative analytical infor-mation of plasma parameters. We develop a reliable colli-sional-radiative model for the laser produced Zn plasma and coupled it with the intensity measurements performed using the laser-induced breakdown spectroscopy (LIBS) by Smijesh et al. [3]. Since electron impact excitation of zinc is a leading process in the plasma generation, we need to incorporate it in a detailed and reliable manner along with the radiative processes in the CR model.

Electron impact excitation cross sections of zinc are available for very few transitions viz. for excitation from the ground state (4s2) to 4s4p fine structure state only [4]. Therefore, the detailed excitation cross section calcula-tions are performed for the present or future diagnostics of zinc plasma.

We have studied the electron impact excitation of neu-tral zinc atom using fully relativistic distorted wave (RDW) theory. In the present work calculations are per-formed for the transitions from their ground state 4s2 (J=0) and excited states to various fine-structure levels of

4s4p, 4s5s, 4s5p 4s4d, 4s6s, 4s6p, 4s5d, 4s7s and 4s7p excited states. The RDW method is very reliable for the electron impact cross section calculations of Zn atom due to the strong relativistic effects such as spin-orbit interac-tion and jj coupling are supposed to play significant role. In our RDW method, the transition matrix is evaluated using, both the bound target atomic wave functions and the projectile electron scattered wave function obtained from the relativistic Dirac equations. The bound states of Zn are represented through multi-configuration Di-rac-Fock wave functions and have been calculated by the GRASP2K program, while for continuum projectile elec-tron wave functions are solved using Dirac equations nu-merically. The detailed cross section results for the elec-tron excitation of Zn from the ground to various higher fine structure levels are calculated as a function of inci-dent electron energy.

In the model, we have considered 30 fine structure lev-els in addition to the ground state and first ionization state of zinc which are interconnected through collisional and radiative transitions occurring in the plasma. The model incorporates various population transfer kinetic processes among fine structure levels such as electron impact exci-tation, ionization and radiative decay along with their reverse processes e.g. electron impact de-excitation and three body recombination. The plasma parameters viz. electron density (ne) and electron temperature (Te) are evaluated by optimizing the model simulated intensities with the five emission lines namely 334.5 nm, 468 nm, 472 nm, 481 nm and 636 nm observed through the meas-urement of Smijesh et al. [3] The plasma parameters are obtained for the pressure range 0.05-10 Torr and at 2.0 mm distance from the target surface. The details of the excitation cross sections results along with the CR model results will be presented in the conference.

References [1] S. Gupta, R. K. Gangwar and R. Srivastava, Spectro-

chimica Acta Part B, 149, 203, (2018). [2] N. M. Shaikh, S. Hafeez and M. A. Baig, Spectro-

chimica Acta Part B, 62, 1311, (2007). [3] N. Smijesh and R. Philip, J. Appl. Phys., 114, 093301,

(2013). [4] T. Das, L. Sharma, R. Srivastava and A. D. Stauffer,

Phys. Rev. A, 86, 022710, (2012).

22

Diagnostics poster

Oxygen 3P atom density and temperature determined by Cavity Ringdown spectroscopy of the forbidden 1D2 3P2 transition

J.P.Booth1, A.Chatterjee1, O. Guaitella1, C. Drag1, K.Manfred2 and G.A.D. Ritchie2

1 Laboratoire de Physique des Plasmas, CNRS, Ecole Polytechnique,

Université Paris-Sud, Université Paris-Saclay, Sorbonne Universités, F-91128 Palaiseau, France 2 Department of Chemistry, The Physical & Theoretical Chemistry Laboratory,

University of Oxford, South Parks Road, Oxford UK, OX2 3QZ

1. Introduction Accurate knowledge of the absolute density of reactive

atoms is key to testing models of plasmas in molecular gases. A range of techniques have been used for O(3P) atoms, including optical emission actinometry and Two-photon Laser Induced Fluorescence with calibration against Xe [1]. However, the former depends on uncertain electron impact cross-sections, and the latter depends on meticulous calibration, and reposes on a two-photon ex-citation cross-section ratio that has been measured only once (and referenced to a titration method). Optical ab-sorption methods are preferable, since the accuracy of the measurement depends only on the accuracy to which the transition line strength is known. However, for O(3P) at-oms the vacuum uv absorption lines at 130nm are difficult to access, and for the atom densities occurring in many practical situations, such absorption measurements occur under optically thick conditions. Therefore we have used the forbidden 1D2

3P2 transition at 630nm. Since in this case the observed absorptions are very weak (ca. 10-5 per pass through the chamber), it is necessary to use cavi-ty-enhancement techniques; the transition was first ob-served using Intra-cavity Laser absorption (ICLAS) [2], followed by pulsed dye-laser CRDS [3] and cavi-ty-enhanced absorption [4]. Here we demonstrate CRDS detection of O(3P) atoms via this transition using a cw single-mode external-cavity diode laser (Toptica DL100L). The high spectral resolution gives improved sensitivity and allows the gas temperature to be deter-mined from the Doppler profile.

2. Experiment

Measurements were made in a DC positive column discharge in pure O2 in a borosilicate glass tube (id 20mm, length 56cm) between cylindrical electrodes located in side tubes. High-reflectivity concave dielectric mirrors (Layertec, R > 99.99% at 630nm) located on adjustable mounts were used to seal the tube ends. The cavity length is repetitively scanned by a piezo-actuator on one mirror. The first order beam from an acousto-optic modulator is injected into the cavity, and switched-off when the optical intensity in the cavity passes a threshold value. The ring-down time is then determined from the (exponential-ly) decaying light intensity that exits the cavity, and aver-aged over 100 shots typically.

3. Results An example of the CRDS spectra obtained is shown in Fig. 1. The O atom density is obtained from the integrated area, whereas the Doppler width gives the gas temperature. The non-zero background absorption is due to the O- negative ion photodetachment continuum. Fig. 2 shows the O density as a function of pressure and current, reaching a maximum of 15% of the gas density at 0.75 Torr 40 mA. Systematic results will be presented, and compared to the values given by other techniques

Fig. 1. CRDS spectrum at 1 Torr and 10 mA.

Fig. 2. O atom density vs. pressure and current. [1] K. Niemi, V. Schulz-von der Gathen, and H. F. Dobele,

Plasma Sources Sci Technol 14, 375 (2005). [2] S.J. Harris and A.M. Weiner, Opt Lett 6, 142 (1981). [3] A. Teslja and P. J. Dagdigian, Chem Phys Lett 400,

374 (2004). [4] M. Gupta, T. Owano, D. Baer, and A. O’Keefe, Appl

Phys Lett 89, 241503 (2006); G. Hancock, R. Peverall, G. A. D. Ritchie, and L. J. Thornton, J. Phys D: Applied Physics 40 , 4515 (2007).

23

poster Diagnostics

Floating harmonic probe for diagnostics of pulsed discharges

M. Zanáška1, 2 , Z. Turek1, Z. Hubička2, M. Čada2, P. Kudrna1 and M. Tichý1

1 Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.2 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic.

1. IntroductionFloating harmonic probe (FHP) represents a relatively

novel probe method for measurement of the electrontemperature and positive ion density in processingplasmas, where the probe tip gets coated by an insulatinglayer. In such conditions, the IV characteristics of aconventional Langmuir probe method are distorted andthus, in many cases, the evaluated data render unreliable.In the FHP method, an ac harmonic voltage in the kHzrange is applied to the probe constructed in standardmanner. Assuming that the capacitance of the depositedfilm is high enough, the electron temperature and positiveion density can be evaluated from the respective probeharmonic currents [1]. No expensive microwaveinstrumentation is needed. Recently, this concept has beenapplied to pulsed discharges using the PDHAM (PhaseDelayed Harmonic Analysis Method) [2]. We have testedseveral modifications of the FHP method applicable topulsed dc discharges with short (100 μs) pulses. Thiswork is motivated by the increasing interest in scientificcommunity in HiPIMS (High-power Impulse MagnetronSputtering).

2. ExperimentalMeasurements were carried out in two systems

designed for reactive sputtering applications: a low-pressure hollow cathode plasma jet sputtering system andin a planar magnetron. Argon was used as sputtering gas.Both systems were operated in dc pulsed excitation(typically 100 Hz, 1% duty cycle) and in both reactiveand non-reactive regime. In the latter, oxygen gas wasintroduced to deposit iron oxide thin films. When possible,standard IV Langmuir probe characteristics were recordedto compare it with the FHP data. Detailed information onthe experimental system can be found in [3].

3. ResultsResults of comparative measurements show that FHPmethod is capable to achieve a good agreement with theLangmuir probe method during measurement in non-reactive conditions. Furthermore, the FHP measurementwas several times faster and the results were less noisy.However, the FHP results and its reliability can highlydepend on the dc blocking capacitor used and thetemporal evolution of the plasma potential. To deal withthis inherent problem, we have also studied somemodifications of the FHP – double probe FHP and

actively biased FHP. Illustration of measurement withsimple FHP in HiPIMS discharge is shown in Fig. 1. Inreactive regime, the FHP method was applicable evenwhen the probe was covered by an insulating film andLangmuir probe could not be used.

This research has been supported by the Grant Agency ofCharles University, grant No. 1188218, by the CzechScience Foundation, the project 19-00579S andEUROfusion.

References[1] M. Zanáška et al., J. Phys. D-Appl.,51, 025205, (2018).

[2] Yu-Sin Kim et al., J. Appl. Phys., 117, 243302, (2015).

[3] M. Zanáška et al., Surf. Coat. Technol., 357, 879–

885, (2019).

a)

Fig 1. The time evolution of the electron temperature a) and iondensity b) as measured by the FHP in HiPIMS discharge togetherwith comparison of two different dc blocking capacitors Cb used.Conditions: 6 Pa, 80 W average power, 100 Hz, 1% duty cycle

b)

24

Diagnostics poster

Gas heating in the ignition phase of pure molecular plasmas assessed by Thomson and Raman scattering A. van de Steeg1, T. Butterworth1, G. van Rooij1

1Dutch Institute for Fundamental Energy Research, De Zaale 20, 5612AJ

Plasma reforming of stable molecules might be an attrac-

tive alternative to conventional gas reforming techniques it is especially suited for converting small and stable mole-cules, such as N2, CH4 and CO2. High efficiencies can be obtained in plasma processes by driving chemistry through vibrations, where the chemical bonds can be broken at ex-actly the bond enthalpy without excess heat production. Crucial here is limiting the heat production, which can partly be done by having the plasma electrons depositing their energy very selectively into the vibrations of the mol-ecules. However, inherent in these plasmas is heat produc-tion through V-T relaxation processes.

The rotational and vibrational temperatures in the igni-tion phase of pure CO2, N2 and CH4 microwave plasmas are measured by Raman scattering to study whether this V-T relaxation is the main heating mechanism. All three gases show a large vibrational non-equilibrium, with rotational temperatures being near ambient, as can be seen in figure 1. Towards the end of the pulse all three gases show rapid gas heating and an equilibration of vibrational and rota-tional temperatures.

Next to Raman scattering, Thomson scattering is applied in the N2 and CH4 plasmas to study the electron tempera-ture. Methane has such a symmetry that it has no rotational Raman activity, making it an ideal molecule for Thomson experiments, the results of which are shown in figure 2. For N2 no complete scan of electron temperature is possible, since the strong rotational Raman signal obscures the Thomson scattered light at low electron densities. It is found that there is a limited selectivity of electron energy to vibrations in N2 and CH4.

V-T relaxation mostly explains the observed temperature evolution of CH4, the characteristic V-T time coincides al-most perfectly with our measurements [1], showing that the limited selectivity to vibrational excitation of the free plasma electrons has a limited effect in the heating rate of CH4.

The characteristic timescales for V-T processes in N2 are orders of magnitude larger [2], which can therefore not be the prime heating mechanism. A more likely cause of the rapid heating observed is atomic nitrogen recombination, or quenching of electronically excited N2 molecules, re-leasing heat. For CO2, the timescale of V-T relaxation ap-proaches the observed equilibration times, however it can-not fully explain the observed heating rate at these condi-tions [3][4]. The results obtained with the combination of Thomson and Raman scattering therefore hint at the im-portance of electronic excitation in heat production during the ignition phase of the plasma for CO2 and N2, but not for CH4.

References [1] M. H. Cabral, “Vibrational Relaxation Times in Methane,”, PhD thesis, 1976.

[2] V. Blackman, J. Fluid Mech., vol. 1, no. 1, pp. 61–85, 1956.

[3] C. B. Moore, R. E. Wood, B.-L. Hu, and J. T. Yardley, vol. 4222, no. 1967,

1973.

[4] E. H. Carnevale, C. Carey, and G. Larson, J. Chem. Phys., vol. 47, no. 8, p.

2829, 1967.

Figure 1 Rotational and vibrational temperature evolutions for N2, CO2 and CH4.

Figure 2 Electron temperature and density in a pulsed CH4 plasma measured by Thomson

scattering.

25

poster Diagnostics

Precautions for using krypton as a calibration species for Two-Photon Absorption Laser Induced Fluorescence of hydrogen and nitrogen atoms

C. Y. Duluard1*, X. Aubert1

1LSPM, CNRS, Université Paris 13, 99 av J.-B.Clément, 93430 Villetaneuse, France

*Contact e-mail: corinne.duluard@lspm.cnrs.fr

1. Introduction Two-Photon Absorption Laser Induced Fluorescence

(TALIF) is a spatially-resolved technique that probes the ground state of atoms directly, providing absolute atomic densities with an adequate calibration method. TALIF measurements of H and N atoms are performed at a simi-lar excitation wavelength; both their densities can be in-ferred by normalizing their TALIF signal intensity by the TALIF intensity from Kr atoms in a pure krypton gas at a known pressure. This study focuses on secondary effects induced by two-photon laser excitation of Kr atoms. The objective is to draw a full picture of the mechanisms in-volved in TALIF of Kr atoms, to provide recommenda-tions for calibrating H and N TALIF measurements.

2. Experimental details

Krypton gas was admitted in a vacuum chamber to a pressure ranging from 0.25 to 10 mbar. Krypton atoms were excited from the 4s²4p6 1S0 ground state to the 4s²4p5(²Po

1/2)5p 2[3/2]2 state (also denoted 2𝑝2 in Paschen’s notation) by two photons at 204.2 nm. A dye laser pumped by a Nd:YAG laser was used for photon flux generation around 204.2 nm (up to 5mJ/pulse, with a pulse duration of ~9 ns). The laser wavelength was cali-brated using an I2 reference cell. The fluorescence signal was collected perpendicularly to the beam path with a photomultiplier tube (PMT) through an interference filter. Two bandpass filters were used: a bandpass filter, with a 10 nm FWHM centered at ~831 nm, and a bandpass filter with a 9 nm FWHM centered at ~587 nm.

3. Results

Two fluorescence lines emitted from the Kr 2𝑝2 state could be detected, the line at 826.32 nm and the line at 587.09 nm. The former one is usually chosen for calibrat-ing H and N TALIF measurements [1] due to the high spontaneous emission coefficient associated [2]. We have compared the TALIF signal intensity dependence on laser energy and the fluorescence decay time for the two lines.

When the filter centered at 587 nm was placed in front of the PMT, a quadratic regime of excitation was obtained for the full range of laser pulse energies tested (25 – 200 µJ). With the filter centered at 831 nm, the quadratic regime was limited to energies lower than 80 µJ.

The Kr TALIF temporal decay curves were recorded for several pressures in the 0.25 – 10 mbar range, and com-

pared for the two interference filters placed alternatively in front of the PMT. The experimental decay curves at 2 mbar and 10 mbar are depicted in Figure 1, along with simulated Kr TALIF signals. The Kr TALIF model was built solving a simple rate equation for the Kr 2𝑝2 state, assuming no significant depletion of the ground state and neglecting secondary effects (amplified spontaneous emission (ASE), multi-photon ionization…). The laser intensity temporal profile was simulated using the meas-ured response of the photodiode. The Kr 2𝑝2 state radia-tive lifetime and collisional quenching rate by Kr atoms, taken from the Stern-Volmer plot of the experimental data with the 587 nm filter, were 39.3 ns and 1.64×10-10 cm3s-1

respectively. At a pressure of 2 mbar, the normalized Kr TALIF decay curves are very similar for the two interfer-ence filters and accurately reproduced by the model. At a pressure of 10 mbar, the decay curve recorded with the 831 nm filter clearly deviates from a single exponential decay. In order to explain the differences observed for the two fluorescence lines, a spectral analysis of the fluores-cence emitted will be performed to highlight possible collisional intramultiplet or intermultiplet energy transfers at the highest pressures. Concerning the differences in the excitation regime, we will study the onset of ASE for the two fluorescence lines.

References

[1] K. Niemi, V. Schulz-von der Gathen and H.F. Döbele, J. Phys. D: Appl. Phys. 34 2330–2335 (2001)

[2] K. Dzierżega, U. Volz, G. Nave, and U. Griesmann, Phys. Rev. A, 62, 022505 (2000)

Figure 1 Kr fluorescence decay curves at 2 and 10 mbar

26

Diagnostics poster

Electric field measurements in the INCA discharge

C. Lutke Stetzkamp1∗ , P. Ahr1 , Ts. V. Tsankov1 , U. Czarnetzki1

1Institute for Plasma and Atomic Physics, Ruhr University Bochum, 44780 Germany∗Contact e-mail: Christian.LuetkeStetzkamp@rub.de

1. IntroductionRecently a new plasma source for collisionless

electron heating and plasma generation at low pres-sures was proposed. In this inductively coupled ar-ray (INCA) discharge the periodic structure of theelectric field is vital for its operation in the stochas-tic mode. Here we show results from measurementsof the induced electric field in INCA: In-situ mea-surements with RF modulation spectroscopy (RF-MOS) are compared to B-dot measurements (withoutplasma) and numerical calculations.2. Theory

The time dependent electric field modulates theelectron velocity distribution function, which in turnmodulates the excitation to an atomic state (excitationenergy εexc, lifetime τ ). Therefore, the spontaneousemission from this level is also modulated. From thismodulation in the RF period, the RFMOS diagnosticmethod allows to infer the oscillation velocity of theelectrons from the second harmonic η2ω of the mod-ulation [1]:

η2ω =1

2√

1 + (2ωτ)2

(2

3

εexc

kBTe− 1

)m~u2osc

2kBTe(1)

(kB – boltzmann constant, Te – electron temperature).Under locality conditions the distribution of the

electric field E can then be obtained from the oscilla-tion velocity: uosc = eE/(m

√ω2 + ν2) (e,m – elec-

tron charge and mass, ω, ν – RF and electron collisionfrequencies).3. Experimental

The INCA discharge consists of an array of 6× 6coils. The current in the coils is measured with aself-made pick-up coil placed beside the feed-throughconnecting the matchbox and the array [2].

The RFMOS measurements are performed in ahydrogen plasma at moderate pressures of about10Pa and rf powers of about 600W. The CCD cam-era records the emission of the Hα line in front of thecentral 2× 2 coils. The B-dot measurements are per-formed with a homemade measurement system with-out plasma. From these results the electric field of thecentral 2× 4 coils is obtained.4. Results and discussions

For better comparison of the two diagnostics,the electric field measured with the B-dot system issquared and shown in fig. 1. Fig. 2 shows the am-plitude of the second harmonic modulation, together

with contour lines of the data from fig 1. It is propor-tional to u2osc, i. e. to E2 (eq. (1)). The agreement ofthe two independent diagnostics is very good. Alsoa comparison with a numerical model for the elec-tric field shows excellent agreement. The slight dis-crepancies are due to the different spatial resolutionsof the techniques and the difference in the distancefrom the plane of the coils. All results consistentlyreveal the periodic structure of the induced electricfield. However, certain deviations from the initiallyexpected field distribution are present due to the spa-tial structure of the individual coils.

Fig. 1: E2 (in arb. u.) from B-dot measurements. The redcircle marks the region visible in Fig. 2.

Fig. 2: Amplitude (in percent) of the second harmonicmodulation at 15Pa. The contour lines represent the data

from Fig. 1.References

[1] Ts. V. Tsankov, U. Czarnetzki 2011 AIP Conf.Proc. 1390 pp 140

[2] P. Ahr, Ts. V. Tsankov and U. Czarnetzki 2018Plasma Sources Sci. Technol. 27 105010

27

poster Diagnostics

Ro-vibrational distribution measurements in transient atmospheric pressureplasmas by coherent anti-Stokes Raman scattering

J. Kuhfeld1∗, D. Luggenholscher1, U. Czarnetzki1

1Ruhr University Bochum, Faculty of Physics and Astronomy, Institute for Plasma and Atomic Physics, Germany∗Contact e-mail: jan.kuhfeld@rub.de

1. IntroductionRo-vibrational excited molecules govern the

plasma wall interaction and the chemical reactionsin atmospheric pressure plasmas. Excitation of amolecule can occur by an energetic electron or bycollisional transfer from an already excited molecule.One of the key goals in this work is the investiga-tion of these excitation processes in order to achievemaximum energy efficiency in chemical conversion,e.g. CO2 into CO, which is connected to the pop-ulation of high vibrational states. The experimentalapproach for investigating these processes introducedhere is based on a particular coherent anti-Stokes Ra-man scattering (CARS) scheme. This single shot dualpump CARS scheme provides in parallel informa-tion on the ro-vibrational population densities of twomolecular species, here N2 and CO2[1, 2].2. Theory

CARS is a non-linear optical process of third or-der, which is depicted in figure 1. ωpu, ωpr and ωS

denote the angular frequencies of the three incominglaser beams, here called the pump, probe and Stokesbeam. ωAS is the frequency of the anti-Stokes beamcreated in the process. The generation of the anti-Stokes beam is resonant if ωpu−ωS equals the energydifference of a ro-vibrational transitions. Its intensity,IAS , contains information about the population den-sities of the participating ro-vibrational states, Nu,l,through the non-linear susceptibility, χCARS :

IAS ∝ |χCARS(Nu −Nl)|2IpuIprIS (1)

Ipu, Ipr and IS are the intensities of the pump, probeand Stokes beams, respectively. In this work a specialscheme called dual pump broadband CARS is used.The general scheme is shown in figure 2. The pumpand probe wavelength are chosen in a way, that theAS signal for two different molecular species is inthe same wavelength region and can be detected withjust one spectrometer. By using a broadband dyelaser to produce the Stokes beam measuring multiplero-vibrational states with one laser shot is possible.

3. Experimental setupOne pump beam is produced by an injection

seeded, frequency doubled Nd:YAG laser, the otherby a narrowband dye laser. The Stokes beam is pro-

duced by a broadband dye laser. Space and time res-olutions are determined by the interaction volume ofthe lasers and the pulse length, respectively. Thoseare in the order of 10 µm and 10 ns.

ωpu

ωpr

ωS

ωAS

ωvib

Fig. 1: General CARS scheme for degenerate pump andprobe beams.

532nm

560nm6

07nm

496nm

2330cm-1

N2 CO2

1380cm-1

532nm

496nm

607nm

560nm

Fig. 2: Scheme of the single shot dual pump CARSapplication. The role of the pump and probe beams is

interchange for the two species N2 and CO2. Therectangle symbolizes the broadband operation of the

Stokes laser.

References[1] P. R. Lucht 2012 Opt. Lett. 12 p 78-80

[2] D. Bruggemann, B. Wies, X. Zhang, T. Heinze,K. F. Knoche, in: D. F. G. Duraom, M. V. Heitor,J. H. Whitelaw, P. O. Witze (Eds.), Combus-tion Flow Diagnostics, Kluwer Academic, Dor-drecht, 1992, p 495

28

Diagnostics poster

Sub-ns electric field measurements in a nanosecond pulsed atmospheric plasma jet

N. Lepikhin, D. Luggenhölscher, U. Czarnetzki

Ruhr University Bochum, Faculty of Physics and Astronomy, Institute for Plasma and Atomic Physics, Germany

1. Introduction

Knowledge of the reduced electric field is extremely im-portant for plasma kinetics studies since it determines the electron energy distribution function (EEDF). The EEDF controls the electron energy branching through internal de-grees of freedom of atoms/molecules. This work is dedi-cated to electric field measurements in a nanosecond pulsed atmospheric plasma operated in N2, H2 and CO2 ad-mixtures to noble gas.

2. Experimental setup

The discharge is generated by high-voltage (HV) pulses with 6 kV amplitude at a repetition rate of 1 kHz between two electrodes with a length of 20 mm and an inter elec-trode distance of d = 1 mm. The electric field induced Sec-ond Harmonic Generation (SHG) is the chosen measure-ment technique [1, 2]. A Nd-YAG laser with pulse duration of 100 ps is used as the radiation source at 1064 nm. In the presence of an electric field, the intensity of the generated second harmonic at 532 nm is proportional to the square of the electric field strength, E, and to the square of the laser beam intensity, IPump:

𝐼𝑆𝐻𝐺 ∝ 𝐼𝑃𝑢𝑚𝑝2 𝐸2. (1)

In addition, the direction of the electric field can be de-

termined by varying the polarization direction of the fun-damental laser.

The method works with arbitrary gases, for example, CO2, N2, CH4, Ar and air [1]. Due to the properties of the laser, high temporal (100 ps) and spatial (100 m) resolu-tion can be achieved.

The schematic view of the setup is presented in Fig. 1.

In order to obtain absolute values of the electric field in the discharge, calibration with a known field value is nec-essary. The calibration is made by measuring the second harmonic signal at known voltages applied to the elec-trodes. The calibration curve is shown in Fig. 2. It is clearly seen from Fig. 2 that the value √𝐼𝑆𝐻𝐺/𝐼𝑃𝑢𝑚𝑝 is propor-tional to the applied voltage according to equation (1).

Temporally and spatially resolved measurements of the value and the direction of electric field in the discharge are under development. Due to the high temporal resolution, especially the ignition phase of ns-pulsed discharge will be studied in details.

References [1] A. Dogariu et. al., Physical Review Applied 7 (2017). [2] B. Goldberg et. al., Applied Physics Letters 112 (2018).

Fig. 2. Calibration curve obtained for DC voltages. ISHG is the intensity of generated second harmonic at 532 nm, IPump is the laser beam intensity, U is the value of the ap-plied voltage.

0 200 400 600 800 1000 1200 14000

1

2

3

4

5

6

I0.5SHG / IPump, a.u.

Linear fitI0

.5S

HG / I

Pu

mp (

a.u

.)

U (V)

Fig. 1. Schematic view of the experimental setup.

29

poster Diagnostics

Ultra-fast dynamics of a pulsed microwave surface wave discharge in argon

E. Carbone1, E. van Veldhuizen2, G. Kroesen2, N. Sadeghi3

1Max Planck Institute for Plasma Physics, Boltzmannstr. 2, D-85748 Garching, Germany

2Dept. of Applied Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands 3LIPhy (URA5588) & LTM (URA5129), Univ. Grenoble-Alpes & CNRS, Grenoble, France

1. Introduction

Surface wave plasmas are a particular kind of micro-wave discharges where the plasma absorbs locally the power but also serve as conductor/guide for the propaga-tion of the microwave with the generation of additional segments of plasma. Extended plasma columns can be generated that scale up with the applied power and can be tens of times longer than the plasma radius itself. Plasma column properties depend mainly of the nature of the gases used as well as pressure and reactor geometry. In previous work, it was shown that the pulse repetition fre-quency and more particularly, the OFF-time has a strong impact on the spatio-temporal evolution of light emitted by an ultra-fast pulsed surfatron plasma [1]. Additional measurements were later done by measuring electron mean energy and density as well and resonant and meta-stable states densities by absorption [2]. Two main differ-ent spatio-temporal evolution modes were identified: one where large amounts of electrons remain from the previ-ous power pulse and the microwave power is coupled “into the volume” and another where an ionization wave propagates from the launcher towards the end of the plasma column. The second case corresponds to the situa-tion where the electron density decays in the afterglow (OFF-time of the pulsed power) and new electrons are created when the power is applied again. Due to their in-trinsic low electric fields, ionization front velocities in the order of 104m/s are typically observed experimentally in the literature and also in ref. [2]. In the present contribu-tion, we report however the observation that, at the mo-ment of the application of the microwave power, an ul-tra-fast excitation wave propagates along the plasma column with velocities in the order of several 106m/s be-fore the propagation of the usual slow ionization front. 2. Experimental setup The plasma is generated inside a quartz tube with in-ner/outer radii of 3 and 4 mm respectively using a surface wave launcher belonging to the category of surfatrons. The plasma is operated in pure argon in a pressure range be-tween 2 and 40 mbar using powers from 2 up to 130 W. Square microwave power pulse shapes are used with rep-etition frequencies in the range of 1 up to 100 kHz with rise and fall time in the order of 50 ns. The power increases from 0 to 80% of the peak power within approximately 100 ns. Measurements are performed along the plasma column using a fast avalanche photo-diode and by tunable diode laser absorption spectroscopy (TDLAS) [2]. All four

lowest Ar states, the 1s5,3 metastable and 1s4,2 resonant states are detected by TDLAS as a function of plasma parameters with high spatial and temporal resolutions. 3. Results An example of the temporal evolution of the 1s5 argon metastable density at 6 cm from the launcher is shown in figure 1 for different applied peak powers. At the moment of power application, a sudden increase of density is ob-served that later decays before a large density increase occurs when the main ionization wave front arrives at the position of observation. Similar time evolutions are seen for the other 1s states and in the visible light. One can see that the power is initially coupled very rapidly (t<0.5µs at distances of several 10s of cm) before that a slow excita-tion wave takes over and propagates towards the end of the column. The latter propagation velocity is very sensi-tive to the plasma parameters and position while the fast excitation wave is not (within experimental accuracy).

References [1] E. Carbone, S. Nijdam. “Ultra-fast pulsed micro-

wave plasma breakdown: evidence of various ignition modes”. Plasma Sources Sci. Technol. (2014) 23 012001.

[2] E. Carbone, N. Sadeghi, E. Vos, S. Hübner, S. Nijdam, E. van Veldhuizen, J. van Dijk, G. Kroesen ”Spatio temporal dynamics of a pulsed microwave argon plasma: ignition and afterglow” Plasma Sources Sci. Technol. (2015) 24, 015015.

Fig. 1: Temporal evolution of the 1s5 metastable state of argon measured 6 cm away from the launcher at a pressure of 10 mbar at a pulse repeti-tion rate of 5 kHz with 100 µs ON- and OFF-time.

30

Diagnostics poster

Laser-Induced Fluorescence Measurements of Ion Implantation in Inductively Coupled Plasma PIII Device

J. Moreno, L. Couëdel, M. Bradley1

1University of Saskatchewan, Saskatoon, SK, Canada

1. Introduction

Plasma Immersion Ion Implantation (PIII) is an ion im-plantation method which has applications in semiconduc-tor processing, materials surface treatment, and nanofabri-cation [1]. PIII provides a high, uniform dose rate over large-area targets and a PIII machine has a smaller footprint than a beamline ion implanter, for the same target size, which presents significant advantages for industrial scala-bility. In PIII the implantation target is immersed in a dense plasma. Negative high-voltage pulses (~ -10 kV with pulse duration 1-100 µs) are applied to the target, causing posi-tive ions to accelerate to high energies in the high-voltage sheath, and be implanted into the target surface.

PIII does not easily permit m/q selection, so contami-nants are easily implanted. Also, plasmas for this applica-tion typically consist of compound gases, and can cause sputtering, which introduces new elements and further complicates the sheath kinetics. Thus, it is critical to de-velop a precise spatially and temporally resolved model to predict ion dose implantation that considers sheath charac-teristics as well as secondary emission.

2. Purpose

This study will make use of an Inductively Coupled Plasma (ICP) device at the University of Saskatchewan Plasma Physics Lab configured for PIII applications. The current diagnostics (emissive probes and RF-compensated Langmuir probes) measure bulk plasma characteristics and the potential spatial distribution profile. However, these di-agnostics are highly intrusive and cannot be used to diag-nose the sheath in front of the target. A new diagnostic for this research is Laser-Induced Fluorescence (LIF). LIF has the capacity to measure absolute and relative densities as well as highly-resolved Ion Velocity Distribution Func-tions (IVDF). Along with the many benefits of this non-invasive technique, LIF allows characterizing PIII in an ICP with time-resolved measurements, a novel approach that may provide new insight.

3. Experimental Set-Up

A layout of the experiment is detailed in Figure 1. It is modeled after a setup by Dongsoo, et al. [2]. The ICP (Plasmionique ICP II-600, maximum RF power 600 W at 13.56 MHz) system produces a plasma with 109 < n0 <1011 cm-3, 1 eV < Te <5 eV, and a plasma potential between 10-30 V. It operates at a base pressure of 0.1 – 1 µTorr and a working pressure between 1-10 mTorr.

The LIF apparatus uses a Littrow-type grating-stabilized

external cavity diode laser head (maximum laser power 25 mW with a maximum mode-hop-free tuning range of 57 GHz). Wavelength fine-tuning occurs via a piezoelectric-controlled grating within the laser cavity. The laser light will traverse an Ar lamp to enable wavelength calibration, and then be mechanically chopped. The 3-level LIF schemes for Ar-I and Ar-II are shown in Figure 2 [3]. Flu-orescence collected orthogonal to the incident beam will pass through an interference filter centered at 442.6 nm for Ar-II and 750 nm for Ar-I and be detected using a photo-multiplier tube. Lock-in amplification will be used to im-prove the signal-to noise ratio. For the initial tests, we ex-pect to see a high peak of intensity around the central wave-length of Ar-I and Ar-II, from which we can calculate IVDF, electron temperature, and density. We will then search for changes to the IVDF when the PIII pulse modulation is ap-plied to the target.

References [1] M. Risch and M.P. Bradley 2009, phys stat. solidi (c) 6,

S210. [2] Dongsoo Lee et al. 2006 J. Phys. D: Appl. Phys. 39

5230. [3] Amy M. Keesee et al. 2004 Rev. Sci. Instrum. 75, 4091.

Figure 1: (1) PC, (2) Toptica laser, (3) Iris (4) Argon lamp (5) Mechanical chopper, (6) Beam splitter, (7) Chopper controller, (8) Wave meter, (9) ICP contain-ment vessel, (10) Gas inlet, (11) Exhaust, (12) Beam dump, (13) Interference filter, (14) Photomultiplier tube, (15) Lock-in amplifier, (16) Oscilloscope.

Figure 2: LIF schemes for a) neutral Ar-I and b) Ar-II.

31

poster Diagnostics

Broadband afterglow emission as a diagnostic tool in CO2 microwave plasma

F. J. J. Peeters1, H. J. L. Hendrickx1, A. W. van de Steeg1, T. W. H. Righart1, A. J. Wolf1, G. J. van Rooij1,2

W. A. Bongers1, M. C M. van de Sanden1,2

1Dutch Institute for Fundamental Energy Research, DIFFER, Eindhoven, The Netherlands 2Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands

1. Introduction

Microwave plasmas offer promising efficiency and scalability for the conversion of CO2 to CO in industrial systems for the production of solar fuels [1].

The configuration of the CO2 microwave plasma reactor is shown in Fig.1 and consists of a quartz tube with a tan-gential injection nozzle on one end, with pumping occur-ring on the other end. The quartz tube passes through a WR-340 waveguide, where 2.45 GHz microwave radiation is used to sustain the plasma. Typical operating conditions are pressures of 50 – 500 mbar, absorbed microwave pow-ers of 400 – 1400 W and CO2 flow rates of 4 – 24 standard liters per minute.

The tangential injection of CO2 produces a cold, outer vortex near the tube wall, with streamlines spiraling down-wards towards the pumped end of the tube. Within the cen-ter of the tube, a secondary, recirculating vortex is formed which has no streamlines connecting directly to the exhaust. This approach is made necessary by the high gas tempera-tures produced by the plasma, which are typically in the range of 3500 – 6000 K [2].

While this configuration allows for stable, high-power-density plasma (up to 1 GW/m3), with energy efficiency of CO production up to 45% and conversion yields of 15% of CO2 input flow [1], it is difficult to assess the rate-limiting steps for CO production. The plasma itself is sufficiently hot to ensure full dissociation of CO2 into CO and O, but the eventual yield of CO from the reactor is dependent on its trajectory within the inner recirculating vortex, the mix-ing of species between inner and outer vortex, as well as the gas temperature gradients encountered along the way.

To improve our understanding of the overall CO produc-tion process, a diagnostic which allows for mapping the composition and gas temperatures in the regions surround-ing the plasma is proposed: absolute broadband emission spectroscopy of the afterglow.

2. Afterglow emission in CO2 microwave plasma Typical absolute intensity-calibrated afterglow emission

spectra are depicted in Fig.2. By simultaneously measuring local gas temperatures using rotational Raman scattering, the dependence of this emission on temperature is revealed. Furthermore, it can be shown that the afterglow emission is composed of roughly equal contributions from two chemiluminescent recombination processes:

O + O O2 + hν and

CO + O CO2 + hν. By measuring absolute emission rates, and fitting the re-

sulting spectra to temperature dependent models of the two emission processes, it becomes possible to determine both temperature and local CO and O densities within the after-glow. This method can be expanded to 2D imaging of the afterglow at various wavelengths, allowing for a relatively fast mapping of afterglow composition over a wide range of operating conditions.

References [1] W. A. Bongers, et al, Plasma-driven dissociation of

CO2 for fuel synthesis Plasma Process. Polym. 1–8, 2016

[2] N. den Harder et al, Homogeneous CO2 conversion by microwave plasma: Wave propagation and diag-nostics Plasma Process. Polym. 1–24, 2016

Fig.1 Photograph of the vortex-stabilized microwave plasma. Tangential injection of CO2 occurs on the left-hand-side, plasma is visible within the waveguide at the center, while pumping occurs on the right-hand-side. Afterglow emission is visible throughout the reactor tube.

Fig.2 Typical afterglow emission spectra normalized at λ = 450 nm, obtained at various gas temperatures within the af-terglow. Gas temperatures are measured independently using rotational Raman scattering on CO2.

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32

Diagnostics poster

Diagnostics of hollow cathode plasma jet plasma for deposition of iron oxide thin films

K. Tuharin1, M. Zanáška1,2, P. Kudrna1, M Tichý1

1Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic

2Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic

1. Introduction The iron oxide can serve as the convenient precursor for

iron sulfide (FeS2) also known as pyrite which is now be-coming extremely popular for study. The band gap energy of FeS2 is ~1.0 eV is close to the ideal band gap range of 1.1-1.5 eV for a single-junction photovoltaic device [1]. Since it is nontoxic it is affordable for solar electricity gen-eration. Unfortunately the traditional pyrite synthetic pro-cess could lead to intermediate phases during formation of the pyrite, such as FeS or Fe1−xSx. These conductive metal-lic phases destroy the semiconductive properties of FeS2 hurting the quality of the resultant material. The Fe-O-S phase diagram shows that the pyrite synthesis in the ab-sence of the oxygen crosses the FeS phase. However, if the sulfur is added to oxides Fe3O4 or Fe2O3, one does not cross the FeS phase field. The substitution of the oxygen by the sulphur can be relatively easily performed in the furnace with sulphur vapors at the temperature above 350 C.

The iron oxide thin films could be prepared by plasma deposition in magnetrons, electric arc sprayers etc. In this presentation we utilize the hollow cathode plasma jet with the admixture of the oxygen in the argon working gas with the iron nozzle as the sputtering target.

2. Experimental system

The hollow cathode plasma jet is installed in the ultra-high vacuum chamber. The anomalous glow discharge cre-ated by the hollow cathode is able to reach up to 1000 times higher ionization degree compared to a normal glow dis-charge and, as a consequence, a higher deposition rate. The discharge is powered by the constant current source in the

continuous mode and by the constant voltage source in the pulsed mode.

The cylindrical hollow cathode system is isolated by the ceramic cylinder. The pure Fe (99.96%) nozzle is sur-rounded by the copper blocks cooled by the water flow. While the argon is introduced into the system via the hol-low cathode the oxygen inflow tube is positioned few cm nearby the cathode. The apparatus is described in detail in previous studies [2]. The FTO glass substrate is positioned 4–6 cm below the nozzle exit.

3. Results

The thin films created in the continuous mode show dif-ferent properties in the central part corresponding to the hollow cathode axis and the off-center part while the dc pulsed mode creates the homogeneous film, see Fig. 1. We compare the plasma parameters in both modes. As an ex-ample, the Fig. 2 shows the ion energy measured by the mass spectrometer with the energetic analyzer. The higher ion energy in the pulsed mode is probably caused by the changes of the plasma potential during the period of the pulsed discharge.

Research has been supported by the Grant Agency of the

Czech Republic, grant No. 19-00579S and by the Grant Agency of Charles University, grant No. 1188218. References [1] Xia et.al., Phys. Letters, A: General, Atomic and

Solid State Physics, 377 (31–33), 1943–1947. [2] P. Kudrna et.al., Contrib. Plasma Phys. 50, No. 9,

886– 891, 2010.

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Hematite RRUFFID=R110013

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films at dc a), b) and pulsed c) modes. The blue line shows

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rent 300 mA, Ar flow 150 sccm, pressure 15 Pa.

33

poster Diagnostics

Mesenchymal Stem Cells Behavior After Nanosecond Capillary Cold Atmospheric Plasma Helium Jet Treatment

I. S. Orel1, S. Celik2, S. Audonnet3, S. M. Starikovskaia1, H. Kerdjoudj2

1Laboratory of Plasma Physics (CNRS, Ecole Polytechnique, Sorbonne Universities, University of Pierre and Marie

Curie - Paris 6, University Paris-Sud), Ecole Polytechnique, Route de Saclay, 91128 Palaiseau, France

2EA 4691, Biomatériaux et Inflammation en Site Osseux (BIOS), SFR CAP Santé (FED4231), Université de Reims Champagne Ardenne, Reims 51100, France

3Plateau Technique URCACyt, Université de Reims Champagne Ardenne, Reims 51100, France

1. Introduction During the last decade, mesenchymal stem cells (MSCs) have been proven effective for tissue regeneration as evi-denced through in vitro, in vivo animal experiments and clinical trials [1]. Following tissue damage, MSCs have the capacity to migrate from their niche and to home at damaged site, where they provide powerful regenerative effects by increasing healing rates, modulating immune response and promoting angiogenesis. Clinical investiga-tions have identified the positive effects of Cold Atmos-pheric Plasma (CAP) on tissue healing and regeneration [2]. For regenerative medicine applications, the current study aims to investigate CAP effect on MSCs behavior. 2. Experimental The discharge is initiated in a glass capillary with an internal diameter of 2 mm, between a high voltage copper needle at the axis of capillary and the ground wrapped around capillary slightly below the needle’s end (Fig. 1). Helium is flown through the capillary at the flow rate of 28 sccm. Plasma afterglow is thus flown on Petri dish with a cell suspension with nozzle-surface distance of 3-4 mm. Voltage and gas support are provided via side tubes. The discharge in the tube is initiated using positive polar-ity high-voltage nanosecond pulses of 6 kV, pulse rise/fall time of 4 ns, and pulse duration (FWHM) of 30 ns, at a repetition rate of 300 Hz, produced by an FPG 12-1NM (FID GmbH) high voltage pulse generator.

Human MSCs at density of 50 × 103 cells were exposed to CAP for 10, 30, 60 and 180 sec. Following MSCs ex-posure, the intracellular accumulation of reactive oxygen species (ROS) and cell apoptosis were assessed by flow cytometry. After 24h of culture cell cytotoxicity was fol-lowed, according to ISO10993-5 norm. Finally, MSCs proliferation was assessed after 7 days by WST-1.

3. Results and Discussion Following cells exposure to CAP for studied times, apoptosis was not observed but intracellular accumulation of ROS after 180 sec increased significantly. After 24h, our results showed that 180 sec was cytotoxic, involving MSCs death. Furthermore, cell morphology following 180 sec highlighted a round and unattached MSCs to plastic culture whereas, MSCs exposed to 10, 30 and 60

sec kept their fibroblastic shape (Fig. 2). Interestingly, after 7 days of culture, proliferation of MSCs, exposed to CAP for 10 sec, was close to unstimulated cells, whereas, MSCs exposed for 30 sec and 60 sec showed an arrest on cell proliferation.

4. Summary In this study, our CAP device seems to be a versatile tool for MSC stimulation. In perspective, regenerative proper-ties of MSCs after CAP exposure would be investigated. 5. References [1] G. Lloyd, Weinheim, Plasma Processes and Polymers 7, 194-211, (2010). [2] X. Lu, Netherlands, Physics Reports 630, 1–84, (2016).

Fig. 1. (a) Picture of the CAP devise in use. (b) Scheme of the CAP devise. HV: high voltage, GR: ground ring.

Fig. 2. MSCs after being exposed to CAP device for (a) 10 and (b) 180 sec. Scale: 200 um.

(a) (b)

(b) (a)

34

Diagnostics poster

Ideal Multipole Resonance Probe: a Spectral Kinetic Approach

J. Gong1, M. Friedrichs2, S. Wilczek1, D. Eremin1, J. Oberrath2 and R. P. Brinkmann1

1Institute of Theoretical Electical Engineering, Ruhr-University Bochum, Bochum, Germany2Institute of Product and Process Innovation, Leuphana University of Luneburg, Luneburg, Germany

1. IntroductionActive Plasma Resonance Spectroscopy (APRS)

denotes a class of industry-compatible plasmadiagnostic methods. One particular realization ofAPRS with a high degree of geometric and electricsymmetry is the Multipole Resonance Probe (MRP).The ideal MRP is an optimized model which issuited for theoretical investigations. In this work, aspectral kinetic scheme is presented to study thebehavior of the ideal MRP in the low pressure regime.The proposed scheme reveals the kinetic behavior ofthe plasma which is of importance to the resonancestructure.

R

R-dE

E

+

_

electrodes

dielectric (D)

plasma bulk (P)

Fig. 1: The schematic of the idealized MRP

2. Kinetic effectsTo understand the behavior of the surrounding

plasma and the influence of the probe, the study basedon a fluid model of MRP has been proposed [1].However, the kinetic effects on the damping of theresonances are not taken into account. Fig. 2 showsthe comparison between the simulation results basedon the fluid model (square) and the measurement(rhombus) which indicates that the energy loss dueto the escape of the free particles can not be captured.Therefore, a kinetic approach of MRP is necessarilyexpected.

p [Pa]

Δω [10⁸s⁻¹]

Fig. 2: Comparison of half-width of the measured spectraof the MRP and the calculated electron-neutral collision

frequency based on a fluid model

3. Spectral kinetic modelThe kinetic scheme consists of two modules, the

particle pusher and the field solver, which is similarto the particle-in-cell method. The dynamics of theparticles can be described in Hamiltonianformalism. A Poisson problem is solved in Green’sfunction method to provide the solution of fieldsolver, where the influence of each electrode isdefined. The description of Green’s function in thefield solver can be expanded in spherical harmonicsas an infinite series. The prominent features of theresonance spectrum of the MRP are the absorptionpeaks. According to [2], the first absorption peak isthe dominant one. Considering the symmetry of theideal model as well, the truncation can be made tocome up with a compact solution in the field solver.Consequently, the explicit Green’s function with thecontribution of the dipole mode is determined..4. Summary

A numerical algorithm is defined to describe theinteraction of the plasma around the ideal MRP. Thecharge on the electrodes are recorded and the dampedoscillation in the time domain is observed. Fig. 3shows the captured broadening of the resonance curvein frequency domain. The presented kinetic modelprovides the possibility to overcome the limitation ofthe fluid model especially in the low pressure regime.

0.2 0.4 0.6 0.8 1.0

2

4

6

8

Te = 2eV

Te = 3eV

Te = 5eV

ω/ωpe

Re{F

[Q]}

Fig. 3: The resonance curve is captured in spectral kineticsimulation, the broadening phenomenon is interpreted as

the kinetic effect.

References[1] M. Lapke, T. Mussenbrock and R.P. Brinkmann

Appl. Phys. Lett. 93, 051512 (2008)

[2] J. Oberrath and R.P. Brinkmann PSST 23,065025 (2014)

35

poster Diagnostics

Experimental study of a cold plasma column by spectro-tomography V. Gonzalez-Fernandez1, A. Escarguel1, Y. Camenen1, A. Poyé1, R. Baude2, P. David3

1 Aix Marseille University, CNRS, PIIM UMR 7345, Marseille, France 2 APREX Solutions, campus ARTEM, 2 allée A. Guinier 54000 Nancy,France

3 Max Planck Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching, Germany

Low temperature magnetized plasma sources are present in numerous applications: Hall thrusters for satellites propulsion, ion sources, plasmas pulverisation, Penning gauges… Their optimisation requires the development of non intrusive diagnostics to observe non-stationary and spatially inhomogeneous plasmas. Spectro-tomography is particularly interesting since it gives access to the time evolution of 2D spatially resolved spectra by combining the advantages of emission spectroscopy (spectral resolution) and tomography (spatial resolution). A spectro-tomography diagnostic is presently being tested on the MISTRAL experiments.

The MISTRAL experiment produces a 1 m long, 20 cm diameter stable magnetized plasma column. The wide range of accessible plasma parameters, the reproducibility and the stability of the plasmas produced in MISTRAL are ideal for the development of new plasma diagnostics. The spectro-tomography diagnostic consists of 49 lines of sight coupled to an imaging spectrometer. The spectral and spatial resolutions of the system are 0.3 nm and 15 mm, respectively.

The possibility to directly image the plasma with a spectrometer coupled to a collimated line of sight at the end of the plasma column [1] is exploited to validate the results of the tomographic inversion [2].

The measurement of different plasma parameters in a low temperature magnetized plasma will be presented.

Fig. 1: Spectro-tomography on Mistral: experimental setup (left) – results on HeI emission lines (right).

References

[1] R. Baude et al., 2nd ECPD conf., Bordeaux, France (2017); P. David et al., 12th FLTPD

conf.,Zlatibor, Serbia (2017); R. Baude, et al., 43rd EPS Conf. , Leuven, Belgium (2016).

[2] P. A. S. David et al., Rev. Sci. Instrum. 88, 113507 (2017); P. A. S. David, et al., Physics of Plasmas 23, 103511 (2016).

36

Diagnostics poster

Formation pathways of HO2 in a cold atmospheric pressure plasma jet investigated by cavity ring-down and two-photon laser induced fluorescence

spectroscopy

S.-J. Klose1, A. Schmidt-Bleker1, K. Manfred2, H. Norman2, M. Gianella2, S. Press2, F. Riedel3, T. Gans3, D. O’Connell3, G. A. D. Ritchie2, J. H. van Helden1

1Leibniz Institute for Plasma Science and Technology (INP), Felix-Hausdorff-Straße 2, 17489 Greifswald, D

2Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK 3Department of Physics, University of York, Heslington, York, YO10 5DD, UK

1. Motivation

Cold atmospheric pressure plasma jets are often employed in biomedical applications, but their mode of action, in particular the plasma to cell interaction, is not fully understood, yet. Key species like H2O2 were investigated in the gas phase and in liquids, but the chemical reaction network remains an incomplete picture [1]. In order to solve a part of the puzzle, we analysed the production and destruction pathways of species involved in the formation of H2O2, namely H, O, and HO2, within the effluent of an in argon operating atmospheric pressure plasma source, the kINPen. The kINPen was equipped with a shielding device, that allows to adjust the effluents’ surrounding gas. From Schlieren diagnostics and gas flow simulations it is known, that the effluent has a cylindric shape with a diameter of 4 mm [2]. The small diameter makes particle diagnostics like absorption spectroscopy challenging. Moreover, highly reactive species like radicals have to be measured directly in the effluent due to their short lifetime. Collecting them in an external reservoir to increase the absorption signal, as it is commonly done for laser absorption spectroscopy, makes it difficult to calculate the real density. 2. HO2 measurements with CRDS

An option to increase the optical path length through a small plasma volume is cavity ring-down spectroscopy (CRDS). In [3], the reaction kinetics of HO2 in the kINPen with water admixture to the Ar feedgas were investigated. It was shown, that the surrounding gas composition plays an important role in the chemistry occurring. However, CRDS gives the line averaged density and thus, no spatial information about the radial HO2 distribution could be provided. A common approach to overcome this lack of information is to perform Abel inversion. We will present radial HO2 densities obtained from Abel-inverted CRDS-spectra measured at different distances from the nozzle and provide a revised chemical kinetics model. The main production and destruction pathway of HO2 includes the reaction with atomic hydrogen formed by the dissociation of water molecules to H and OH. As atomic oxygen can be formed by two

OH radicals or dissociated from oxygen molecules from the gas shielding, it is as well a species worthy to be determined. 3. H and O measurements with TALIF

Atomic oxygen and hydrogen can be detected spatially resolved with two-photon laser induced fluorescence (TALIF). Due to the high amount of collisions at atmospheric pressure, we used a ps laser system described in [4]. The quenching rate for oxygen could be determined from the effective lifetime measurements; the rate for atomic hydrogen was higher than the detection limit of the system, so we calculated the quenching coefficients to obtain an upper limit for the quenching rate. Knowing the radial and azimuthal distribution of H and O, we will also present a reaction kinetics model for the consumption of H and O in the plasma effluent to answer the question, if dissociation mechanisms in the effluent by Ar excimers and Ar metastables are still relevant. References: [1] J Winter, H Tresp, M U Hammer, S Iseni, S Kupsch, A Schmidt-Bleker, K Wende, M Dünnbier, K Masur, K-D Weltmann, S Reuter, Tracking plasma generated H2O2 from gas into liquid phase and revealing its dominant impact on human skin cells, J. Phys. D: Appl. Phys. 47, 285401, (2014) [2] A Schmidt-Bleker, S Reuter, K-D Weltmann., Quantitative Schlieren diagnostics for the determination of ambient species density, gas temperature and calorimetric power of cold atmospheric pressure jets, J. Phys. D: Appl. Phys. 48, 175202, (2015) [3] M Gianella, S Reuter, S A Press, A Schmidt-Bleker, J H van Helden, G A D Ritchie., HO2 reaction kinetics in an atmospheric pressure plasma jet determined by cavity ring-down spectroscopy, Plasma Sources Sci. Technol. 27, 095013, (2018) [4] S Schröter, PhD thesis: Reactive oxygen and hydrogen species generation in radio-frequency atmospheric pressure plasmas - Experimental and numerical investigations, University of York, (2017)

37

poster Diagnostics

Laser-induced breakdown spectroscopy (LIBS) in open air applied to the elemental analysis, specifically Si, in pig iron samples. Comparison from a

handheld instrument and a bench-top apparatus

G.S. Senesi1, G. Dilecce1, A. Bove2, O. De Pascale1

1CNR - Istituto di Nanotecnologia (NANOTEC) - PLasMI Lab, Bari, Italy 2Laboratori di prova Ilva in AS, Stabilimento di Taranto, Italy

1. Introduction

Pig iron is an intermediate product of the steel industry originating from a mixture of iron ore, coke and limestone in a blast furnace. This material, which is a fluid hot metal with high carbon content and brittle in its solid state, is then refined into steel in a basic oxygen furnace. The primary qualitative factor of pig iron is its chemical com-position, which specifies its class and fitness to the char-acteristics required by the steel plant. To evaluate the quality of pig iron, its Si content is usually measured on samples in the laboratory. Laser-induced breakdown spectroscopy (LIBS) has been in use for decades and has been proven to be a versatile diagnostic technique. In par-ticular, the application of LIBS in steel industry, e.g. for iron ore selection, process control and iron slag analysis has been investigated widely by many groups [1]. Re-cently, the LIBS technique has progressed so to allow the use of efficient handheld, self-contained commercial in-struments.

In this work, the performance of a handheld LIBS in-strument in the identification of elements, specifically Si, in pig iron was compared to that of a typical bench-top LIBS apparatus.

2. Samples and instrumentation

Four different pig iron samples originated from indus-trial and manufacturing plant were used in this work. A portable, compact handheld LIBS instrument (B&W Tek) consisting of a miniature-diode pumped, solid-state, short-pulsed laser emitting at the wavelength of 1064 nm and a classical bench-top apparatus were used to analyse the samples.

3. Results

The full broadband LIBS emission spectra recorded di-rectly within few seconds by a point and shoot operation was used to analyse the elements present in the pig iron samples. Although all the spectra featured an intense background, especially in the wavelength range of 200–300 nm, where multiple lines of the major matrix element, Fe, appeared, LIBS spectral data were able to reveal the main elemental components of the four samples, i.e. C, Fe, Mn, Si and Ti. In particular, the Si emission line could be rapidly identified with a high degree of confidence by means of either the commercially available, portable, handheld LIBS instrument (Fig. 1) or a traditional

bench-top system without requiring any sample prepara-tion.

Fig. 1 A pig iron sample (upper left corner) and rastering

beam spot (upper right corner) obtained by the handheld LIBS instrument. LIBS spectra in the range 280-315 nm (bottom) of the four pig iron samples obtained by the handheld LIBS. 4. Conclusions

Results of this study demonstrate that broadband LIBS analysis is able to identify different elements in pig iron samples, and specifically Si, using different LIBS instru-ments. However, the capacity of portable handheld in-struments to perform spatially resolved, multi-element, in-situ, online analysis of element concentrations in open air without any sample pretreatment provides an obvious advantage, with respect to a traditional, laboratory, bench-top apparatus. References [1] R. Noll, H. Bette, A. Brysch, M. Kraushaar, I. Monch, L. Peter, V. Sturm, Laser-induced breakdown spectrometry - applications for production control and quality assurance in the steel industry, Spectrochimica Acta Part B 56, (2001), pp. 637-649.

38

Tuesday May 14th

Diagnostics

Diagnostics invited

Time-correlated single photon counting on transient plasmas at atmospheric pressure

R. Brandenburg1,2, S. Jahanbakhsh1

1Leibniz Institute for Plasma Science and Technology, Greifswald, Germany 2University of Rostock, Institute of Physics, Rostock, Germany

1. Introduction

Atmospheric pressure plasmas gain more and more at-tention in applied plasma research. But, these discharges are also challenging objects for plasma diagnostics. Non-thermal plasma in molecular gases or mixtures with molec-ular gases often appear in the filamentary regime. The plasma consists of individual discharge channels. In case of Dielectric Barrier Discharges (DBD) in air microdis-charges (MDs) with a diameter of about 100 µm and a du-ration of about 30 ns are generated. The investigation of the discharge development of single DBD microdischarges and corona discharge with sufficient spatial and temporal resolution is possible by means of time-correlated single photon counting (TC-SPC). TC-SPC is the detection of sin-gle photons of a selected wavelength from the repetitive discharge event by a photomultiplier, the determination of the time information of the photons relative to a reference pulse from a second photomultiplier and its accumulation in time-corresponding memory segments. Through the ac-cumulation the waveform of the repetitive signal is finally reconstructed [1]. Beside high resolution, the advantages of TC-SPC are highest sensitivity, high signal-to-noise ra-tio and spectrally selected measurements in a broad spec-tral range (UV and Vis). Furthermore, statistical (or errati-cally) occurring discharge events can be studied.

The presentation will introduce the principle of TC-SPC and give an overview about studies, which have been per-formed with this method within the last about 10 years [2].

2. Multi-Dimensional TC-SPC

In sinusoidal and pulsed operated plasmas several dis-charge events can occur during one half-cycle of the ap-plied voltage, but with an undetermined inception time. In order to investigate the spatio-temporal development of microdischarges in such situations a multi-dimensional TC-SPC set-up has been realized [3, 4]. In this set-up, the discharge gap is scanned automatically by a mirror, which is driven by a scanning device. This scanner is operated by a controller, which also delivers an output voltage for the generation of the high voltage for plasma generation. Both scan controller signals operate the recording of the single photons in different time- and position-corresponding memory segments of the TC-SPC module. Consequently, the TC-SPC module records the time-resolved photon dis-tributions of MDs over the phase of the high voltage as well as the position along the discharge gap simultaneously. The presentation will show the realization of the multi-dimen-sional TC-SPC.

3. Examples

Examples of multi-dimensional TC-SPC will be dis-cussed for a sinusoidal operated metal pin-to-dielectric coated electrode arrangement [4]. The discharge is oper-ated in dry air with and without an admixture of toluene by applying the high voltage to the dielectric coated electrode (i.e. pin is grounded). Several subsequent MDs are gener-ated in the same voltage cycle [6]. The phasial resolution enables the study of MDs appearing at different times in the cycle as demonstrated in Fig. 1. It will be shown that the MDs development is quite different within the cycle, which is determined by volume and surface memory ef-fects. A detailed discussion will be given in the presenta-tion References [1] K.V. Kozlov et. al, J. Phys. D: Appl. Phys. 34, 3164-

3176 (2001). [2] R. Brandenburg, Plas. Sour. Sci. Technol. 26, 053001

(2017). [3] R. Brandenburg and A. Sarani, Eur. Phys. J. Special

Topics 226, 2911–2922 (2017). [4] S. Jahanbakhsh et al., Plas. Sour. Sci. Technol. 27,

115011 (2018).

Fig. 1: Discharge configuration (left middle), current (purple) and voltage (green) oscillograms (bottom) and the measured MD de-velopment (top) for different time windows in the period (for dry air with toluene admixture).

41

invited Diagnostics

Electric field measurements in atmospheric pressure non-thermal plasmas: Pockels-based Mueller polarimetry

A. Sobota1, E. Slikboer1,2,3, O. Guaitella2, E. Garcia-Caurel3

1Eindhoven University of Technology, Eindhoven, the Netherlands

2LPP, CNRS, Ecole Polytechnique, UPMC, Université Paris-Saclay, 91128 Palaiseau, France 3LPICM, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91128 Palaiseau, France

The question of electric fields is central in plasmas, as it

determines many other plasma properties. Although the methodology for its determination in low pressure plasmas has long been established, atmospheric pressure non-ther-mal plasmas have proven themselves to be a challenge in this respect. Because of their small size, probes cannot be used, as they are disruptive to the plasma. Furthermore, be-cause of their highly transient nature, the methods to deter-mine electric fields have to be time resolved at the ns time-scale. In addition, non-thermal atmospheric pressure plas-mas often exhibit complex features when in interaction with surfaces, which require methods with high spatial res-olution. It was precisely the nature of plasma-dielectric in-teraction that inspired the development of a novel diagnos-tic for the determination of electric fields.

The new method uses ellipsometry to get a complete pic-

ture of the influence of the plasma on the dielectric surface through measuring the entire Mueller matrix. As an ellip-sometric method, it uses polarized light to probe matter, or in this case a target under plasma exposure. Formally speaking, the Mueller matrix represents the relationship between the incoming probing beam of polarized light and the resulting beam, after light has interacted with the target, when using the Stokes formalism to describe (partially) po-larized light. As such, it totally characterizes the optical properties of the sample by the interaction of polarized light with matter and provides the most general and com-plete description of the response of a medium to excitation by polarized light. When the interaction is between plasma and an electro-optically active target, the Pockels effect can be used to further analyze the elements of the Mueller ma-trix.

With respect to previous work where the Pockels effect

was used for non-thermal atmospheric pressure plasmas (characterization of surface streamers in the 1990s and DBDs later on), this method offers an entirely new dimen-sion in the diagnostics. By analyzing the Mueller matrix, it is simple to extract at least 1. all three electric field components (in previous

Pockels-based measurements only the axial compo-nent was measured)

2. the change of temperature induced in the target with the precision under 1 K

3. the level of depolarization of the target.

The last (3rd) point becomes relevant when the ideal crys-tal target is replaced by a target that has at least one layer of complex material on it (e.g. biological tissue). In that case it is essential to have the ability to independently measure depolarization from e.g. electric field components, to arrive at the correct interpretation of the results.

In addition to the explanation of the method, an overview

of results is available that shows the structure of the electric field footprint of an ionization wave from a He kHz jet in all three electric field components, under varying plasma conditions. In addition, the change of temperature in the target has been measured when under plasma exposure, which is in the range of several degrees K.

42

Diagnostics oral

Two-Dimensional Electron Density Measurement over Streamer Discharge in Atmospheric-Pressure Air Using Laser Wavefront Sensor

Y. Inada1

1Division of Mathematics, Electronics and Information Sciences, Saitama University, Saitama, Japan

1. Introduction

In the non-thermal plasma, the electrons accelerated by the electric fields collide with ambient gas species and gen-erate the reactive species through electron-impact excita-tion, dissociation, ionization, recombination, attachment and/or detachment. Therefore, the electron is the essential determinant factor in the generation processes of reactive spices, and detailed diagnostics of electron density Ne is re-quired to understand the radical production mechanisms. However, electron density measurement remains a formi-dable challenge for atmospheric-pressure non-thermal mo-lecular plasmas [1], such as the streamer discharge in air under atmospheric pressure. The measurement difficulty for the atmospheric-pressure air streamer arises mainly from the irreproducibility of the discharge paths and streamer initiation jitters; their combined effects prevent the use of conventional measurement systems, which pro-vide only zero- or one-dimensional electron density distri-bution from a single-shot recording. In this paper, we de-scribe the utilization of laser wavefront sensors capable of realizing single-shot two-dimensional electron density de-termination of the streamer discharges generated in atmos-pheric-pressure air.

2. Experiment

Pulsed positive streamer discharges were generated in a 13-mm gap installed in open air. The air gap was composed of a plate cathode and a pin anode, whose tip radius was 80 m. The voltage waveform rose from zero to a peak of 35 kV in 70 ns, and the voltage rise-rate was 0.83 kV/ns.

A detailed description of electron density measurement using laser wavefront sensors was previously reported [2, 3]. The sensors measure two-dimensional distributions of laser wavefront gradients, which reflects the electron den-sity gradients in the plasma. The spatiotemporal resolution of the sensors was 300 m and 2-5 ns.

3. Result and Discussion

Figure 1(a) shows a two-dimensional Ne distribution over the primary streamer discharge at t = 8 ns [2]. The origin of t corresponds to the time at the primary streamer initiation at the anode tip. The primary streamer had a branching structure around the gap center. Figure 1(b) shows a datum over the secondary streamer discharge at t = 57 ns [3]. Notably, the upper and lower parts of the elec-tron density image were recorded for different streamers.

Figure 2 shows a comparison of axial Ne distribution over the primary streamer between (a) experiment and (b)

simulation. The profiles of Fig. 2(a) were reproduced under the consideration of the dissociative recombination reac-tion of electrons with cluster ions: O4

+ + e → 2O2, which had not been included in conventional simulation models.

The secondary streamer propagated in the gap while maintaining an almost uniform Ne of 0.7-1 × 1015 cm-3 along the propagation direction. This temporally stable Ne suggested that the reduced electric field inside the secondary streamer was 120 Td during the propagation.

Acknowledgement The author likes to thank Dr. A. Komuro of Tohoku Uni-versity, R. Ono, A. Kumada, K. Hidaka of the University of Tokyo and M. Maeyama of Saitama University for their support throughout the duration of this study. References [1] I. Adamovich, et al., J. Phys. D: Appl. Phys. 50,

323001, (2017). [2] Y. Inada, et al., J. Phys. D: Appl. Phys. 50, 174005,

(2017). [3] Y. Inada, et al., J. Phys. D: Appl. Phys. 51, under re-

view, (2018). [4] A. Komuro, et al., J. Phys. D: Appl. Phys. 51, 445204,

(2018).

(a) primary (t = 8 ns) [2] (b) secondary (t = 57 ns) [3]

Fig. 1 2D Ne image over positive streamer in air.

(a) experiment [2] (b) simulation [4]

Fig. 2 Comparison of axial Ne distribution over primary streamer.

different shot

43

oral Diagnostics

CO detection by two-photon absorption laser induced fluorescence (TALIF) in a CO2 glow discharge

Mark Damen, Desmond Hage, Luca Matteo Martini, Richard Engeln

Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands

1. Introduction

Efficient reduction of CO2 to CO is a key step in the process of storing renewable energy in the form of a hy-drocarbon fuel, i.e. a so-called solar fuel. A preferred dis-sociation method, which is finding much attention, makes use of the non-thermal nature of plasma, or ionized gas. Exploiting this phenomenon, the most efficient method to reduce CO2 to CO is in literature proposed to be by selec-tively exciting the asymmetric stretch vibration of CO2, designated as ν3. [1] In this so-called vibrational lad-der-climbing process, the distance between CO and O becomes ever larger, after which at 5.5 eV dissociation happens without significant heat formation. We employ different diagnostics to study the CO2 dissociation process under plasma conditions.

2. Diagnostics

Previously we have developed two diagnostics to in-crease our current level of understanding of the vibration-al kinetics within CO2 discharges, with the intention to ultimately contribute to a controlled and efficient dissoci-ation process. These diagnostic techniques are (1) time resolved in situ Fourier transform infrared (FTIR) spec-troscopy and (2) spatiotemporally resolved in situ rota-tional Raman spectroscopy [2, 3]. We now focus on the detection of CO, i.e. one of the products in the dissocia-tion process of CO2 by means of two-photon absorption laser induced fluorescence (TALIF). The two-photon transition that is used to excite CO is the (𝑋 Σ!! →𝐵 Σ!! )-transition [4]. The 230 nm photons that are need-ed for the two-photon excitation are generated from a Nd:YAG pumped dye laser, operating on Coumarin-460 dye and doubled in a BBO crystal. The fluorescence after CO excitation is from the (𝐵 Σ!! → 𝐴 Σ!! )-transition, i.e. between 450 and 750 nm, and is detected by means of a photomultiplier tube. The B-state is used for the TALIF measurements as this state has a rather long natural life-time (22 ns), which should allow for absolute density measurements under not too high-pressure conditions. 3. Results

The first results have been obtained in a gas cell at pressures between 10 and 100 mbar. These results have been used to validate the simulations of the excitation spectra. A typical example is shown in Figure 1. The structure visible in the spectrum originates from the rota-tional distribution of the CO molecule in the ground state. Using this distribution the rotational temperature, i.e. the

gas temperature, can be determined. Our results on the density measurements at different

pressures show that up to pressures in the 10 mbar range the absolute number density can be deduced from the TALIF measurements. The reason is mainly the less and less accurate determination of the effective lifetime of the B-state, a number that is necessary for calculating the absolute density from the TALIF signal.

Figure 1: Measured (black) and simulated (red) TALIF

excitation spectrum of CO in a static cell at a pressure of 10 mbar.

During the meeting results on CO production in a pulsed CO2 glow discharge will be reported. First preliminary measurements show a fast temperature rise at the start of the plasma pulse, and slower cooling after the plasma pulse. Number densities show a drop at the start of the plasma pulse, which is most probably due to the tempera-ture increase.

References [1] A. Fridman, Plasma Chemistry (Cambridge University Press, New York, 2008). [2] B.L.M. Klarenaar, O. Guaitella, R. Engeln, A. Sobota,

Plasma Sources Sci. Technol. 27, 085004 (2018) [3] B.L.M. Klarenaar, R. Engeln, D.C.M. van den

Bekerom, M.C.M. van de Sanden, A.S. Moril-lo-Candas and O. Guaitella, Plasma Sources Sci. Technol. 26, 115008 (2017)

[4] S. Linow et al. Appl. Phys. B 71 (5) 2001

44

Diagnostics oral13th Frontiers in Low-Temperature Plasma Diagnostics, Bad Honnef, Germany, May 13-16, 2019

Surface production of negative ion in low pressure H2/D2 plasmas: measurement of the absolute negative ion flux

Lenny Tahri1, Dmitry Kogut1, Ane Aanesland2, Dmytro Rafalskyi2, Alain Simonin3, Jean Marc Layet1, Gilles Cartry1

1 Aix-Marseille University, CNRS, PIIM, UMR7345, F-13013 Marseille, France 2 ThrustMe, 4bis, rue des Petits Ruisseaux 91370 Verrières-le-Buisson, France

3 CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France (*)) gilles.cartry@univ-amu.fr

In low-pressure hydrogen plasmas, there are two

main negative ion (NI) formation processes: volume production by dissociative attachment of electrons on molecules and surface production through the electron capture by a neutral atom or an ion when colliding with a surface [1]. In most cases, the most efficient formation process is the volume production, so that the surface one is usually neglected. However, NI formation on surface is enhanced when certain materials, such as low work-function metals or carbon-based surfaces (graphite or diamond for instance), are put in contact with the plasma. The aim of our study is to characterize NI production on carbon-based surfaces in hydrogenous plasmas [2] in the framework of the development of NI sources for fusion applications.

Measurements are performed in an ICP reactor. The sample is placed in the middle of the plasma chamber 37 mm away from a NI detector. So far, Hiden EQP 300 mass spectrometer (MS) was used as diagnostic in such a way that the center of the sample lies on the axis of the spectrometer. The sample is negatively biased with respect to the plasma potential. Positive ions from the plasma are attracted toward the sample and form NI, which are then accelerated toward the plasma and self-extracted from the plasma to the MS. The measurement gives the Negative-Ion Energy Distribution Function (NIEDF). Unfortunately, since the transmission function of the MS cannot be easily determined, only relative flux of NI is obtained by the integration of NIEDF. The goal of the present work is to calibrate the MS relative measurements using a measurement of the absolute NI flux with a MRFEA. This MRFEA is a small-sized ion energy analyser made of a magnetic barrier (MB), a grounded grid and a collector. The main idea of the MRFEA is to use the MB for electrons suppression while ions motion is not affected. The collector potential is scanned and I-V curves are obtained. By differentiating I-V curves, positive and negative ions distribution functions are determined. The grounded grid limits perturbation of the plasma by the MRFEA when the collector voltage is scanned. The MRFEA has been used previously with success for measurements of heavy-mass NI in SF6 plasmas [3]. In this work we study its capabilities for light NI such as H- and D-.

First the light-positive-ions transmission inside the MRFEA has been measured. Currents measurements

were performed with both MRFEA and RFEA at different positive-ions energies. Current ratio between the former and the latter gives straight the transmission ratio. For low mass ions, the transmission ratio at 130 eV is 0.5 for mass 4 and 0.3 for mass 2 and strongly decreases at energy below 100 eV. This transmission has been compared with calculations of ions transmission through the magnetic barrier. Second, the MRFEA has been put under UHV conditions facing an electron gun to calibrate its suppression ratio for high energy electrons. This suppression ratio is 70 at 3 keV, reaches 1800 at 1.5 keV and becomes then non-measurable due to the extremely low current passing through the analyser. For plasma electrons (energy of few eVs to few tens of eV), the suppression ratio is 6000 [3]. Third, I-V curves from the MRFEA have been acquired both for hydrogenous and noble gases with a Boron Doped Diamond (BDD) sample biased between -60V and -130V. In noble gas plasmas, NI cannot be produced, only secondary electrons can be emitted from the surface and no negatively-charged-particles signal was detected. It is therefore believed that secondary electrons emitted from the sample and reaching the MRFEA at energy of few tens of eV (60 – 130eV) are efficiently suppressed by the MB. On contrary, with hydrogenous plasmas, it was possible to get the energy distribution function for surface produced negative particles which are believed to be NI since surface-produced electrons are efficiently suppressed by the MB. A total current on the order of hundreds of nA have been obtained.

Finally, NI current has been related to the NI flux emitted by the surface by considering both the MRFEA transmission and the transmission through the plasma between the sample and the detector. This last transmission is obtained by computing trajectories of negative ions leaving the sample. Those missing the detector are eliminated from the calculation. References [1]: Bacal M., Physics aspects of negative ions, Nucl. Fusion 46 (2006) S250–S259. [2]: Cartry Get al A 2017 New Journal of Physics 19 025010. [3]: D Rafalskyi et al Rev. Sci. Instrum., 86, 053302 (2015).

45

oral Diagnostics

Densities of He*(3S1) and He2*(a 3Σu+) metastable species in an atmospheric

helium RF discharge measured by Broad-Band Absorption Spectroscopy

G. Nayak1, N. Sadeghi2, P. Bruggeman1

1University of Minnesota, Minneapolis, MN 55455, USA 2LIPhy (URA5588) & LTM (URA5129), Univ. Grenoble & CNRS, Grenoble, France

1. Introduction

In atmospheric pressure micro-plasmas (APµPs) with helium as support gas, metastable species of helium, which carry an energy of about 20 eV, play an important role in ionization and fragmentation of molecules present in the gas. Through Penning ionization of trace molecules, like O2, N2 or H2O, they also are responsible for the igni-tion stability of nanosecond pulsed discharges generated at high repetition rates. Atomic, He*(23S) and molecular He2*(a

3Σu) metastables are commonly included in model-ing of APµPs and they play a crucial role in the kinetic of the discharge and for the energy transport [1]. Several groups have used laser absorption spectroscopy to meas-ure the density of He*(23S) atoms in APµPs [1-3] but, to our knowledge, the density of He2*(a3Σu) molecules has never been measured in these plasmas. We recently applied the Broad-Band Absorption Spectroscopy [4] to record the density of both metastable species of helium and deduce their density distribution within the 1.8 mm gap of an atmospheric pressure capacitively coupled RF (13.56 MHz; 15 W) helium µ-discharge.

2. Experimental

Copper electrodes of 9.5 x 19 mm2, separated by 1.8 mm gap are inside a housing continuously flushed with 5 slm of high purity helium. A laser-stabilized lamp serves as white light source. Lenses and a diaphragm provide a less than 0.2 mm diameter spot that after crossing the plasma is focused on the entrance slit of a 1 m spectrom-eter equipped with a 1800 g/mm grating and backed by a CCD camera. Densities of He*(23S) and He2*(a

3Σu) are deduced from the absorption rates on 388 nm (33PJ¬ 23S1) and 465 nm (e3Πg¬ a3Σu) transitions, respectively. With a 50 µm slit width, the spectral resolutions is around 40 pm.

3. Results and comments

Figure 1 shows the fractional absorbance recorded in the spectral region 462-469 nm. Rotational lines of the He2 (a

3Σu-e3Πg) v’=0-v”=0 (in black) and 1-1 (in red)

transitions are identified. Given that the spectral resolu-tion is larger than the pressure broadened width of the lines (18 and 22 pm for 388 nm and 465 nm lines, respec-tively) the density of He*(23S) atoms and that in rotation-al levels of He2*(a

3Σu;v=0) molecule are deduced from the recorded fractional absorptions by using the procedure described in [4]. The population density of He2*(a

3Σu; v=1) is estimated to be about 20% of that of the v=0 level and the Boltzmann plot of the rotational levels’ popula-tions of v=0 provides a gas temperature of 330 K. The distribution between electrodes of both metastable densi-ties is illustrated in Fig. 2. It corresponds to the well known profile of the gas excitation when the RF dis-charge operates in α-mode [3], with maximum excitations at about 0.25 mm from electrodes. The density of He2*(a

3Σu) metastable molecules is 4 times lower than that of He*(23S) atoms and the density of atoms in the other He*(21S) state, monitored with 501.7 nm line, was below the detection limit.

References [1] K. Niemi, et al, PSST, 20 (2011) 055005 [2] C. Douat, et al, J. Phys. D, 49 (2016) 285204 [3] B. Nierman, et al, Eur. Phys. J, 60 (2010) 489 [4] M. Kogelschatz, G. Cunge and N. Sadeghi, J. Phys.

D: Appl. Phys. 37 (2004)1954

462 463 464 465 466 467 468 469

0.00

0.01

0.02

0.03

0.04

P5Q13

Q11

P3

Q9

Q7Q5

Q1R3 P9Q15

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Q7

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Q3

Q1

R1

R3

R5

R7

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Fra

ctio

nal

Abs

orb

anc

e

Wavelength (nm)

R13

He2 (a 3Σ+

u ® e 3Π

u) (0-0)

He2 (a 3Σ+

u ® e 3Π

u) (1-1)

Fig.1 Absorption spectra on He2 (a3Σu→e3Πg) transition

-0.8 -0.4 0.0 0.4 0.8

0

1

2

3

4

5

6

7

He(23S) He

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Radial distance (mm)Fig.2 Metastables density profiles between electrodes

46

Diagnostics invited

Water-contacting Micro-discharge: Diagnostics of Gaseous Thermal Field and Reactive Species

Q. Xiong1, L. Xiong1, QH. Huang1, Z. Shu1

1State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing Univer-

sity, Chongqing 400044, PR China

1. Introduction Plasma interacting with liquids receives great attentions

in the two decades, not only stimulated by fundamental re-search interests at interaction of plasma and soft materials (liquids and polymers), but also by attractive novel appli-cations such as plasma medicine and nano-material synthe-sis [1]. The interactions, particular in the gas-and-liquid in-terface, involve plenty of physical and chemical processes which greatly complicates this multidisciplinary area. Fun-damental parameters including gas temperature, electron density and temperature are desired for further experi-mental and modelling investigations. And, the transfer be-haviors of important active gaseous species, are of high in-terests for getting sights of plasma activating process of contacting water solutions.

2. Methods and results

In this work, multiple advanced diagnostic approaches are applied to visualize the physical and chemical details of a pin-to-water micro-glow discharge. Because of the un-known gas composition due to water evaporation, a precise determination of gas temperature (Tg) distribution is diffi-cult. Although the spatial-resolved optical emission spec-troscopy (OES) is able to provide a rough estimation of Tg, it is line-integrated and more importantly it only character-izes the hot discharge core of a size much smaller than that of the full Tg field. As compared to the discharge pattern by schlieren (CS) photography for a pin-to-pin micro-glow discharge, as shown in Fig. 1, the plasma emitting zone co-vers less than one third of the full Tg map obtained from the schlieren image. Therefore, other advanced routines in-cluding laser-induced fluorescence (LIF) and broadband UV absorption of OH radicals, are applied as well for com-parison to determine the Tg distribution accurately.

Based on the Tg distribution, the water vapor above the

solution surface was probed through the method of tunable diode laser absorption spectroscopy (TDLAS) at wave-length of 1392 nm. Further, reactive species including OH, HO2, H, were diagnosed through approaches of (TA)LIF and TDLAS. The aqueous OH and H2O2 were measured and combined with above gaseous concentration, the trans-fer behavior of gaseous OH was studied in detail together with its contribution to the production of H2O2 under posi-tive and negative polarities of water solutions.

Fig. 1 Schlieren images of a micro-glow argon discharge with increasing water vapor contents at a fixed current of 16 mA. The boundaries of both the OH(A-X) luminous pat-tern and the discharge schlieren are outlined by red and white dash lines respectively for comparison [2]. References [1] P J. Bruggeman, M J. Kushner, B R. Locke, et al 2016

Plasma Sources Sci. Technol. 25 053002. [2] Q. Xiong, L. Xu, L. Xiong, Q H. Huang, et al 2018

Plasma Sources Sci. Technol. 27 095010.

47

oral Diagnostics

Two-photon spectroscopy applied to study of the cathode fall characteristics in a hollow cathode glow discharge operated in hydrogen and deuterium

V. Gonzalez-Fernandez1,2, K. Grützmacher1, C. Pérez1, M. I. de la Rosa1 1Dpto. de Física Teórica, Atómica y Óptica, Universidad de Valladolid, Paseo Belén 7, 47011, Valladolid, Spain

2Aix-Marseille Université, St Jérôme, CNRS, PIIM UMR7345, Case 322, 13397 Marseille, France

Low pressure as glow-discharges have been widely studied due to its many applications in industry and re-search environments; so its study and comprehension is undoubtedly justified. A complete understanding requires theoretical and experimental deep studies. One of the most important parameters to be analyzed is the electric field (E-field) in the plasma, due to the discharge dynam-ics is strongly conditioned by it. High spatial and tempo-ral resolution E-field measurements are presented in this work to determine its dependence with the cathode ge-ometry and material, the buffer gas, and different pres-sures and currents. The measurements are performed in the cathode fall region of a hollow cathode discharge (HCD) operated in pure hydrogen and deuterium. The technique is based in the shifting and splitting of the 2S level of hydrogen and deuterium caused by the Stark ef-fect. Two photon excitation of the 1S - 2S transition is induced by two counter propagating circularly polarized laser beams of opposite directions (DL=0), providing Doppler free measurements, and followed by optogal-vanic detection. The local E-field strength value is deter-mined measuring the separation in GHz of the 2P1/2 and 2P3/2 components and comparing it with the theoretical calculations. The UV radiation is generated by an injec-tion seeded Q-switched Nd:YAG laser (repetition rate of 10 Hz), pumping a second laser based of non-linear crys-tals (OPO-OPA-SFG). The 243 nm obtained radiation works in single-longitudinal mode, with energy up to 5 mJ, a temporal duration of 2.5 ns and 300 MHz band-width. The two counter propagating beams are focalized in the upper central part of the discharge, in a tiny over-lapping volume of 100 mm in diameter and 10 mm in length and parallel to the cathode surface.

The plasma is generated in a home-made HCD, with two stainless steel peaked anodes, and a cylindrical cath-ode placed between the anodes. All the pieces have an

axial perforation to allow performing end-on spectro-scopic measurements at different distances from the cathode surface. The discharge can be operated in a wide range of pressures (from 400 to 900 Pa) and currents (from 50 to 300 mA), with cathodes of different diameter and material, even changing the buffer gas (hydrogen or deuterium). A complete study of the cathode fall charac-teristics was carried out with this experimental setup; ob-taining very interesting results about the influence of the cathode diameter and the behaviour that sputtering in-duces in the plasma. Comparison of the obtained results in hydrogen and deuterium is of special interest because all their main parameters are identical, except the mass. See Fig 1 (a) where hydrogen and deuterium E-field falls are compared, both measured in a 15 mm tungsten cathode, with 600 Pa of pressure and 150 mA. It can be seen clearly how the maximum E-field is larger for deuterium, meanwhile the length of the cathode region is shorter.

Also the fittings of the theoretical functions give the values of interesting parameters, which can provide useful information about the discharge characteristics. Actually two classical models had been applied to the measured E-field distributions: Davis and Vanderslice [1] with Rickards modification [2], and second, the theory by Wronski [3]. As an example, Fig. 1 (b) shows the mean free path of the particles (understood as the net charge) in the proximities of the cathode surface, obtained from [3], showing that the mean free path is almost identical for both isotopes.

References [1] W. D. Davis and T. A. Vanderslice, Phys. Rev., 131 (1) 219, 1963 [2] J. Rickards. Vacuum, 34(5):559-562, 1984. [3] Z. Wronski. Vacuum, 40(4):387-394, 1990.

Fig.1. (a) E-field strength vs. the distance from the cathode surface in a discharge operated in a 15 mm tungsten cathode, a pressure of 600 Pa and 150 mA in hydrogen and deuterium. (b) Estimation of the mean free path of the net charge in the cathode fall proximities for hydrogen and deute-

rium in a discharge of 600 Pa, and a 15mm tungsten cathode

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Absolute H density measurement in an RF driven Ar + 0.27% H2O plasma

V. S. S. K. Kondeti, and P. J. Bruggeman Department of Mechanical Engineering, University of Minnesota, Minneapolis, USA

1. Introduction

Atmospheric pressure plasma jets (APPJs) are widely employed for applications including synthesis of nanopar-ticles, material processing and disinfection [1]. APPJs for such applications are typically operated in a noble gas such as argon or helium with a small admixture of a mo-lecular gas such as oxygen, water vapor or air. The efflu-ent of the APPJ surrounded by ambient air consists of a wide variety of reactive species at close-to-room temper-ature. Plasmas with admixture of water vapor generate the highly reactive H and OH radicals. The H density in water containing plasmas has been shown to be orders of mag-nitude higher than the OH density in a high electron den-sity plasma, while it has been shown that the H and OH densities are similar for a low electron density diffuse plasma [2,3]. In this work, we report the H density in an RF plasma jet whose electron density is in between the electron densities of Ref. [2] and [3] for which the H den-sity has previously not been studied.

2. Experiment The variation in the radio frequency driven plasma jet

used in this work is described in Ref. [4]. Argon with 0.27% H2O is used as a feed gas and the plasma dissipat-ed power was maintained at 2.5 W. The H density gener-ated by the plasma jet is measured by two photon absorp-tion laser induced fluorescence [2, 4]. Briefly, a combina-tion of a pump-dye laser system generates photons of wavelength 205.08 nm. Two such photons excite the ground state H atoms (1s 2S1/2) to the excited state (3d 2D3/2, 5/2). Fluorescence is observed at 656.28 nm when they de-excite to the 2p 2P1/2, 3/2 state. An ICCD camera with a filter was used to detect these photons. An absolute calibration of the H density was obtained by measuring the fluorescence by a known concentration of krypton gas. The gas temperature in the APPJ effluent is measured by Rayleigh scattering.

3. Results The variation in the H density and the gas temperature

in the effluent of the plasma jet along the central axis with the increase in distance from the APPJ nozzle is shown is figure 1. The gas temperature reduces from 480 K close to the nozzle to 350 K 15 mm away from the nozzle. The H density close the nozzle is approximately 9 x 1015 cm-3, corresponding to a dissociation degree of 21% of H2O in the feed gas. H radicals are expected to be produced in the core of the discharge and transported out of the APPJ nozzle by the gas flow. The H density can be expected to higher inside the APPJ. The observed H density is one order of magnitude higher than the OH density for the

same plasma jet [5]. The O density in the same APPJ cal-culated by a plug flow model is of the same order as the measured H density [5]. The H density decays exponen-tially with the increase in the distance from the APPJ noz-zle with a decay constant of ~ 4 mm. This fast recombina-tion of H during its transport in the jet effluent is due to reactions of H with O and OH radicals in addition to self-recombination. The result shows a fast-reducing H flux with increasing distance from the jet nozzle.

Figure 3. H density and gas temperature as a function of the distance from the APPJ nozzle.

3. Conclusion The highest H density measured in the APPJ was 9 x 10

15 cm-3. The H density measured in the APPJ is one or-der of magnitude higher than the previously determined OH density. This large H density leads to a fast decrease in H density with increasing distance from the jet nozzle due to radical-radical recombination reactions.

4. Acknowledgement This work is supported by the US Department of Ener-gy through the Plasma Science Centre (DE-SC0001939) and Office of Fusion Energy Sciences (DE-SC0016053). References [1] P. J. Bruggeman, U. Czarnetzki, S. Samukawa, M.

Hori, and M. Armelle, J. Phys. D. Appl. Phys., 50, 323001 (2017).

[2] Y. Luo, A. M. Lietz, S. Yatom, M. J. Kushner, and P. J. Bruggeman, J. Phys. D. Appl. Phys., 52, 044003 (2019).

[3] D. X. Liu, P. Bruggeman, F. Iza, M. Z. Rong, and M. G. Kong, Plasma Sources Sci. Technol., 19, 2 (2010).

[4] V. S. S. K. Kondeti, U. Gangal, S. Yatom and P. J. Bruggeman, J. Vacuum Sci. & Tech. A, 6, 35 (2017).

[5] K. Wende, et al., Biointerphases, 10, 2 (2015).

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Separated effects of plasma species and post-treatment on the properties of barrier layers on polymers

M. Böke1, B. Biskup1, M. Brochhagen1, J. Benedikt2

1Experimental Physics II, Ruhr-Universität Bochum, Germany

2Experimental Plasma Physics, Christian-Albrechts-Universität Kiel, Germany

1. General Analyses of a-C:H /a-Si:H multilayers on polymer sub-

strates indicated that prolonged ion bombardment influ-ences negatively the properties of the barrier layer, while a short plasma pretreatment can improve the barrier effect [1]. This work is motivated by these results and investi-gates the influence of different reactive plasma compo-nents, namely ions, metastables and (V)UV-photons, on the properties of the grown barrier layer.

To separate the different species and their influence on plasma pretreatment and film growth, we build an ion-re-pelling grid system (IReGS), which repels the ions from the substrate, so that only metastables and (V)UV-photons have an effect on the layer. In a second approach we sepa-rate the effect of argon metastables from the effect of (V)UV photons. In addition to the before mentioned ion-repelling grid we use a collimator with a high aspect ratio and an argon or helium gas-shower in front of the substrate. With this setup it is possible to reduce the metastable and ion density in front of the substrate, so that only (V)UV-photons have an effect.

An integral part of this investigation is to measure the photon fluxes to the substrate by an absolutely calibrated VUV monochromator. For that, a differentially pumped VUV spectrometer with a spectral range 30 – 300 nm is used, where, for example, the absolute emission intensities of the two most prominent argon lines at 104.9 and 106.8 nm can be measured. In this approach we are able to study the different effects of the plasma species and also possible synergy effects, to improve the properties of the barrier layer. [1].

In a second part, the deposition of SiOx films from O2/hexamethyldisiloxane (HMDSO) or Ar/HMDSO mix-tures in an inductively coupled plasma is investigated. Sub-strate temperature and electron density are measured dur-ing the deposition process.

Furthermore, the deposited layers are analyzed with a profilometer (thickness), infrared absorption spectroscopy

(FTIR), and X-ray photoelectron spectroscopy (XPS). Pro-cesses with continuous and pulsed HMDSO flows are com-pared to characterize the effect of surface post-treatment of a grown layer. Pure O2 or Ar plasmas between the HMDSO gas flow pulses can offer a “post-oxidation” or “posttreat-ment” of the grown films. The discharge dynamics during the different phases are also investigated by time-resolved electron density measurements. This approach has led to formation of carbon and Si-OH group free SiOx films even without addition of O2 gas under atmospheric pressure conditions.

Acknowledgement This work is supported by DFG within SFB-TR 87. References [1] H. Bahre et al, J. Phys. D: Appl. Phys. 46, (2013)

084012 [2] B. Biskup et al, J. Phys. D: Appl. Phys. 51, 115201,

(2018), doi:10.1088/1361-6463/aaac15 [3] M. Brochhagen et al., PPP 15. e1700186 (2018)

Fig.2 Monitoring Ne with a plasma absorption probe

Fig.3 FTIR spectra with different O2 post treatment

Ar Ar+

Fig.1 Ion Repelling Grid System.

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Laser Absorption Spectroscopy for H (D) (n = 2) Density Measurements

F. Merk1, R. Friedl1, C. Fröhler1, S. Briefi1, 2, U. Fantz1, 2

1AG Experimentelle Plasmaphysik, Universität Augsburg, 86135 Augsburg, Germany 2Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany

1. Introduction

To investigate the population processes of excited hydro-gen (or deuterium) atoms in low pressure low temperature (LPLT) plasmas the knowledge of the state resolved den-sity is necessary. For the states with n ≥ 3 this can be done by optical emission spectroscopy (OES) measurements of the Balmer transitions. For the n = 2 density the Lyman- transition in the vacuum ultra-violet (VUV) range can be used. However, an absolutely calibrated VUV spectrome-ter is required. In addition, opacity effects might be rele-vant such that the obtained emissivity is reduced by orders of magnitudes. Although the latter problem can be over-come by applying the escape factor method [1], this proce-dure raises large error bars which disturbs the accuracy of the n = 2 density determination via the L emissivity.

Another method to access the n = 2 density is using ab-sorption spectroscopy on the Balmer- transition at 656 nm. Therefore, a TDLAS (tunable diode laser absorp-tion spectroscopy) setup has been installed at a planar ICP. In the present contribution density measurements are per-formed at LPLT discharges in both hydrogen and deuter-ium and compared with VUV spectroscopy measurements and model calculations.

2. Experimental setup and the TDLAS system

The measurements are performed at a laboratory experi-ment, which consists of a cylindrical vessel with a height of 10 cm and a diameter of 15 cm. The inductively coupled plasma is heated via a planar solenoid on top of the exper-iment which is connected to an RF generator (2 MHz) with powers up to 2 kW.

The TDLAS setup consists of a diode laser (laser power 10 mW) in the Littman/ Metcalf configuration which is tunable in the wavelength range between 652 and 660 nm. Using fiber optics, the beam is split in two arms one of which is used to irradiate the plasma. To acquire the trans-mitted laser intensity this beam hits a photodiode which is covered by an interference filter to block the radiation com-ing from the plasma. The second arm is used to monitor the frequency change of the emitted photons using an etalon in the optical path. By removing the etalon, it is also used to monitor the frequency dependent change in the emitted la-ser intensity which is required for data evaluation. At the end of the second arm, a photodiode is installed to acquire the signal.

Additionally, VUV spectroscopy at the Lα line is performed with an absolutely intensity calibrated VUV spectrometer [2]. Further, high resolution OES at the

Balmer lines and the Fulcher- transition and Langmuir probe measurements are used to determine the H (D) (n ≥ 3) densities, the electron density and temperature, the gas temperature [3] and the H/H2 (D/D2) density ratio [4]. These parameters act as input parameters for the collisional radiative (CR) model Yacora H [5] and the calculation of escape factors for the L emission line. 3. Measurements

First, a characterization of the TDLAS setup is carried out. The obtained line profiles are in good agreement with calculated ones where the fine structure and natural broad-ening as well as Doppler broadening were considered. Fig. 1 shows an exemplary profile measured in hydrogen and the corresponding simulation with T = 780 K.

Investigations of the H (n = 2) density are carried out for plasmas with varying pressure between 2 Pa and 10 Pa at constant power absorbed by the plasma and for plasmas with varying absorbed power between 350 W to 650 W at constant gas pressure. The obtained n = 2 density values are compared to escape factor corrected VUV spectroscopy measurements and the CR model Yacora H.

In deuterium the D (n = 2) density is measured for vary-ing pressure between 2 Pa and 10 Pa at a constant RF power. Those densities are as well compared to escape factor cor-rected VUV spectroscopy measurements. References [1] K. Behringer, IPP report 10/11, Max-Planck-Institut

für Plasmaphysik, (1998). [2] U. Fantz et al., Plasma Sources Sci. Technol., 25, 4,

(2016). [3] S. Briefi et al., J. Quant. Spectrosc. Radiat. Transf.,

187, 135 – 144, (2016). [4] U. Fantz, Contrib. Plasma Phys., 44, 5-6, 508 – 515,

(2004). [5] D. Wünderlich and U. Fantz, Atoms, 4, 26, (2016).

Fig. 1: measured line profile of H compared with a simulation at 780 K.

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poster Diagnostics

Plasma chemical studies of nitrocarburizing with an active screen made of carbon in laboratory and industrial scale reactors

A. Puth1, S. Hamann1, L. Kusýn2, I. Burlacov3, A. Dalke3, H.-J. Spies3, H. Biermann3, J. Röpcke1, J.H. van Helden1

1Leibniz Institute for Plasma Science and Technology, 17489 Greifswald, Germany

2Masaryk University, 60200 Brno, Czech Republic 3Institute of Materials Engineering, TU Bergakademie Freiberg, 09599 Freiberg, Germany

Active screen plasma nitrocarburizing (ASPNC) is an

advanced technology for the surface treatment of steel components. Compared with conventional methods it re-duces sooth formation and the risk of cementite precipita-tion in the compound layer of the treated materials. A new approach for this method is the usage of an active screen made of solid carbon as a substitute for carbon-containing gas supplements.

In the laboratory-scaled plasma nitriding monitoring re-actor (PLANIMOR), see Fig.1, low-pressure pulsed DC N2-H2 plasmas have been studied by infrared absorption spectroscopy (IRLAS) techniques. Similar studies have been conducted at an industrial-scaled plasma nitrocarbu-rizing reactor in Freiberg, see Fig.2. Tunable diode lasers (TDL) and external-cavity quantum cascade lasers (EC-QCL) were used as radiation sources to monitor the con-centrations of ten stable species, CH4, NH3, H2O, C2H2, HCN, C2H4, CO, C2H6, CO2, C2N2, and of the CH3 radical. In dependence on plasma power at the active screen, the gas pressure and flow, and the feed gas composition, the concentrations ranged between 1012 and 1016 cm-3. The gas and rotational temperature of select species have been de-termined using Boltzmann plot and line profile analysis.

A detailed analysis of the surface microstructure of sam-ples treated in the industrial-scaled reactor in plasma chem-ical monitored atmosphere has been performed. This in-cluded glow discharge optical emission spectroscopy (GDOES), optical microscopy, micro hardness measure-ments, as well as X-ray diffraction analysis.

References [1] Spectroscopic investigations of plasma nitrocarburiz-

ing processes using an active screen made of carbon in a model reactor, A. Puth et al., Greifswald, Plasma Sources Sci. Technol. 27, 075017, (2018).

[2] Solid carbon active screen plasma nitrocarburizing of 316L stainless steel: influence of N2-H2 gas composi-tion on structure and properties of expanded austenite, A. Dalke et al., Freiberg, Surf. Coat. Technol., ac-cepted manuscript, available online, (2018).

Fig. 1 Schematic top view of the experimental setup at PLANIMOR. The plane of the White cell is parallel to the carbon screen and displayed 90° turned for illus-tration. Modified figure originally from [1].

Fig. 2 Schematic top view of the experimental setup at the ASPNC reactor. The line of sight for IRLAS meas-urements is marked as A. Modified figure originally from [2].

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Tracking the evolution of rotating electrode gliding arc discharge channels using fast-framing camera and electrical diagnostics

J. Čech1, L. Dostál2, M. Zemánek1, Z. Navrátil1, J. Valenta2, P. Sťahel1

1CEPLANT, Faculty of Science, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic

2 Faculty of Electrical Engineering and Communication, Brno University of Technology, Technicka 12, 616 00 Brno, Czech Republic

1. Introduction Non-thermal plasmas have been studied for pollutants

and hazardous substances remediation [1] and gliding arc discharges [2] were found promising for such applications [3]. The industry-scale applications demand high through-put at low pressure drop, which were addressed in new pro-peller-shaped rotating electrode gliding arc (RGA) [4]. Its plasma-properties are co-driven by the propelled evolution of discharge channels in the reactor. To reveal the behavior of the propelled discharge channels the high-speed optical and electrical diagnostics needs to be performed.

2. Experimental & Results

The discharge channels evolution was observed using fast-framing camera Photron FASTCAM SA-X2 type 1000K-M2 and digital oscilloscope LeCroy WaveRunner 6100A with Tektronix P6015A and Pearson 2877 probes. At full resolution the 0.9 s long intervals were acquired (1024×1024 pixels, 12bits @ 12500 fps @ 16 GB file per acquisition). Efficient data analysis was developed to ena-ble big-data processing and tracking of discharge channels pulled by rotating electrode.

The evolution of the rotating gliding arc has the similar-ities to the simple stationary horn electrodes design, see e.g. [5]. The RGA discharge starts as a short channel in the elec-trode gap. It then evolves in successive steps being repeti-tively reignited by alternating HV, prolongating the dis-charge channel locked to the hot-spot on revolving rotating electrode. The channels distort by the action of electromag-netic forces and the turbulent gas flow and finally breaks at some point and the next channel starts to evolve.

The complete fast-camera image analysis at such condi-tions gives valuable results but is a highly demanding pro-cess. The analysis of the fast-camera data will be presented enabling tracking of the evolution of discharge channels, following the inception points and estimating the channels length.

At the same time, we searched for simpler on-line tool for the channels length estimation, enabling efficient dis-charge parameters tuning. We would like to propose the voltage waveform tracking as the tool for on-line estima-tion of discharge channels length, correlating it to the ap-plied voltage at the re-ignition of the gas breakdown.

The diagnostics was performed on the mid-scale RGA reactor (gas flow rate 200 m3/hr) consisting of metal tube of 20 cm diameter and 5-blade rotating electrode powered by alternating HV at frequencies 50 Hz, 5 kHz, resp. 30 kHz and amplitude between 1 to 10 kV. The frequency of electrode rotation was varied in the range 10 to 25 Hz and the electrode-gap distance was kept at approx. 5 mm.

Fig.1 Evolution of discharge channel length and image of the channel, at 1500 W power input and 5 kHz HV frequency.

Acknowledgement Authors JC, MZ, ZN and PS gratefully acknowledge finan-cial support from the project LO1411 (NPU I) funded by Ministry of Education, Youth and Sports of the Czech Re-public and project CZ.01.1.02/0.0/0.0/15_019/0004703 Operational Programme Enterprise and Innovations for Competitiveness. Authors LD and JV gratefully acknowledge financial sup-port from the Ministry of Education, Youth and Sports of the Czech Republic under NPU I program (project No. LO1210) and OP VVV Programme (project No. CZ.02.1.01/0.0/0.0/16_013/0001638 CVVOZE Power La-boratories - Modernization of Research Infrastructure). References [1] A. M. Vandenbroucke, R. Morent, N. De Geyter, Ch.

Leys, Journal of Hazardous Materials 195, p.30-54, (2011).

[2] F. Richard, J. M. Cormier, S. Pellerin, J. Chapelle, Journal of Applied Physics 79 (5), 2245-2250, (1996).

[3] A. Czernichowski, Pure and Applied Chemistry 66 (6), p. 1301–1310, (1994).

[4] Patent application No.: WO2016177353A1. [5] L. Potočňáková, J. Šperka, P. Zikán, J. J. W. A. Van

Loon, J. Beckers, V. Kudrle, Plasma Sources Science and Technology 26, 045014, (2017).

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Rate Coefficients of OH(A,v=0,1) quenching and vibrational relaxation for non-thermal rotational distributions

M. Ceppelli1, L.M. Martini1,3,*, M. Scotoni1, G. Dilecce 2,1, P. Tosi1,2

1Dipartimento di Fisica Università di Trento, via Sommarive 14, Povo, Trento, ITALY

2P.Las.Mi Lab NANOTEC - CNR, via Amendola 122/D, Bari, ITALY 3present address: Department of Applied Physics - Eindhoven University of Technology, The Netherlands

*Contact e-mail: l.m.martini@tue.nl

1. Introduction Electronic quenching and vibrational relaxation processes in the OH(A2u

+, v=0,1) vibronic manifold are important when using Laser Induced Fluorescence in atmospheric pressure discharges. These processes have been quantita-tively well characterized only in conditions of thermal ro-tational distributions of the OH(A) state, and at Trot=Tgas=300 K [1]. Such conditions are met in discharge gas mixtures made of buffer He or Ar with a small addition of molecular gases. Relevant examples are the atmospheric pressure plasma jets (APPJ) largely used in Plasma Medi-cine. In molecular gases, rotational relaxation is far from being complete, and the resulting rotational distributions in the OH(A,v) manifold are strongly non-Boltzmann (Fig.1). Due to the large dependence of collision rate constants on the rotational level, the thermal rate constants measured in [1] cannot be used in these non-equilibrium conditions and new data are required to quantify the collision processes. In this contribution, we address the experimental apparatus we have implemented to this aim. In particular, we report the measurement of rotational distributions and rate coeffi-cients by collision with CO2, CO and O2. The interest in these collision partners arises from the use of Collision En-ergy Transfer LIF [2] for measuring the time-resolved CO2 dissociation degree in ns pulsed discharges [4].

Fig. 1 OH LIF spectrum in CO2 after laser excitation of P1(3) line of (1,0) band. Both (1,1) and (0,0) bans are visible, the latter being due to vibrational relaxation.

Fig. 2 Drawing of the experimental apparatus. The LIF outcome is measured by a photomultiplier (PMT) for the temporal LIF pulse, and by an ICCD spectrograph for the LIF spectrum. 2. Experimental apparatus The experimental apparatus is shown in Fig. 2. The colli-sion cell is filled with a small amount of vapour from 50% H2O2 solution in water and variable amounts of collider gas. The fourth harmonic of a Nd-YAG laser at 266 nm – about 10 mJ/pulse, is used for the production of OH(X) by photo-dissociation of hydrogen peroxide. A second laser beam, at about 281 nm (SH of a Nd-YAG pumped dye laser) pumps the OH(A, v=1) level. Collision rate constants are derived from the LIF pulse temporal decay and from the LIF spec-trum (see Fig. 1). Since photo-dissociation results in translationally very hot OH fragments, the 281 nm laser beam is fired with a delay of 10 s after the 266 nm one, to ensure thermalisation of OH(X). Variation of this delay will eventually allow exploring the kinetic energy depend-ence of the collision rate constants. 4. References [1] L. M. Martini, N. Gatti, G. Dilecce, M. Scotoni and P. Tosi, 2017 J. Phys. D:Appl. Phys. 50 p. 114003 [2] L. M. Martini, N. Gatti, G. Dilecce, M. Scotoni and P. Tosi, 2018 Plasma Phys. Controlled Fusion 60 p. 014016 and L. M. Martini, M Ceppelli, M. Scotoni, G. Dilecce and P. Tosi, 2019 This book of abstracts [4] L. M. Martini, S. Lovascio, G. Dilecce and P. Tosi, 2018 Plasma Chem. Plasma Process. 38 p.707-718

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Formation of Nanoparticles of Copper and Zinc in a Magnetic Field

Vyacheslav F. Myshkin1, Valeriy A. Khan1,2, Milan Tichy1,3, Anna Kapran3

1Tomsk Polytechnic University, 634050, Tomsk, Lenina av., 30, Russian Federation 2V.E. Zuev Institute of Atmospheric Optics SB RAS, 634055, Tomsk, Akademicheski av., 1, Russian Federation

3Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Praha 8, Czech Republic

1. Abstract We present the results of the development of dispersed

particles formed upon cooling of Zn and Cu vapors. The parameters of the particle size distribution (the width and the maximum) of the formed ensemble of the dispersed particles are temporarily dependent in a random manner and lie in a certain range. Our experiments have established that in a magnetic field of 44 mT and 76 mT, the granulo-metric composition of the formed Zn nanoparticles in the range of 100–200 nm is more stable over time than without a field. When cooling down the Cu vapor without a mag-netic field, much more conglomerates containing dispersed particles with a diameter of less than 300 nm are formed than in a magnetic field of 30 mT.

2. Introduction

Phase transitions and chemical reactions are associated with the formation or destruction of molecular (intermolec-ular) bonds formed by paired electrons. The valence elec-tron and the nuclei of some isotopes have a magnetic mo-ment (spin) involved in magnetic interactions. As a result, the magnetic effects can affect the rates of chemical reac-tions, the thermal conductivity of materials or the plasticity of crystals. Further, the magnetic effects can facilitate con-trol of the isotopic selectivity of chemical reactions in so-lutions or purification of water from dissolved minerals.

3. Formation of disperse particles from gas phase

The nucleation process begins with the collision of three particles and the formation of condensation nuclei — clus-ters of atoms. At low supersaturation, clusters can sponta-neously evaporate. The frequency of the fluctuation for-mation of condensation nuclei is established in a time less than 1 ns. One should expect a different action of the mag-netic field on the desublimation from the high-temperature flow of different types of magnetics. The magnetic field promotes the formation of larger ferromagnetic particles of iron oxides. In a magnetic field, the ferromagnetic particles of iron oxide follow the field lines of an external magnetic field. That accelerates the process of agglomeration of fer-romagnetic particles in a magnetic field.

4. Experimental facility

At the first facility, the evaporator, we studied the phase transition from the high-temperature flow, see Fig.1a. In the second facility, shown in Fig.1b, we created an arc dis-charge between the copper electrodes in a cylindrical quartz chamber, through which argon was pumped at a rate

of 60 slm. In both systems, the magnetic field was formed by two permanent magnets in the range 30 mT-76 mT.

5. Results Fig. 2 shows the typical granulometric composition, of

the dispersed Cu particles in the arc discharge facility with-out and with the magnetic field. That was determined by

counting the number of particles of differ-ent sizes in SEM im-ages with the magni-fication 60,000. It is clearly seen that in a magnetic field 30 mT the size distribution function is signifi-cantly narrower and its maximum has shifted to larger sizes.

4. Conclusion The rate of condensation nuclei formation and the prob-

ability of clusters reaching a critical size increases with the nucleation of supersaturated Zn and Cu vapors in an exter-nal constant magnetic field. As a result, the degree of vapor supersaturation decreases faster to a level at which the fre-quency of new condensation nuclei formation decreases significantly. In a magnetic field, a large fraction of dis-persed Zn or Cu particles also grows faster than without a field. The formation of conglomerates from larger particles having a lower concentration is less likely. Therefore, in a magnetic field, a decrease in the width of the size distribu-tion function of dispersed zinc particles is observed from 40 nm without magnetic field down to 10 nm at 44 mT or 76 mT as well as a shift of its maximum towards larger sizes with increasing field strength. The patterns of the nu-cleation process of copper vapor in a magnetic field are similar.

Fig. 2. Dispersity of the Cu particles formed in the arc discharge facility without and with a magnetic field.

Fig. 1. Scheme of the experimental facilities for the for-mation of dispersed particles from a high-temperature flow containing metal vapors. a - Zinc: 1 - Zinc melt, 2 - Zinc vapors, 3 - heater, 4 – permanent magnets, 5 – probing laser beam. b - Copper: 1 - arc electrodes, 2 - permanent mag-nets, 3 - substrate, 4 - quartz tube, 5 - argon flow.

55

poster Diagnostics

Automated determination of pressure profile generated bydischarges in contact with liquid phase by interferometric technique

L. Kusyn1, P. Hoffer2, Z. Bonaventura1, T. Hoder1

1 Department of physical electronics, Masaryk University, Brno, Czech Republic2 Institute of Plasma Physics, Czech Academy of Sciences, Praha, Czech Republic

The development of discharge in dielectric liquids isstill centre of discussion to this day. The propagationof streamer is closely accompanied by time-dependentradial expansion of pressure wave in surrounding liq-uid. The phenomena of shock waves produced by ex-panding corona-like discharges in aqueous solutions iswell known, nevertheless the determination of pres-sure profile of investigated inhomogeneity can be quitechallenging (eg. [1]). To investigate aforementioneddischarges in distilled water or highly conductive saltaqueous solutions the Mach-Zehnder interferometer isused to reveal fast micro-physics by study of changeof refractive index. Consequentially, the distributionof pressure and electric field generated by nanosecondhigh-voltage pulse in water can be estimated.

To effectively achieve these distributions, we pro-pose the use of automatized procedure to analyze largeamount of experimental data by advanced statisticaltechniques. An example of interferogram used in thiswork can be seen in Fig. 1. The evaluation of inter-ferometric images is based on isolation of construc-tive interference pattern and ability to select individ-ual light rays, see Fig. 2. As the light ray propagatesthrough inhomogeneous region the deviation of refrac-tive index is determined by ”onion-peeling method”.The method separates investigated inhomogeneity intoseveral layers where the resulting distributions areachieved by iterative procedure. Result of such evalu-ation can be seen in Fig. 3.

The results of automatized process will be criticallycompared with standard approach to evaluate its pre-cision and improve it further. Ultimately we believethat this approach will lead to novel insight into thephenomena of nanosecond discharges in liquid environ-ment and significantly increase the efficiency of exper-imental data evaluation. This contribution is fundedby Czech Science Agency grant no. 18-04676S.

References

[1] W. An, K. Baumung, and H. Bluhm. “Underwa-ter streamer propagation analyzed from detailedmeasurements of pressure release”. In: Journal ofApplied Physics 101.5 (Mar. 2007), p. 053302.

[2] P. Hoffer. “Shock waves generated by corona-likedischarges in water”. PhD thesis. CTU, 2014.

Figure 1: Typical interferogram of the pulsed dis-charge in water media, taken from [2]. The framedarea is used for following calculations.

Figure 2: An example of detection of interference pat-tern in interferometric image.

Figure 3: An example of pressure and density profileinside the inhomogeneity as dependence of distancefrom its centre.

56

Diagnostics poster

Some Problems in Today’s Diagnostics of Low Temperature Plasma

V. Godyak

RF Plasma Consulting, Brookline, MA, USA

In the following presentation we will discuss a number of misconceptions found in the literature related to experi-mental measurements in low temperature gas discharge plasmas. Among troubling issues found in some recent publications on rf discharges are: a) Neglecting vacuum hygienics - Seems, that some au-thors are not aware of the necessity of degazing the dis-charge vessel and achieving the ultimate vacuum pressure being few orders of magnitude lower than the working gas pressure. Otherwise, due to gas desorption by heating and ion bombardment of the discharge vessel, the content of the discharge working gas becomes unknown. b) Undefined rf power delivered to plasma Pp - which is usually considerably less (and sometimes much less) and is not proportional to the measured the output power Ps of a rf source. A significant portion of Ps dissipates in the matcher, induction coil and (due to eddy current) in metal chamber. Therefore, Pp has to be measured as the rele-vant parameter defining the discharge condition, and not Ps which is usually measured instead. c) Probe rf compensation - It is widely recognized that meaningful application of Langmuir probes in rf plasmas requires mitigation of rf voltage across the probe sheath, i.e. Vrfs < Te, where Vrfs is the peak-to-peak probe sheath voltage and Te is the electron temperature in Volt. In or-der to determine, whether this requirement is satisfied one has to measure the plasma rf potential Vprf to make an ade-quate rf filter with certain impedance characteristics. Using just some filter does not mean that the probe is rf compen-sated. Probe rf compensation is required not just for Lang-muir probes, but also for any probe used in rf plasma diag-nostics, including ion current, emissive, microwave, mag-netic probes and greed analyzer immersed into plasma. d) Probe diagnostics based on the ion current – There is a wide spread opinion that the plasma parameters infer- red from the ion part of the probe characteristics are equiv-alent to those found from its electron part. There are few collisionless theories for the ion current to the probe, and all of them give different plasma densities which differ up to order of magnitude from densities found from classical

Langmuir procedure, or as an integral of the measured EEDF. Furthermore, the temperature of the bulk of the EEDF does not coincide with the electron temperature found from the probe characteristic around the probe float-ing potential, where due to inelastic electron-atom colli-sions the EEDF usually changes its shape. Because, typ-ically, li << le, and because the ion current is very sensitive to ion-neutral collisions, the upper limit of gas pressure for applicability of the ion current diagnostics is about two or-ders of magnitude lower than in the case of Langmuir-Druyvestein diagnostics. Here, li and le are the mean free paths of ions and electrons. d) EEDF measurements – It was known for a long time, and now is widely accepted, that electron energy distribu-tion function (EEDF) in gas discharge plasma is not a Max-wellian. That put under question the validity of classical diagnostics based on assumption of Maxwellian EEDF and promoted the measurement EEDF with finding plasma macro-parameters as appropriate integrals of the measured EEDF. Unfortunately, many published EEDFs measured with home-made and commercial probe instruments are missing its low energy and high energy parts. That makes meaningless such EEDF measurements, since the infor-mation about low energy electrons (that are majority of electron population) and about the EEDF tail (responsible for excitation and ionization) are missing. Such EEDF measurements have no any advantage comparing to the classical Langmuir probe diagnostics based on assumption of Maxwellian EEDF.

f) Magnetic probes measurement – Encapsulated (non-transparent for plasma rf current) magnetic probes have been in use for long time for measurement of electromag-netic fields and rf currents in plasma. However, it has been shown that the data obtained with such probes are heavily distorted due to plasma density depletion and current path obstruction due to the presence of the probe. The difficulties in the topics (a-f) mentioned above have been already addressed in the literature, but for some reason are not well known. The goal of this presentation is to alert researchers about those problems, point to the right literature and to help them to get correct measurements.

57

poster Diagnostics

Dissociation in nanosecond capillary pulsed discharge in CO2 G.V. Pokrovskiy1 and S.M. Starikovskaia1

1Laboratory of Plasma Physics, Ecole Polytechnique, Palaiseau, France

1. Introduction

The dissociation of carbon dioxide by plasma has been an object of scientific interest since 1960s. First, this pro-cess gives the clue for purification of ambient air with a presence of carbon dioxide. The latter is one of the main reasons of global warming. Second, CO2 is one of the most abundant gases in planetary atmospheres. For instance, 95 % of the atmosphere of Mars consist of carbon dioxide and thus better comprehension of the mechanisms of its dissociation could be useful for perspective colonization of the Red planet.

The conventional mechanism of dissociation of CO2 in

plasma is the so called ladder mechanism: it is excitation of its vibrational degrees of freedom by electron impact with consequent V-V exchange of vibrational quanta and eventual dissociation onto CO and O. This mechanism is dominant at relatively low values of reduced electric field (E/N = 20-50 Td) [1,2]. However, the excitation of higher (7.2 and 10.5 eV) electronic degrees of freedom of carbon dioxide with their consequent dissociation takes the lead-ing role within the increase of the reduced electric field up to hundreds of Td. This mechanism gives more dissocia-tion yield but provides less energy efficiency because higher electron energies are required.

The goal of the study described here is to understand the

case of the dominant role of the mechanism of CO2 disso-ciation by means of excitation of electronic degrees of free-dom. The nanosecond discharge has been chosen for the announced purpose as far as it provides relatively high val-ues of reduced electric field.

2. Experiment

The nanosecond pulses of positive polarity were pro-duced by high voltage generator. The pulses were delivered to the discharge capillary by a system of high voltage ca-bles. The diameter of the capillary was 1.7 mm and the dis-tance between the electrodes was 75 mm. The discharge was generated by a high voltage pulse with a maximal volt-age of 10 kV. The rise time of the pulse was 4 ns and the FWHM was 30 ns. The highest electric current (130 A) was observed at the pressure of 15.5 mbar.

The fast ionization wave was observed in the capillary,

the values of the E/N of 1200 Td were registered in the front of the wave. The values of the electric field of approx-imately 300 Td were registered behind the front of the fast

ionization wave. The synchronized in time waveforms of the electric field and the electric current are given in the Fig.1.

Fig. 1. The profiles of synchronized electric field and the

electric current. The plasma components were studied by the means of

optical emission spectroscopy. The optical emission spec-tra were recorded in the spectral range of 200-900 nm. The molecules were represented by CO2+ systems (the so called “flame system” at 288-289 nm and the system of Fox, Duf-fendack and Barker at 310-420 nm) and the 4th positive sys-tem of CO (200-270 nm). The atomic lines of C (248 nm) and atomic oxygen (777 nm, 844 nm) were observed.

3. Conclusion. The stable nanosecond discharge in the atmosphere of

pure CO2 was observed at the pressure of 15-20 mbar as like as the formation of the fast ionization wave in the ca-pillary. The optical emission spectra have shown the pres-ence of the CO2+ ions which gives an evidence of intensive formation of this ions by the means of electron impact from the ground state of CO2. The numerical modeling of the discharge has shown that the most probable reaction which can follow this process might be dissociative attachment of electrons to positive ions. References

[1] A. Fridman. Plasma Chemistry, Cambridge University

Press (2008)

[2] D.Rusanov, A.Fridman, G.Sholin, UFN (1981)

58

Diagnostics poster

Oxygen effects on streamer-to-filament transition in nanosecond surface dielec-tric barrier discharge

Ch. Ding, S. A. Shcherbanev and S. M. Starikovskaia

CNRS, Ecole Polytechnique, Sorbonne University, University Paris-Sud, Observatoire de Paris, University

Paris-Saclay, Palaiseau, 91128, France

1. IntroductionPlasma of nanosecond surface dielectric barrier dis-

charges (nSDBD) is found to be efficiently used for plasma assisted ignition/combustion (PAI/PAC) [1] and plasma as-sisted aerodynamics [2]. The filamentary nSDBD at high pressure in a single short regime was obtained in 2014 when the transition points (voltages and pressures) from streamer mode to filamentary mode was first measured [3]. Recent experiments show that oxygen plays a crucial effect in the streamer-to-filament transition process.

2. Experimental setupThe experiments of streamer-to-filament transition were

carried out with the cylindrical configuration of the elec-trode system [4] with the disk-like high voltage electrode connecting to the high voltage pulse generator FPG 12-1PM (FID GmbH) provided single shot pulses of positive polarity, 20 ns FWHM, 2 ns rising time. The discharge was installed into a closed high-pressure chamber (up to 12 bar). The images were taken by Pi-Max4 Princeton Instrument ICCD camera.

3. Results and discussionThe transition curve was measured in different composi-

tion of nitrogen and oxygen gas mixtures (pure nitrogen, with 1%, 2%, 5% and 20% of oxygen). The voltage ampli-tude when only 3-5 filaments are observed during the first 12 ns of the discharge was considered as a transition volt-age.

Fig.1 Transition curve in different gas mixtures.

The voltage-pressure diagrams for positive polarity are given by figure 1. It can be seen that the effect of molecular oxygen addition is quite strong. For instance, at the pres-sure P=6 bar, in pure nitrogen the transition occurs when the voltage reaches up to 30 kV while after adding 2% of oxygen, the transition required voltage increases nearly 50% up to 44 kV. A progressive increase of oxygen addi-tions to 5% and then to 20% changes smoothly the transi-tion point: at P=6 bar, the transition voltage is 48.5 kV for 5% of oxygen in the mixture and 49 kV for air. At lower pressures, 2.5-5 bar, the discharge is in the streamer mode even with the smallest used additions of oxygen, 2%, no filamentation is observed for the voltages up to U=+52 kV on the high-voltage electrode.

It is shown experimentally that, for positive polarity, the streamer-to-filament transition is a strong function of the gas mixture composition. Oxygen influence the transition process which can be explained as photoionization.

4. AcknowledgementsThe work was partially supported by French National

Research Agency, ANR (ASPEN Project), LabEx Plas@Par and the French–Russian international laboratory LIA KaPPA ‘Kinetics and Physics of Pulsed Plasmas and their Afterglow’ (including RFBR project 17-52-16001 and CNRS financial and organization support). The support of China Scholarship Council (CSC) for Chenyang Ding is gratefully acknowledged. The authors are thankful to Ali Mahjoub and Bruno Dufour for engineering support.

References [1] S. M. Starikovskaia, Journal of Physics D: Applied

Physics, 47(2014), 353001.[2] S. B. Leonov, V. Petrishchev and I. V. Adamovich,

Journal of Physics D: Applied Physics, 47(2014),465201.

[3] S. A. Stepanyan, A. Y. Starikovskiy, N. A. Popov & S.M. Starikovskaia, Plasma Sources Science & Technol-ogy, 23.4(2014), 045003.

[4] S. A. Shcherbanev, S. A. Stepanyan & S. M.Starikovskaia, 22nd International Symposium onPlasma Chemistry, July 5-10(2015), Antwerp, Bel-gium.

59

poster Diagnostics

Analysis of homogenous nanosecond discharge at moderate pressure: dissocia-tion of oxygen for plasma assisted detonation

M. Ali Cherif1, SM. Starikovskaiai1

1 Laboratory of Plasma Physics, UMR764 , Ecole Polytechnique, Palaiseau, France

1. Introduction

Combustion of a fuel-oxidizer mixture in a pulsed det-onation engine (PDE) is characterized by significantly higher thermodynamic efficiency than the conventional constant pressure combustion. A detonation wave may be formed by a direct initiation or through a deflagration to detonation transition (DDT) process. The typical DDT length amounts to several tens of tube diameters. DDT length and time are crucial parameters for PDE applica-tions. Detonation significantly depends on the parameters of the gas, in particular – on gas composition. Plasma community has been intensively working on the problem of plasma assisted ignition [1,2], that is reduction of delay time of ignition of combustible mixtures with the help of non-equilibrium low-temperature plasma, and plasma assisted combustion [3] where gas mixture is prepared by the action of plasma right before combustion demonstrat-ing significant extension of combustion limits and capac-ity to burn lean mixture in a stable regime.

2. Experimental set-up and results

The discharges were made in O2:Ar:Air (50:40:10) mixture with pressure ranging between 50 and 200 mbar. The mixture was contained in a rectangular-parallelepiped cell made of Plexiglas, with inner length and squared sec-tion 200 mm and 50x50 mm², respectively. Breakdown was found to occur for all pressures in the considered range. Homogeneous plasma was obtained for all pres-sures.

Figure 1. Energy deposition versus pressure for 25 and 30 kV. Maximum are observed for 120 and 170 mbar.

Electrical diagnostics were made using classic back

current shunt techniques. The energy deposited to the plasma was measured for several pressures and two gen-erator tensions 25 kV and 30 kV. Figure 1 shows that

energy deposition increases with increasing pressure and reaches the maximum 180 mJ at 120 mbar for 25 kV, and 300 mJ at 170 mbar for 30 kV.

3. Discussion and conclusions

Figure 2. Numerical modeling of the densities of O and O2 for 120 mbar and 25 kV.

These measured values of the deposited energy were

key parameters for oxygen dissociation. The experimental electrical fields were used as input data to ZDPlaskin calculations of the O density produced by the plasma. Figure 2 shows the O2 and O densities as a function of time for the case 25 kV-120 mbar, and indicates that the ratio of the O/O2 dissociation is 0.1%. This is a small value compared with those achieved for DDT. Therefore, the present nanosecond single pulse is not supplying enough energy to dissociate more than 0.1% oxygen in our conditions. Using a repetitive nanosecond discharge appears to be necessary and will be investigated in next experiments. 4. References [1] Starikovskiy, A. and Aleksandrov, N. 2013 Plas-

ma-assisted ignition and combustion Prog. Energy Combust. Sci. 39 61-110

[2] Starikovskaia SM, Plasma-assisted ignition and com-bustion: nanosecond discharge and development of kinetic mechanisms 5topical Review) J. Phys. D: Appl. Phys. 47(2014) 353001 (34pp)

[3] Ju, Yu., Sun, W. Plasma assisted combustion: Dynam-ics and chemistry, Progress in Energy and Combus-tion Science 48 (2015) 21-83

[4] Adamovich, I. et al. Plasma assisted ignition and high-speed flow control: non-thermal and thermal effects, Plasma Sources Sci. Technol. 18 (2009) 034018

60

Diagnostics poster

Experimental characterization of CO2/Ar glow discharges and model validation, 13th-16th May 2019, Bad Honnef, Germany

A.Silva1,2,3, A. S. Morillo-Candas2, A. Tejero-del-Caz3, O. Guaitella2, V. Guerra3 1 DIFFER, part of the Netherlands Organization for Scientific Research , Eindhoven, the Netherlands

2 LPP, École Polytechnique, Sorbonne Université, CNRS, Palaiseau, France 3 IPFN, Instituto Superior Técnico, Universidade de Lisboa, Portugal

1. Introduction The use of plasmas for activation of the highly endother-

mic dissociation of CO2 has been under investigation in the context of the CO2 reduction and reutilization. This work concerns the characterization of CO2/Ar mixture glow dis-charges at low pressures using several diagnostic tech-niques. The experimental results are then used to validate a model with which further insights about the discharge can be obtained. 2. Diagnostics

The diagnostics used in this work are mainly optical. With in-situ Fourier transform infrared (FTIR) spectros-copy it is possible to quantify gas and CO2 vibrational tem-peratures as well as conversion (α) of CO2 into CO [1]. Moreover, with tunable diode laser absorption spectros-copy (TDLAS), scanning the 772.42 and 772.38 nm ab-sorption lines corresponding to electronic excitation of the Argon metastable sates, the densities and temperatures of these states are measured. Also, inside the reactor, are two metal rods placed 8 cm apart, pointing radially inside the positive column of the plasma. Probing the potential drop between the two, the electric field inside the plasma is es-timated. A representation of the set-up is shown in Fig.1 and some of the measurements are plotted in Fig.2 and 3. 3. Conclusions

The mentioned methods returned reproducible measure-ments that enable the characterization of the plasma at high and low Argon content. Given the homogeneity of the dis-charge and the comprehensive amount of experimental data, the measurements are then further used for validation of a kinetic scheme implemented on the 0D simulation tool LoKI [2]. A good agreement between model and experi-ment is observed and some of the preliminary results are plotted in Fig.4. The modelling and experimental effort constitutes a useful tool in understanding what are the main dissociation mechanisms in the CO2 plasma, how is the ad-dition of argon affecting those, and how to further enhance conversions.

References [1] B. L. M. Klarenaar, et al., P. S. Sci. Technol. 26 (2017) [2] A. Tejero-del-Caz et al., "The LisbOn KInetics tool suit", contribution to the 24th ESCAMPIG (2018)

Fig. 1 Representation of the experimental set-up.

Fig. 2 Measured densities and temperatures of Arm using TDLAS.

Fig. 3 Gas and asymmetric stretch mode vibrational temperature measured with FTIR. Fig. 4 Simulated and measured E/N and α.

61

poster Diagnostics

CO2 splitting in low temperature atmospheric plasma sustained with nanosec-ond microwave pulses

S. Soldatov1, A. Navarrete2, R. Dittmeyer2, J. Jelonnek1,3, G. Link1, C. Schmedt2…

1IHM, 2IMVT, 3IHE, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

1. Introduction

The plasma assisted conversion of CO2 into synthetic fuels based on renewable energies is considered as very promising approach for mitigation of CO2 emission and en-ergy storage [1]. Among different plasma discharges, the CO2 splitting reaction CO2 CO + 1/2 O2 has shown to be most efficient in microwave sustained plasmas. Due to high electrical energy density and preferential activation of vibrational excitation states of CO2 molecules, an energy efficiency of up to 80 % was demonstrated, though at sub-atmospheric pressure [2]. At atmospheric pressure, the en-ergy transfer from vibrational (Tvib) to rotational (Trot) tem-perature increases and results in the deterioration of reac-tion energy efficiency [3]. The supply of microwave not continuously but rather in short pulses may increase the en-ergy efficiency by shifting a thermal equilibrium in plasma towards Tvib.

In present work, the efficiency of CO2 splitting in atmos-pheric microwave plasma has been studied if the micro-wave power is modulated at nanosecond time scale.

Fig. 1. Scheme of experiment.

Fig. 2. Plasma experiment.

2. Experimental Setup Figure 1 shows the scheme of the experiment. The corre-sponding lab setup is shown on in Fig. 2. A solid-state mi-crowave source from HBH Microwaves GmbH with a

maximum output power of 250 W and an operating fre-quency between 2.4 and 2.5 GHz is used. The microwave is fed by a coaxial line into the plasma reactor. Using a bi-directional coupler and a calibrated fast oscilloscope al-lows to control the microwave power absorbed in the plasma. A plasma torch PS-Cle from company Heuermann HF-Technik GmbH is utilized as the reactor. An Emerson X-STREAM gas analyzer is used to measure concentra-tions of the reaction products CO2, CO and O2.

In the experiment, the microwave operated in a pulsed regime with a constant power level of 200 W. The pulse duration time (tON) and the interval between pulses (tOFF) were varied to get the variation of the duty cy-cle 𝑡𝑂𝑁 (𝑡𝑂𝑁 + 𝑡𝑂𝐹𝐹)⁄ between 0.01 and 1. The experi-ments were performed with three different levels of CO2 flow at the input: 12 slm, 15 slm and 18 slm. The estimated CO2 conversion is based on the measured concentration of the reaction products.

3. Energy efficiency The energy efficiency is defined as the ratio of the energy

spent for the dissociation of CO2 molecules to the total mi-crowave energy consumed:

. Here , 𝛥𝐻𝑅

0, Peff are the CO2 conversion, reaction en-thalpy and effective microwave power, which is calculated as averaged power.

Within the parameter scan a maximum energy efficiency of about 42% was reached for a duty circle of 0.2 and input flow of 12 slm. It corresponds to a specific energy input of SEI=0.05 eV/mol. Towards lower SEI, the energy effi-ciency drops fast. For values of SEI > 0.05 the decreases almost linearly.

It was found that a plasma sustained with continues mi-crowave power (tOFF=0) is less energy efficient as com-pared to a plasma sustained with pulsed microwaves. This is a direct indication that the pulsed energy supply shifts the thermal equilibrium in plasma towards Tvib. For further verification the high resolution OES is planned.

References [1] A. Navarette et al, Energy Technology 2017, 5, 796-811, Willey-VCH Verlag GmbH & Co. [2] V.D. Rusanov, A.A. Fridman, G.V. Sholin, Usp. Fiz. Nauk, Vol. 134, Issue 2, 185-235,1981 [3] R. Snoeckx and A. Bogaerts, Chem. Soc. Rev., 2017, 46, 5805-5863

Gas FlowController CO2

CW or PWM tON> 30 ns, tOFF> 1 ns

CO2 CO + ½ O2

Ignition at 2.49 GHz, 150 W, 1 barOperation at 2.45 GHz, 200 W, 1 bar

0.01 < Duty Circle < 1

Coaxial Plasma-Jet

Gas AnalyzerCO2

COO2

Solid State Generator

𝜂 =𝐹𝑙𝑜𝑤 (𝑚𝑜𝑙/𝑠) ∙ 𝜒 ∙ 𝛥𝐻𝑅

0(𝐽/𝑚𝑜𝑙)

𝑃𝑒𝑓𝑓(𝑊)

62

Diagnostics poster

Study of particle transport above the target in high power impulse magnetron sputtering plasmas using a marker technique

S. Thiemann-Monjé1, M. Sackers1, A. von Keudell1

1Experimental Physics II, Ruhr-University Bochum, Bochum, Germany

1. General

High power impulse magnetron sputtering (HiPIMS) has established itself as one of the premier methods for depos-iting high quality hard coatings [1,2]. Nevertheless, the plasma discharge itself is not fully understood yet. Espe-cially the potential structure of the so-called spokes, which are believed to be rotating instabilities, is a part of current research. These spokes seem to play an important role in the particle movement inside the plasma. This movement leads not only to the development of a coating on the sub-strate but sputtered particles from the surface as well return to the magnetron target surface and are redeposited there. In this work an indirect method for the analysis of the par-ticle transport inside the plasma is introduced. It is based on the use of marked magnetron targets and the analysis of the redeposited marker material on the targets.

2. Setup

Figure 1 shows the experimental setup made up of a plasma discharge chamber containing the magnetron and viewports to enables the plasma analysis using a fast inten-sified CCD-Camera. This chamber is connected to the X-Ray Photoelectron Spectroscopy via a transport chamber to avoid contaminations prior the surface analysis. The struc-ture of the marker targets is shown in Figure 2. These are

made of 50 mm Al magnetron targets with a marker metal in form of a cylindrical insert placed in the middle of the

racetrack. 3. Measurements and results

The distribution of marker material on the surface is an-

alyzed by spatially resolved X-ray photoelectron spectros-copy (XPS) after various plasma discharges. These anal-yses are connected with electrical and optical measure-ments of the used plasmas.

It could be shown that the distribution of marker material contains information about the plasma discharge. Particu-larly correlations with the marker material as well as the discharge power were found. It was as well possible to find evidence for the assumed potential structure [3] of the spoke phenomenon

4. Referee and publication References [1] J. Alami et al. J. Vac. Sci. Technol. A 23 278–80

(2005). [2] K. Sarakinos et al. J. Phys. D 40 2108 (2007). [3] A. Anders et al. Appl. Phys. Lett. 103, 144103 (2013).

Fig. 1 Sketch of the experimental setup with the plasma discharge chamber on the right, containing the magnetron and optical viewport for the use of an PIMAX 3 ICCD camera. An on the left the connection to a PHI 5000 X-Ray Photoelectron Spectroscopy system

Fig. 2 Sketch of the used marker targets. Made of an Al magnetron target of 50 mm diameter with a marker in form of a cylindrical insert of 3 mm diameter placed in the mid-dle of the racetrack.

63

Tuesday May 14th

Simulations

Simulations invited

Benchmarks for two-dimensional electrostatic particle-in-cell simulations

M. M. Turner1, P. Chabert2, Z. Donko3, A. Derzsi3, D.Eremin4, P. Hartmann3, R. Lucken2, T. Mussenbrock5 and P. Stoltz6

1National Centre for Plasma Science and Technology and School of Physical Sciences, Dublin City University, Dublin

9, Ireland

2LPP, CNRS, Ecole Polytechnique, UPMC Univ Paris 06, Univ. Paris-Sud, Observatoire de Paris, Université Paris-

Saclay, Sorbonne Universites, PSL Research University, Ecole Polytechnique, 91128 Palaiseau, France

3Department of Complex Fluids, Institute for Solid State Physics and Optics, Wigner Research Center for Physics , Bu-

dapest, Hungary

4Lehrstuhl für Theoretische Elektrotechnik, Fakultät für Elektro- und Informationstechnik der Ruhr-Universität, Ger-

many

5Institute of Electrical Engineering and Information Science, Brandenburg University of Technology, Cottbus, Germany

6Tech-X Corporation, Boulder, Colorado, U. S. A.

1. Introduction

Correctness of computer simulation codes is a matter

of increasing interest in many fields of science and engi-

neering, and low-temperature plasma physics is no excep-

tion. The preferred method is to show that the computer

simulation programme under test converges to an exact

solution of the equations of the underlying physical

model. Of course, this presupposes that such a solution is

known, but there are ingenious methods for finding such

solutions, based on the insight that for testing purposes

physical significance is not mandatory. However, for var-

ious technical reasons this approach cannot yet be applied

to all categories of simulation code, and at the present

time particle-in-cell simulations are one of these excluded

categories. An alternative procedure is benchmarking, in

which one shows that several independent codes can pro-

duce the same solution of a prescribed problem. This is a

less robust procedure than comparison with an exact solu-

tion, but better than no correctness testing at all.

In this work we extend a previous benchmark study [1]

of one-dimensional particle-in-cell codes with Monte

Carlo collisions by considering two-dimensional prob-

lems. Of course, the range of geometries and boundary

conditions that might be thought of practical interest is

wide, and we limit our attention to Cartesian geometry

with combinations of periodic, Dirichlet and von Neu-

mann conditions on the edges. We discuss the benchmark

cases more specifically in the next section.

2. Benchmark Cases

The benchmarks feature a plasma confined in a rectan-

gular region in the x-y plane, and the cases differ by the

boundary conditions applied along the four edges. In all

cases, the plasma is sustained by a uniform electric field

applied along the z axis, normal to the simulation plane.

The boundary condition that determines this electric field

prescribes the total current passing through the simulation

region in the z direction, which is assumed to oscillate at

10 MHz with a fixed amplitude. All the benchmark cases

assume a fixed neutral background density with collision

67

oral Simulations

1

Challenges in the modeling of low temperature partially magnetized plasmas

J.P. Boeuf

LAPLACE, Université de Toulouse, CNRS, INPT, UPS, 118 Route de Narbonne, 31062 Toulouse, France

In applications such as ion sources (Hall thrusters for space propulsion, gridded ion sources, negative ion sources for fusion applications) or plasma processing (e.g. magnetron discharges) the gas pressure is relatively low, typically between 0.1 and 10 mtorr, and plasma confinement by a magnetic field is required. In these plasma sources the external magnetic field is perpendicular to the discharge current and the resulting cross-field drift in the EXB or PXB direction (E is the discharge electric field, P the electron pressure and B the magnetic field) must be closed, i.e. in the azimuthal direction of a cylindrical geometry, to allow efficient confinement.

We call these plasmas “partially magnetized plasmas” because electrons are strongly magnetized (Larmor radius much smaller than the plasma dimensions) while ions are not. For this reason, the properties of these plasmas are specific and different from those of fusion applications, but the presence of the magnetic field can be, as in fusion plasmas, responsible for a variety of instabilities that are difficult to describe quantitatively. Turbulent current across the magnetic field is often due to azimuthal fluctuations. These fluctuations can be triggered by various physical mechanisms including the large EXB electron drift, density gradients, ionization, neutral depletion etc… Unstable fluctuations across different time and length scales as well as self-organized coherent structures are ubiquitous in EXB devices. The origin and interactions of fluctuations transport and structures are not clear but have a dramatic effect on the regimes and performance of these devices.

Fluid models of these plasmas are complex because of the strong anisotropy of the electron conductivity induced by the magnetic field. Particle models although simpler to implement than fluid models, must be used with caution because of the possibility of numerical diffusion which can not only be responsible for significant errors in the quantitative predictions of the device properties but can also lead to a wrong qualitative description of the physics.

In this talk we will focus on the challenges in the use of Particle-In-Cell Monte Carlo Collisions (PIC-MCC) simulations of low temperature partially magnetized plasmas. We will present an ongoing project on benchmarking PIC-MCC simulations in the

context of instabilities and anomalous electron transport in Hall thrusters, the Landmark project1. We will also illustrate on this example that the purpose of modelling is not always to describe a device in all its complexity but that an important aspect of modeling is to help understand a specific (and sometimes essential) feature or physical property of this device. In that case the art of modelling is to simplify the problem in order to address the specific issue and to get rid of the non-relevant complexity (see Fig. 1 and Ref. [2]).

Finally, we will briefly describe some examples of challenging problems in the modeling of low temperature partially magnetized plasmas that remain to be solved.

Figure 1: Axial-azimuthal distributions of the fluctuating ion density (top) and azimuthal electric field (bottom) in a Hall thruster, obtained with a simplified, non self-consistent model of ionization2. The axial distributions of the given radial magnetic field B and ionization source term S, and of the calculated axial electric field Ex are shown on top of the figure.

References

[1] https://www.landmark-plasma.com/[2] J.P. Boeuf and L. Garrigues, Phys. Plasmas 25,

061204 (2018)

68

Simulations oral

3D simulations of magnetron discharges with an energy-conservingimplicit particle-in-cell code

D. Eremin∗

Theoretical Electical Engineering, Ruhr University Bochum, Germany∗Contact e-mail: denis.eremin@rub.de

1. IntroductionMany different low-temperature plasma dis-

charges (such as those in magnetrons or Hallthrusters) resort to use of the magnetic field for en-hanced electron confinement, with the magnetic fieldoriented mainly perpendicular to the direction of thedischarge current flowing from the cathode to the an-ode. The electron confinement allows to sustain suchdischarges through the electron impact ionizationprocesses even at low plasma pressures, whereas ionstypically remain virtually unaffected by the magneticfield. This results in a population of magnetized elec-trons moving with a large E × B drift velocity rela-tively to ions, representing a strong source of free en-ergy which, complemented by other possible sourcesof free energy (such as the gradient of plasma density,temperature and the magnetic field) can be tapped byvarious instabilities, such as the electron-cyclotron-drift instability (ECDI) or the modified two-streaminstability (MTSI). The instabilities are associatedwith the so-called anomalous particle/energy trans-port across the magnetic field, observed in experimentand impossible to explain based on the classical col-lisional transport paradigm. In turn, understanding ofthe anomalous transport is indispensable for the dis-charge optimization and scaling studies.

Whereas the E × B drift velocity in the afore-mentioned discharges is typically oriented in theazimuthal direction and the anomalous transport isthought to result from the δEθ × Br corrections tothe particle velocity, which makes inclusion of theazimuthal direction mandatory in models describingthe anomalous transport. However, any correspond-ing 2D model geometry (r − θ or θ − z) seems tobe missing an important ingredient: for the former itis the ion flow and plasma/magnetic field gradientsin the z direction, and for the latter it is the com-ponent of the unstable mode wavevector parallel tothe magnetic field and plasma sheaths in the case ofHall thrusters. Even if an instability exists in a par-ticular geometry without requiring an additional di-rection (such as the case for the ECDI instability inthe θ−z geometry), the additional direction can havea significant influence on the dispersion relation ofthe mode (through k‖), so that dispersion curve of themode starts to resemble the acoustic branch, which isalso seen in experiment. Although such behaviour ofthe dispersion curve is observed in numerical simula-tions of the ECDI mode in θ− z geometry as well, in

the latter case it is believed to occur due to the elec-tron de-magnetization, which is still a topic of muchdebate. If the influence of finite k‖ in 3D dominatesover all other effects, it would unequivocally clarifythe mechanism of emergence of the ion-sound disper-sion curve. Furthermore, a finite k‖ is also observedin experiment. Considering also the low operationalpressures of the discharges in question, a kinetic self-consistent 3D model is necessary to simulate plasmabehaviour of such discharges, and the particle-in-cell(PIC) method is chosen for this purpose.

2. Energy-conserving implicit 3D PIC codeTo enable simulations of magnetron discharges

exhibiting large plasma densities and thin plasmasheaths, an implicit electrostatic energy- and charge-conserving PIC approach with strongly non-uniformcomputational meshes based on the Crank-Nicolsonorbit and field integrator is adopted from [1]. In orderto apply this technique to bounded plasmas, however,the approach is modified to take into account reactorsurfaces. Moreover, the iterative scheme underlyingthe method is modified to allow exact energy conser-vation after a finite number of iterations even with thecentrifugal force term, which is present in cylindricalgeometry used in the model. The energy conservationallows to remove the finite grid instability prohibit-ing simulations of plasmas with large plasma densi-ties and subcycling in the orbit itegration needed forthe charge conservation ensures improved conserva-tion of momentum [2], which is otherwise problem-atic in energy-conserving PIC schemes.

In order to accelerate the code, it is parallelized onfour NVIDIA V100 GPUs connected via NVLINKinterface. The domain decomposition is made in thedirection of the z coordinate.

3. ResultsThe 3D results indeed demonstrate co-existence

and interplay of the various types of instabilities atdifferent stages of discharge development.

References

[1] L. Chacon, G. Chen, D.C. Barnes, J. Comp.Phys. 233, 1 (2013)

[2] G. Chen, L. Chacon, D.C. Barnes, J. Comp.Phys. 230, 7018 (2011)

69

oral Simulations

Cylindrical (2D) PIC/MCC simulations: Acceleration techniques and accuracy compromise

P. Hartmann1, Z. Donkó1, Z. Juhász2

1Wigner Research Centre for Physics of the Hungarian Academy of Sciences, Budapest, Hungary 2 Department of Electrical Engineering and Information Systems, University of Pannonia, Veszprem, Hungary

1. IntroductionParticle in cell with Monte Carlo collisions (PIC/MCC)

simulations represent one of the main techniques for the numerical investigation of low-temperature gas discharges. In principle the method is capable to describe any complex geometry with any number of collision processes with the background gas for which cross-section data is available. Non-local kinetics and self-consistent field calculation are trivial ingredients, and can be extended to include electro-magnetic effects, Coulomb collisions, plasma chemistry, realistic plasma-wall interactions, etc. [1]. The computa-tional cost, however can pose practical limits and often force the use of further simplifications and approximations. One trivial possibility is to reduce the dimensionality of the simulation by taking advantage of the geometrical symme-tries of the setup. One-dimensional simulations (infinite plane-parallel geometry) has been proven to be invaluable to understand the fundamental processes and can be run on single consumer class computers, but sometimes fail the direct experimental comparison due to geometrical effects. Extension to two dimensions can be in Cartesian or curvi-linear coordinates. In many cases the applications are re-alized in setups with cylindrical symmetry and using cylin-drical coordinates in the simulation is the natural choice.

2. The accuracy compromiseThe principal concept of PIC/MCC simulations are: (i)

the application of super-particles representing a large num-ber of real electrons and ions, and (ii) the use of a numerical grid to which the charge density is allocated, and the self-consistent field calculations are performed. For the time-propagation several integrators are available with different orders and with explicit and implicit schemes. The choice of proper number of grid-points and super-particles, as well as the length of the time-step have to be chosen to fulfill the (mostly phenomenologically derived) stability criteria for electrostatic simulations listed in Table 1.

In most 1D cases 1000 grid points and 105-106 super-par-ticles are sufficient, in 2D, the need for 106 cells and 107 particles is more typical. Further task is the choice and implementation of the field solver, which is trivial and fast in 1D (Thomas algorithm), but is highly non-trivial in 2D, as well as the proper treatment of the discharge walls, as those have significant contribution to the spatial and tem-poral distribution of local plasma properties. The peculi-arity of the cylindrical coordinate system is the singularity at r = 0, which results in the radial dependence of the

statistical noise arising from the discrete particle nature of the simulation, peaking at the center, introducing large ar-tificial fluctuations in the electric field and thus numerical heating.

In the presentation we will discuss possible ways to find the acceptable compromise between accuracy and simula-tion time.

Table 1. Stability criteria

criterium example values (GEC ref. cell)

time-step: Dt < 0.2 wp-1 Dt ≈ 10 ps grid spacing: Dx < lD Dx ≈ 100 µm Courant cond.: vmax Dt/Dx < 1 Dt ≈ 1 ps particles in Debye sphere: ND >> 1 Nparticle ≈ 10 Ncell collision probability per time-step: Pcoll < 0.05

Dt ≈ 100 ps

with lD and wp being the Debye length and plasma fre-quency, respectively. Example values are for an argon discharge driven with 10 Watt of 13.56 MHz RF at 1 Pa pressure).

3. Acceleration techniquesTwo-dimensional PIC/MCC simulations are signifi-

cantly more demanding computationally than 1D cases, making it important to optimize the codes to use all avail-able computing capacity and the use of modern hardware architectures and software tools. If available, large scale CPU clusters are a possible option, but in the last decade general purpose graphical processing units (GPGPUs or GPUs) became accessible for reasonable budget with un-precedented computing power. In the presentation we will discuss possibilities and pitfalls, as well as achievable speedups of different parallelization techniques focusing on massively parallel GPU acceleration.

Examples include a long DC neon discharge and a large area electrode RF argon plasma in external magnetic field.

Acknowledgements We gratefully acknowledge the financial support from

the Hungarian National Research, Development and Inno-vation Office NKFIH through projects K-115805 and K-119357.

References [1] C. K. Birdsall, A. B. Langdon (1985). Plasma Physics

via Computer Simulation. McGraw-Hill.

70

Simulations oral

Re-injection schemes for E ×B Particles in Cells simulations.

A. Tavant(∗)1,2, R. Lucken1, T. Charoy1, A. Bourdon1, P. Chabert1

Weakly magnetized low temperature plasmas can presenthighly magnetized electrons, but non-magnetized ions astheir Larmor radii are larger that the system dimensions.The additional presence of an Electric field E in a directionperpendicular to the magnetic field B will accelerate the ionsin its direction, but the electrons will drift in the E × Bdirection, resulting in a net drift of the electrons with respectto the ions. This is the case in magnetrons, magnetizedplasma columns, or Hall Effects Thrusters.

In theses conditions, strong instabilities can rise, forinstance the Electron Cyclotron Drift Instability, the anti-drift mode, or the modified two stream instabilities. Thekinetic nature of this instabilities forces us to use kinetictools in order to study them. With the progress of computers,Particle in Cells (PIC) simulations have become the tool-of-choice for most of their analysis [1], [2], [3]. However,three-dimensional PIC simulations are still too computation-ally expensive for realistic simulations. Hence, most of thesimulations are still uni- or bi-dimensional.

In Hall Effect Thrusters (HET), the instabilities results inan enhanced electron transport in the direction of the electricfield. When this direction of propagation is present in thesimulation domain, the observed instabilities rise and reacha steady state [4].

On the other hand, in the cases when this direction isnot in the simulation plane, the convection and losses ofthe electrons are absent of the simulation. This results inever-increasing electron energy, which makes impossible toreach a steady state [5] (as shown on figure 1).

An attempt to include this electron convection in the PICsimulations has been proposed in 1D simulation [6] andadapted to 2D simulations [7], [8]. This is done by fixinga length in the direction of the electric field, and followingthe particle movement in this direction. When a particle exitsthis domain, it is removed from the simulation. In order tokeep the number of particles constant, another particle is re-injected at the other side of the domain.

Figure 1 presents the effect of the finite length in thedirection of the electric field on the mean electron energyin the case of a 2D PIC simulation of HET [7]. While thismethod allows to reach a steady state, hence to analyse theinstabilities at a steady state, few comments on the numericalaspect and its impact on the simulation have been made.

1 LPP, CNRS, Ecole polytechnique, Sorbonne Universite, Univ. Paris-Sud,Observatoire de Paris, Universite Paris-Saclay, PSL Research University,91128 Palaiseau, France

2 Safran Aircaft Engines, Vernon, France* antoine.tavant@lpp.polytechnique.fr

0 1 2 3 4Time[ s]

0255075

100125150175200

elec

tron

ener

gy [e

V]

Lz = 1cmLz

Fig. 1. Temporal evolution of the mean electron temperature with andwithout the finite length in the electric field direction (adapted from [9]).

Here, we propose a quantitative analysis of the impactof the re-injection scheme with respect to the cases inwhich no re-injection occurs. A model for its impact onthe simulation is proposed, which allows to estimate it. Adiscussion is proposed to compare the re-injection models,and to evaluate whether they can be used in PIC simulations.

AcknowledgementThis work has been partially funded by the Agence

Nationale de la Recherche under the reference ANR-16-CHIN-0003-01 and Safran Aircraft Engines withinthe project POSEIDON. AT is supported by a CIFREPhD fellowship from Safran Aircraft Engines and theAssociation Nationale de la Recherche et de la Technologie(ANRT). This work was granted access to the HPCresources of CINES under the allocation A0020510092 andA0040510092 made by GENCI, and the HPC resources ofCERFACS.

References[1] D. Sydorenko, I. Kaganovich, Y. Raitses and A. Smolyakov, PRL 103

(2009) 145004[2] J. C. Adam, A. Heron, and G. Laval, Phys. Plasmas, vol. 11, no. 1, pp.

295–305, Jan. 2004.[3] J.-P. Boeuf, J. Appl. Phys., vol. 121, no. 1, p. 011101, Jan. 2017.[4] J. P. Boeuf and L. Garrigues, Plasmas, vol. 25, no. 6, p. 061204, Jun.

2018.[5] A. Heron and J. C. Adam, Phys. Plasmas, vol. 20, no. 8, p. 082313,

2013.[6] T. Lafleur, S. D. Baalrud, and P. Chabert, Phys. Plasmas, vol. 23, no.

5, p. 053502, 2016.[7] V. Croes, T. Lafleur, Z. Bonaventura, A. Bourdon, P. Chabert, Plasma

Sources Sci. Technol. 26 (2017) 034001[8] A. Tavant, V. Croes, R. Lucken, T. Lafleur, A. Bourdon, and P. Chabert,

Plasma Sources Sci. Technol., vol. 27, no. 12, p. 124001, 2018.[9] V. Croes, Ph.D. thesis, Universite Paris-Saclay, 2017.

71

oral Simulations

An asymptotic-preserving well-balanced scheme for the fluid plasma equations in low-temperature plasma applications

A. Alvarez Laguna1,2, T. Magin3, P. Chabert1, M. Massot2, and A. Bourdon1

1Laboratoire de Physique des Plasmas, Ecole polytechnique, 91128 Palaiseau, France 2Centre de Mathématiques Apliquées, Ecole polytechnique, 91128 Palaiseau, France

3von Karman Institute for Fluid Dynamics, B-1640 Sint-Genesius-Rode, Belgium

Although particle-in-cell simulations provide a very ac-curate description of the state of a plasma, they are compu-tationally very expensive and remain unaffordable to study full-domain 3-D geometries. For that reason, hybrid and fluid descriptions are a sound alternative in low-tempera-ture plasma discharges that can represent more efficiently the macroscopic scales. Nevertheless, the fluid models rely on the transport models and the accuracy of numerical dis-cretization schemes in order to provide a meaningful solu-tion.

Most of the fluid models assume charge neutrality and neglect the electron inertia. By doing this, the numerical discretization does not need to resolve the electron plasma frequency and the Debye length, which are in most of the cases much smaller than the macroscopic scales of interest. Nevertheless, these assumptions do not allow for capturing the plasma sheath nor electrostatic waves such as the elec-tron cyclotron drift instability that is present in some 𝐸×𝐵 devices. Additionally, some of the fluid codes neglect the ion temperature as it is much lower than this of electrons.

In this paper, we present a finite volume numerical dis-cretization for the fluid plasma equations. We account for charge separation, the electron inertia, and the finite ion temperature. The numerical method that is proposed here features an asymptotic-preserving property which allows for tackling the quasi-neutral behavior of the plasma with-out resolving the Debye length nor the electron plasma fre-quency. Similarly, the scheme preserves the behavior of the fluids with large mass disparity such as this between ions and electrons, similar to the one proposed by Alvarez La-guna et al. [1]. Additionally, the numerical scheme pro-poses a well-balanced discretization in order to avoid nu-merical instabilities in low-temperature applications. The main advantage of the scheme is that it does not require an implicit solver thanks to the asymptotic preserving prop-erty.

The results show that the proposed numerical discretiza-tion is more stable than a first order standard Riemann solver, such as Roe or HLL. As recently published in [2], in a simple 1D discharge, we find that the standard discreti-zation develops spurious plasma instabilities. These are caused by the error caused by the large mass disparity and

the low temperature of the ions. The proposed first-order asymptotic preserving discretization performs similar to a third-order standard discretization with the same resolution and does not develop spurious instabilities. More im-portantly, the scheme allows for cell sizes that are larger than the Debye length and time-steps that are larger than the plasma frequency without the need of an implicit solver. In this paper, we will also present the advantages of this discretization in terms of computational performance.

This work has been partially funded by the postdoctoral fellowship from Fondation Mathématique Jacques Hada-mard, the Agence Nationale de la Recherche under the ref-erence ANR-16-CHIN-0003-01, , the Jean d’Alembert fel-lowship program from Université Paris-Saclay, and Safran Aircraft Engines within the project POSEIDON.

References [1] A. Alvarez Laguna, N. Ozak, A. Lani, H. Deconinck,

S. Poedts, Comp. Phys. Comm., Vol. 231(2018)[2] A. Alvarez Laguna, T. Magin, P. Chabert, A. Bour-

don, M. Massot, NASA technical memorandum,NASA Ames Research Center (2018)

AP scheme for the fluid plasma equationsAlejandro Alvarez Laguna

alejandro.alvarez-laguna@polytechnique.edu

Plasma instability induced by numerical error Numerical results at t = 75

Figure 10: Converged third-order solution with CFL = 0.9 and 104 mesh points. The simulation captures the physicsas predicted by the theory.

References[1] P. Chabert and N. Braithwhaite. Physics of radio-frequency plasmas. Cambridge University Press, 2011.

[2] Christophe Chalons, Mathieu Girardin, and Samuel Kokh. An all-regime lagrange-projection like scheme for the gas dynamicsequations on unstructured meshes. Communications in Computational Physics, 20(1):188–233, 2016.

[3] Giacomo Dimarco, Raphael Loubere, Victor Michel-Dansac, and Marie-Helene Vignal. Second-order implicit-explicit total variationdiminishing schemes for the euler system in the low mach regime. Journal of Computational Physics, 372:178 – 201, 2018.

16

Fig. 1 Comparison between schemes of a DC discharge simulation.

72

Simulations invited

A comprehensive diagnostic study of a DC positive column in O2: a test-bed for models of plasmas in a diatomic gas

J.P.Booth1, A.Chatterjee1,2, O. Guaitella1, C. Drag1, N. De Oliviera2, L. Nahon2,

D. Lopaev3 S. Zyryanov3 D. Voloshin3 and T. Rakhimova3

1 Laboratoire de Physique des Plasmas, CNRS, Ecole Polytechnique, Université Paris-Sud, Université Paris-Saclay, Sorbonne Universités, F-91128 Palaiseau, France

2 Synchrotron SOLEIL, Gif Sur Yvette, France, 3Moscow State University

1. Introduction Despite many decades of study, models of discharges in

molecular gases still lack accurate data on many key col-lisional processes, both in the gas phase and on surfaces, necessary for reliable predictive modeling. Good data is lacking for electron-impact dissociation, surface recom-bination, reactions with metastables, gas heating mecha-nisms, energy transfer, and surface thermal accommoda-tion. This is true even for such “simple” and ubiquitous gases as O2 and N2. Diagnostic techniques have advanced significantly, with an emphasis on accurate absolute den-sity measurements, space and time resolution, and energy distribution functions (translational, rotational and vibra-tional). However, most studies focus on measuring only a few of the modeled parameters for a given system, and often in plasma configurations optimized for applications rather than for model/experiment comparison. Particularly problematic is the presence of large gradients (in temper-ature, density, composition) as well as poorly-controlled surface conditions. Our approach therefore is to use a well-characterized, stable and uniform discharge chosen to be simple to model, namely a DC positive column in pure O2, and to perform a comprehensive set of measure-ments of all accessible parameters using multiple (and sometimes overlapping) diagnostics. The mod-el/experiment comparison is therefore highly constrained, allowing the origin of discrepancies to be probed in un-precedented detail.

2. Experiment

The (10-40 mA) discharge is ignited in pure O2 (0.3-10 Torr) flowing in a borosilicate glass tube (id 20mm, length 56cm) between cylindrical electrodes located in side tubes. The surface temperature is controlled by thermostated fluid flowing in an outer envelope (5-50C). The longitudinal electric field is determined from the space potential measured by two high-impedance floating probes immersed in the discharge. Optical measurements can be made along the tube axis with appropriate win-dows on the tube ends, or transversally through the glass envelope. A range of diagnostic techniques are employed.

Vacuum Ultraviolet Absorption Spectroscopy was per-formed at the DESIRS VUV beamline at Synchrotron Soleil, using the Fourier-Transform Spectrometer to ob-

tain high resolution (106) spectra of O2 in the X, a and b states, and the monochromatic branch for time-resolved kinetic measurements. The O2 a and b kinetics were also followed by optical emission.

The absolute O atom density was determined by optical emission actinometry, by Two-Photon LIF, and finally by cavity ringdown spectroscopy of the forbidden 1D3P transition at 630nm, giving somewhat different results. This variability was traced in part to changes in the sur-face recombination probability, especially at low pressure, as well as to particular issues with some measurement techniques. CRDS also allows the O- negative ion density to be determined from the photodetachment continuum absorption. The oxygen atom loss rate is determined by time-resolved actinometry, allowing the surface recombi-nation probability, . The gas temperature was determined from the O2 b emission spectra (Trot), and from the Dop-pler width of O atom transitions; it reaches 700K at the highest temperature and pressure. 3. Discussion

We will present measurements of the absolute densities and kinetics of all key species in an O2 plasma. The re-duced electric field is deduced from the space potential, the gas temperature and pressure. Afterglow measure-ments in 100% modulated discharges allow the surface reactions to be probed separately, whereas partial modula-tion experiments probe the loss rates during plasma (which including electron impact processes). Whereas the O2 a density depends only on electron impact processes and surface quenching, O2 b shows an additional fast gas-phase loss channel that correlates well with the O 3P density and the gas temperature, indicating the presence of a previously unknown thermally-activated reaction.

The atom surface recombination probability (at pres-sures above 1 Torr) was well correlated with the gas tem-perature, indicating an Eley-Rideal mechanism with an activation energy provided by the incident atoms. At low-er pressures the surface reactivity is strongly enhanced, due to energetic ion bombardment.

The measurements of the absolute oxygen atom density, combined with measurements of the loss frequency, allow the electron impact dissociation rate constant to be deter-mined as a function of the reduced electric field.

73

oral Simulations

Kinetic interpretation of a magnetically enhanced hollow cathode arcdischarge

Liang Xu1, Jens-Peter Heinß2, Ilija Stefanovic3, Denis Eremin1, Peter Awakowicz3 and RalfPeter Brinkmann1

1Institute for Theoretical Electrical Engineering, Ruhr-University, Bochum, Germany2Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology, Dresden, Germany

3Institute for Electrical Engineering and Plasma Technology, Ruhr-University, Bochum, Germany∗Contact e-mail: liang.xu@rub.de

1. IntroductionHollow cathode arc discharges with axially

directed magnetic field attract continuous interestsin industrial applications [1]. By far, there is stillno experimental and theoretical descriptions of theinterior plasma source of the magnetically enhancedhollow cathode arc discharges (ME-HCAD). Theunderstanding of the interior plasma is crucialbecause it controls the characteristics of the plasmaplume. A self-consistent kinetic model is required innature to have insight in the internal plasma column.2. Particle-in-cell/Monte Carlo collision model

The fully kinetic self-consistent model forinternal plasma column in hollow cathode arcs withcrossed electric and magnetic field is developed by1D3V Particle-in-cell/Monte Carlo collision simula-tions [2]. Argon discharges at typical gas pressure10Pa, discharge voltage 100V and magnetic field600G have been simulated to study electron and iondynamics and plasma transport in the radial direction.Anisotropic scattering is applied for all collisions toreproduce the practical discharges.3. Fast electrons kinetics

Our simulations identified three regions in plasmatransport from the cathode to the anode in stronglynonuniform ME-HCAD: ionization region, diffusionregion, and drift region. The regions are labeled inFig. 1.

0 20 40 60 80 100 120 0 1

2 3

4 10

100

1000

10000

EE

PF, a

.u.

U=100V, n=3e21m -3,B=600G

ionizationdiffusion

drift

energy, eV

r, mm

EE

PF, a

.u.

Fig. 1: Spatially resolved electron energy probabilityfunctions (EEPFs) in the radial direction from the middle

of the hollow cathode (r = 0mm) to the cathode(r = 4mm).

As shown in Fig. 1, in the ionization region, fastelectrons result in the bump-on-tail EEPFs while thelow energy region is populated by trapped electrons.Then EEPF successively evolves to a two temperaturedistribution in the diffusion region and a Druyvesteyntype in the drift region.

Fast electron recapture by the cathode canexplain the high voltage and extremely highplasma density in low pressures. Fig. 2shows the effective cathode-emitted electron fluxdecrease with gas density decreasing, which isvalidated by preliminary experiments [1].

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 5 10 15 20 25 30 35 40 45

Γem

it,e

ff/Γ

emit

n,1021 m-3

U=100V,B=600G,Γemit=1.48e23m-2s-1

Fig. 2: Effective cathode-emitted electron flux as afunction of neutral gas density

4. SummaryIn the steady state of ME-HCAD, both

electron and ion energy probability functions stronglydeviate from Maxwellian form. Three regimesin the nonuniform plasma are identified from thecathode to the middle of the hollow cathode:ionization region, diffusion region and drift region.Further, it is found that fast electrons recapture bythe cathode in the magnetic field explains the highdischarge voltage and extremely high plasma densityin the experiments.

References[1] F. Fietzke et. al, Plasma Process. Polym. 6, S242

(2009)

[2] L. Xu et. al., Phys. Plasmas 24, 093511 (2017)

74

Simulations invited

Boltzmann-Chemistry global models: status and future challenges

L.L. Alves and A. Tejero-del-Caz

Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

Modeling and simulation (M&S) activities in low-tem-

perature plasmas (LPT) obtained from gas discharges can

be challenging due to the nature of these media, composed

by charged particles (electrons and ions) and by neutral

species in different excited states, intrinsically in non-equi-

librium as the result of collisional, radiative and electro-

magnetic interactions.

When developing plasma-driven applications (e.g. mate-

rial processing, plasma medicine, environmental control,

energy storage, etc), the focus is usually on the plasma-en-

hanced production of reactive species, and the analysis of

the corresponding chemical reaction pathways for defining

a reaction mechanism, a subject often referred as "plasma

chemistry". In this case, global models are the most logical

choice for the M&S of gas/plasma systems, since they al-

low describing the detailed plasma chemistry in complex

gas mixtures, with little computational effort.

Essentially, global models solve the rate balance equa-

tions of the various gas/plasma k-species [1]

����� = ∑ ���( ) − �(�)��� ∏ ��

���(�)� �� − ����� � (1)

where nk is the density of the k-species; akj(1) and akj

(2) are

the stoichiometric coefficients of the k-species, as they ap-

pear on the left- and right-hand sides of reaction j, respec-

tively; Dk and Λk are the corresponding diffusion coeffi-

cient and diffusion length, respectively, eventually ob-

tained by considering multicomponent transport and ad-

dressing also the reactivity at the walls; and kj is the rate

coefficient of the j-reaction. In the case of electron-induced

mechanisms, the latter writes

�� = � ��

�/ " #$�(#)%(#)&#'( (2)

where me and u are the electron mass and kinetic-energy,

respectively, σj(u) is the cross section of the j-reaction and

f(u) is the electron energy distribution function (EEDF).

Under non-equilibrium conditions, typical of LTPs, the

EEDF should be calculated with a Boltzmann solver, often

integrated in the global model. Usually, the closure of the

model corresponds to the self-consistent calculation of the

power required to sustain the plasma, or any related quan-

tity such as the reduced electric-field. The gas temperature

can also be calculated by solving the power balance equa-

tion for the heavy-species.

The LTP community benefits from several implementa-

tions of global models, such as ZDPlaskin [2], GlobalKin

[3,4], for which a commercial application with a GUI was

developed [5], and the tool within PLASIMO [6]. Recently,

the N-PRiME group with IPFN has also implemented a

global model, using flexible and upgradable object-ori-

ented programming under MATLAB®. The LisbOn KInet-

ics (LoKI) simulation tool [7,8] embeds a Chemistry solver

and a Boltzmann solver, the latter to be released soon as

open source. The development of this platform was also

used as an opportunity to critically review and update sev-

eral reaction mechanisms, namely in rare gases (Ar, He)

and in N2-O2 mixtures [9].

Despite the investment of the community in developing

such models and tools, with considerable predictive perfor-

mance, there are still several open issues that require fur-

ther attention and may pose some challenges. Examples of

those issues are: revisiting the transport models for the neu-

tral and the charged species, considering ambipolar effects

according to the working pressure, and taking multicompo-

nent diffusion (including wall reactions) for the heavy-spe-

cies; bringing global models into hydrodynamic codes; up-

dating the description of radiation; the critical evaluation

of data; strategies for the Boltzmann-Chemistry coupling,

namely in view of self-consistent time-dependent calcula-

tions. On this last issue, a recent analysis for the simple

case of the evolution of the electron kinetics, when excited

by a µs-duration electric-field pulse, shows that results de-

pend on the implementation adopted when solving the elec-

tron Boltzmann equation [10].

References

[1] L.L. Alves et al, Plasma Sources Sci. Technol. 27,

023002 (2018).

[2] S. Pancheshnyi et al, http://zdplaskin.laplace.univ-

tlse.fr (2008).

[3] R. Dorai and M.J. Kushner, J. Phys. D 36, 1075 (2003).

[4] D.S. Stafford and M.J. Kushner, J. Appl. Phys. 96, 2451

(2004).

[5] J.J. Munro and J. Tennyson, J. Vac Sci. Technol. A 26,

865 (2008)

[6] J. van Dijk et al, J. Phys. D 42, 194012 (2009).

[7] A. Tejero-del-Caz et al, Plasma Sources Sci. Technol.,

submitted (2018).

[8] P. Coche et al, J. Phys. D 49, 235207 (2016).

[9] V. Guerra et al, Plasma Sources Sci. Technol., submit-

ted (2018).

[10] A Tejero-del-Caz et al, in FLTP-Simulations (2019).

Acknowledgments

This work was funded by Portuguese FCT, under projects

UID/FIS/50010/2013 and PTDC/FISPLA/1243/2014

(KIT-PLASMEBA).

75

oral Simulations

Modeling for NOx production by gliding arc plasma technology

F. Jardali1, A. Bogaerts1, M. B. Jensen2, R. Ingels2

1Department of Chemistry, Research group PLASMANT, University of Antwerp, Universiteitsplein 1, BE-2610 Wilrijk-Antwerp, Belgium

2N2 Applied, Dronning Eufemias Gate 20, 0191 Oslo, Norway

One of the basic elements responsible for the growth of

living beings on earth is nitrogen. It is a primary compo-nent of amino acids, proteins, nucleic acid and chlorophyll in plants [1]. 78.08% of the earth’s atmosphere constitutes of molecular nitrogen, however atmospheric nitrogen is relatively non-reactive and it remains extremely difficult to break its stable triple bond. Nitrogen fixation, a process by which N2 molecules are converted into simple nitrogen compounds, such as ammonia or nitric oxide (NO), is thus essential to life. For instance, it is crucial for agriculture and the manufacture of crop fertilizer. In order to fulfill the demand of growing population and to continuously feed the planet, the Haber-Bosch process was developed at the beginning of the 20th century to fix atmospheric nitrogen and produce ammonia at high temperature and pressure [2]. However, this industrial ammonia synthesis process con-sumes almost 2% of the world’s total energy production, emits 300 million metric tons of CO2 and utilizes 3-5% of the total natural gas output [3].

Consequently, many efforts have been put into develop-ing and integrating more sustainable nitrogen fixation pro-cesses. Plasma technology has shown great potential in this area, especially because plasma reactors can be operated using renewable energy sources and are generally non-pol-luting. Recent studies have shown that a classical gliding arc plasma reactor provides an energy efficient way for NOx production owing to the vibrational excitation of N2 that contributes significantly to the activation of the mole-cules [4].

In the present study, we investigate the performance of a

reverse vortex flow gliding arc plasma reactor, also called gliding arc plasmatron (GAP), for NOx formation, by means of computer modeling. To this end, a zero-dimen-sional (0D) chemical kinetics model is developed to de-scribe a detailed N2/O2 plasma chemistry in a GAP operat-ing at atmospheric pressure. Special emphasis is given to examine the role of the vibrational kinetics in the model. The conversion, NOx yield, energy cost and energy effi-ciency are calculated for different operating conditions and gas feed ratios and are compared to experiments. A kinetic analysis is performed to investigate the dominant reaction pathways for NOx synthesis. Based on our findings, we propose solutions for further improvements to the N2 fixa-tion process in a GAP.

References [1] B.S. Patil, Q. Wang, V. Hessel, J. Lang, Plasma N2-

Fixation: 1900-2014, Catalysis Today, 256, 49-66 (2015).

[2] V. Hessel et al., Innovation management in the Belle Epoque – How plasma went commercial in 1903, Chemistry Today, 35, 72-75 (2016).

[3] R. R. Schrock, Reduction of dinitrogen. Proc. Natl. Acad. Sci. USA., 103, 17087 (2006).

[4] W. Z. Wang et al., Nitrogen fixation by gliding arc plasma: better insight by chemical kinetics modelling. ChemSusChem, 10, 2145–2157 (2017).

76

Simulations oral

Zero-dimensional volume-averaged modelling formalism:implementations from intermediate- to atmospheric-pressure

E. H. Kemaneci, Y. He

Theoretical Electrical Engineering, Ruhr-University, Bochum, Germany

1. IntroductionPlasma devices used for the purpose of material

processing, such as low-pressure inductive discharges,intermediate-pressure microwave induced surface wavedischarges and atmospheric-pressure plasma jets are oftenassociated with a rich chemical variety. Effective modelsare required to properly describe the regarding chemicalkinetics reducing the resulting computational load to aneligible degree. A commonly used effective model is thezero-dimensional volume-averaged formalism that focuseson the composition space with a cost of spatial resolution.However, existing volume-averaged models often considerlow-pressure inductive or capacitive discharges. Their im-plementations to discharges operating at larger pressureregimes are of interest with a validation by benchmarkagainst measurements.2. Volume-averaged zero-dimensional model

The model contains volume-averaged particle balanceequations of each species denoted by i and electron energybalance equation with an assumption of effective electrontemperature Te

dNi

dt= Si|V + Si|W , (1)

d

dt

(3

2NeTe

)= Q|V + Q|W , (2)

where Ni is the volume-averaged particle density and Si,Q are the net local source terms. The subscript “V ”denotes the homogeneous chemical reactions inside theplasma volume and “W ” losses at the wall due to hetero-geneous reactions at the surface and ion wall flux.

Ion wall flux is conventionally estimated via edge-to-center ratios at Bohm point. However, these estimationsdo not respect the structure of the electronegativity differ-ing at large pressure regimes [1]. The edge-to-center ratiosare analytically derived at such a pressure regime for sur-face wave discharges [2]. These edge-to-center ratios arere-calculated in the Cartesian coordinates to define theirvalue for atmospheric-pressure plasma jets.3. Benchmark against measurements

A variety of surface wave discharges in coaxial andcylindrical structures are simulated either in argon or oxy-gen and an agreement is obtained with the simulations inthe pressure range of 15 − 2000 Pa [2]. The model is alsoimplemented for a feeding gas mixture of oxygen and hex-amethyldisiloxane in a coaxial structure [3]. The simula-tions show an agreement with various types of measure-ments, such as, electron temperature and densities of elec-tron, hexamethyldisiloxane, carbon monoxide, methane,methyl and ethylene. Comparison of the simulation re-sults and measurements for methane, ethylene and methyldensities are given in Fig. 1.

1018

1019

1020

1021

0.5 1.0 1.5 2.0

CH4

C2H4

CH3

Density (

m-3

)

Power (kW)

Fig. 1: Comparison of simulation results (lines) andmeasurements (points) [3] of methane, ethylene andmethyl radical densities in a coaxial surface wave

discharge for a variation of input power.

An atmospheric-pressure plasma jet device is also sim-ulated for a feeding gas of helium and an agreement is ob-tained in electron density as shown in Fig. 2.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Power(W)

10 16

10 17

10 18

ne(m

-3)

He Atom. pres. plasma-Jet

Meas.

Sim.

Fig. 2: The simulation results (line) and measurements ofelectron density (squares) [4] for a variation of absorbed

power in an atmospheric-pressure plasma jet.

4. OutlookAtmospheric-pressure plasma jet devices are to be sim-

ulated for feeding gas mixtures of helium/oxygen andhelium/carbon dioxide with an addition of a Boltzmannsolver to self-consistently calculate the electron energydistribution function. The simulation results are to be com-pared with available measurements.

References[1] C.M. Ferreira et al, J. Phys. D 21, 1403 (1988)

[2] E.H. Kemaneci et al, J. Phys. D 50, 245203 (2017)

[3] A.S.C Nave et al, J. Phys. D 49, 305206 (2016)

[4] J. Golda, PhD Ths., Ruhr-Uni-Bochum, (2018)

77

poster Simulations

The collisionally modified Bohm criterion:Insight or artifact?

R.P. Brinkmann∗

Theoretical Electrical Engineering, Ruhr-University, Bochum, Germany∗Contact e-mail: ralf-peter.brinkmann@tet.rub.de

1. IntroductionAt low collisionality λ�λD (λD: Debye length,

λ: mean free path), the plasma boundary sheath canbe characterized by the famous Bohm criterion [1]:Ions enter the sheath with a directed velocity vi equalto or larger than the Bohm velocity vB =

√Te/mi

(Te: electron temperature,mi: ion mass). Attempts togeneralize the criterion to λ . λD have met criticism.(Franklin even titled a publication “There is no suchthing as a collisionally modified Bohm criterion” [2].)Arguing the opposite, the current author proposed todeduce such a criterion from the presence of a remov-able inner singularity in a certain sheath model [3].Physical insight or mathematical artifact?

2. Algebraic electron-field modelThe smooth step model SSM – a refinement of the

hard step model – gives the electric field in a sheath-plasma transition: Space charge field in the sheath,ambipolar field in the plasma, smooth interpolationin the transition zone in between:

E=−eλDniε0

ΞS

(q

eλDni

)− Te

∂ni∂q

ΞA

(q

eλDni

). (1)

Here, q = e∫ni dx is a transformed spatial coordinate

and λD =√ε0Te/e2ni is the local Debye length [4].

ΞS and ΞA can be understood as “switch functions”.Note that ΞS is insignificant for arguments larger 2,and ΞA for arguments smaller −2.

-2 0 2 4ξ

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 1: Switch functions ΞS (blue) and ΞA (yellow).

3. Ion modelThe ions are described by the continuity equation

(constant flux Ψi) and an equation of motion withinertia, acceleration, and collisional friction:

nivi = −Ψi, (2)

mivi∂vi∂x

= eE − |vi|λmivi. (3)

4. Collisionally modified Bohm criterionTransforming to q-coordinates, and invoking (1)

with ni = Ψi/|vi| yields a differential equation:

L(q, vi)∂vi∂q

= R(λ, q, vi)|vi|eλDni

. (4)

The functions L and R are suggestively written as:

L(q, vi) = v2i − v2B ΞA

(q

eλDni

), (5)

R(λ, q, vi) =λDλv2i − v2B ΞS

(q

eλDni

). (6)

Eq. (4) has an inner singularity (critical point, CP)at (q∗, v∗i ), the intersection of L = 0 and R = 0.Its properties were grounds to call it a Bohm point.

-6 -4 -2 2 4 6q

1

2

3

4

5

Fig. 2: Topologies of (4) for different λ, with curvesL = 0 (blue) and R = 0 (green), and the critical points.

5. Insight or artifact?Eq. (4) gives physically sensible sheath solutions,

and the Bohm point is mathematically well defined.For moderate λD/λ, it is also physically meaningful:q∗ is in the transition zone, ΞS and ΞA are relevant,L = 0 ∧ R = 0 fix the CP. In contrast: For λD � λ,q∗ lies in the ambipolar zone; ΞS loses significance.ΞA becomes unity and L = 0 implies |vi| = vB.Likewise, for λ � λD, q∗ lies in the unipolar region,and ΞA loses significance. ΞS remains significant,R = 0 implies drift behavior, but no velocity is fixed.In summary, the proposed criterion is not an artifact,but its physical interpretation requires some care.

References[1] D. Bohm, in Guthrie/Wakerling (NY, 1949)[2] R.N. Franklin, J. Phys. D 36, 2821 (2003)[3] R.P. Brinkmann, J. Phys. D 44, 042002 (2011)[4] R.P. Brinkmann, PSST 24 064002 (2015)

78

Simulations poster

Kinetic modeling of the electric double layer at the plasma-wall interface

K. Rasek, F. X. Bronold, H. Fehske

Institut für Physik, Universität Greifswald, 17489 Greifswald

A solid in contact with a plasma forms an electric doublelayer, with a positive space charge in the plasma, theplasma sheath, and a negative space charge in the solid.This charge separation arises because more electrons areabsorbed by the wall than ions neutralized by impact. Tounderstand the physics of the double layer, a completekinetic model of charge transport at a solid-plasmainterface is necessary. We develop such a model for aplasma in contact with a dielectric wall based on thePoisson equation for the electric potential and two sets ofBoltzmann equations, one for the electrons and ions in theplasma and one for the conduction band electrons andvalence band holes in the wall. By solving the Boltzmannand Poisson equations we are able to determine the fulldistribution functions of all carriers both inside the walland the plasma, and with them quantities like the chargeprofile or photon emission rate in case the recombinationof charge carriers occurs radiatively inside the solid.

We extend our previous work concerning an idealized,perfectly absorbing, collisionless interface [1] to a morerealistic case where collisions are included in thedielectric wall, while the plasma is still consideredcollisionless. The collisions include both relaxation and(radiative or non-radiative) recombination of conductionband electrons and valence band holes, whereby thetheory becomes fully selfconsistent without relying on anad-hoc condition as in Ref. [1]. Electrons are insertedfrom the plasma at the interface according to the quantummechanical matching conditions, while holes aregenerated by impact neutralization of ions impinging onthe interface, calculated by a semiempirical model. In aquasistationary state, these additional carriers arebalanced by recombination processes of valence bandholes and conduction band electrons. To solve theBoltzmann equations an iterative Grinberg-Luryi-schemeis employed [2], while the Poisson equation can beintegrated once and included into the Boltzmann equationby a coordinate transformation from the position to thevalue of the potential. The merging of the double layerwith the quasi-neutral regions in the dielectric wall andthe plasma is accounted for by selfconsistently imposinginflection points far away from the interface [3]. Theregions beyond the inflection points have no directphysical meaning, but they allow us to implement themerging with the unknown (non-Maxwellian) distributionfunctions of the quasi-neutral regions on both sides of theinterface.

From the inflection point conditions, flux balance at thesource, the potential matching conditions and therecombination balance we are able to derive a set ofequations that fix the free parameters of the system, which

are the two positions of the inflection points, the positionsof the source and the reservoir, and the carrier densities inthe source and the reservoir. We can thus, using onlymaterial parameters such as the dielectric function or theband gap of the wall material, determine the potentialcurve, qualitatively sketched in Fig.1, as well as carrierdistribution functions which are responsible for it.

The emerging picture of our model is thus a (floating)dielectric surface where the potential profile across thedouble layer is the result of a selforganization processbalancing electron-ion generation in the plasma andelectron-hole recombination/relaxation in the solid.

Besides discussing numerical aspects of our kinetictheory for the electric double layer we present results for adielectric wall where charge carriers scatter off opticalphonons and recombine radiatively.

References[1] F. X. Bronold, H. Fehske, J. Phys. D: Appl. Phys. 50

(2017) 294003.[2] A. A. Grinberg, S. Luryi, Solid-St. Electron. 35

(1992) 1299.[3] L. A. Schwager, C. K. Birdsall, Phys. Fluids B 2

(1990) 1057.

Fig.1: Interface model for an electric double layer withnegative space charge inside the solid and positive spacecharge in front of it. Shown are the edges of theconduction (U✻) and valence bands (Uvb), the edge forthe motion of valence band holes (Uh), the potentialenergies for electrons and ions on the plasma side, andthe energetic range, specified by the ion’s ionizationenergy I and its broadening Γ in which hole injectionoccurs due to the neutralization of ions at the interface.Source, reservoir and quasi-neutral regions are indicatedas they will arise in the course of the calculation.

79

poster Simulations

Chemical kinetic modeling of Transient Spark discharge

M. Janda1

1Division of environmental physics, Faculty of mathematics, physics and informatics, Comenius

University in Bratislava, Mlynská dolina F2, 842 48 Bratislava, Slovakia

Fig. 1 Comparison of measured and calculated electron

density during transient spark discharge current pulse.

1. Introduction

The chemical kinetic modeling of the density evolution

of species included in the model is commonly used in

plasma chemistry. The kinetic modeling could also help

us to improve our understanding of the chemistry induced

by the Transient Spark (TS) discharge [1].

The TS is a dc-operated self-pulsing (~1-10 kHz) dis-

charge initiated by a streamer. Thanks to the short (~10-

100 ns) high current (~1-20 A) spark pulses, the TS gen-

erates strongly ionized non-equilibrium plasma.

2. Model description

The model is based on the existing ZDPlasKin module

[2] and set of reactions in N2-O2 mixtures provided by

ZDPlasKin authors (version 1.03). The ZDplasKin pack-

age includes a Bolsig+ solver for the numerical solution

of the Boltzmann equation. The electron scattering cross

sections were taken from the LXCat project database [3].

The additional module compatible with ZDPlasKin was

created, taking into account fast changes of conditions

(reduced electric field strength, gas temperature, ...) dur-

ing the evolution of TS discharge, starting from the pri-

mary streamer till the end of the spark phase.

3. Results and Conclusions

The comparison of calculated electron density (Fig. 1)

with experimental data [4] indicates that created model is

suitable for further study of the TS. The model uses

Maxwellian electron energy distribution functions defined

by the electron temperature Te during the spark phase

characterized by high degree of ionization. The evolution

of Te during the spark phase is calculated by energy bal-

ance equation. Further development of our model is

needed to include the post-spark relaxation phase.

Acknowledgement: Supported by Slovak Research and

Development Agency APVV-17-0382.

References

[1] M. Janda, V. Martišovitš and Z. Machala, Plasma

Sources Sci. Technol. 20, 035015 (2011)

[2] S. Pancheshnyi et al. (2008), Computer Code ZD-

PlasKin, University Toulouse, Toulouse, France,

https://www.zdplaskin.laplace.univ-tlse.fr

[3] http://www.lxcat.net

[4] M. Janda et al. Plasma Sources Sci. Technol. 23,

80

Simulations poster

Self-organized pattern formation and kinetic simulation in dielectric barrier discharge

Weili Fan, Lifang Dong

College of Physics Science and Technology, Hebei University, Baoding 071002, China

From zebra stripes to a honeycomb lattice, nature fea-

tures breathtaking patterns. These mysterious shapes cause for wonder and fascination throughout human his-tory. As a new kind of pattern formation system, dielectric barrier discharge (DBD) has attracted increasing attention in recent years[1-2]. It is capable of producing the most varieties of patterns with simple experimental setup. By using the special designed DBD system with two water electrodes, we have obtained more than 40 kinds of pat-terns through nonlinear self-organization of the filaments, as shown in Fig. 1. Generally, these plasma patterns exhi-bit high spatial-temporal symmetries at the macroscopic level, while are characterized by complex dynamics and interactions from the microscopic view. In this paper, the spatio-temporal dynamics of the patterns, the plasma di-agnostics by light emission spectrum as well as the appli-cations of the patterns as a tunable plasma photonic crys-tal are studied.

To get a further understanding of the physical mechan-ism of pattern formation in DBD, two-dimensional par-ticle-in-cell simulations with Monte Carlo collisions in-cluded (PIC-MCC) have been performed. The formation of multiple filaments and the involved electric fields, electric potentials, plasma densities, and particle temper-atures are studied. The transition of the electron energy probability function (EEPF) from a bi-Maxwellian to a Maxwellian distribution is observed as the discharge proceeds. The evolution of two successive filamentary discharges is studied, as shown in Fig. 2, which displays strong spatio-temporal memory effect. The interplay be-tween the external field, the surface charge field, and the space charge field are studied. It is shown that the space charges always exist in the time interval between two successive discharges, which serve as the seed charges to initiate the second discharge. For the discharges in the full developing stage, the contributions from Ex-space and Ex-external are predominant, while for the discharges in the decaying stage or the initial igniting stage, the contribu-tion from Ex-surface is primary. The discharge occurring at the zero-crossing point of the applied voltage is also ob-served, which results from a joint influence of both the space charges and surface charges accumulated during the preceding discharge. The simulation results explain the dynamical behaviors of the DBD filaments observed in experiments.

In experiment, the spatio-temporally resolved mea-surements of two successive filamentary discharges have been performed in a DBD system with two water elec-

trodes. Experimental observations and numerical simula-tions are in good agreement.

References [1] J. Y. Feng, L. F. Dong, L. Y. Wei, W. L. Fan, C. X. Li,

Y. Y. Pan, Phys. Plasmas 23, 093502 (2016). [2] L. Y. Wei, L. F. Dong, J. J. Feng, W. B. Liu, W. L. Fan,

Y. Y. Pan, J. Phys. D: Appl. Phys.49, 185203 (2016). [3]W. L. Fan, Z. M. Sheng, X. X. Zhong, W. M. Wang, Y.

T. Li and J. Zhang, Appl. Phys. Lett., 102, 094103 (2013)

[4] W. L. Fan, Z. M. Sheng, L. F. Dong, F. C. Liu, X. X. Zhong, Y. Q. Cui, F. Hao, T. Du, Scientific Reports 7, 8368 (2017)

(a) (b) (c) (d)

Fig.1 A rich variety of patterns observed in DBD with two water electrodes

Fig. 2 Evolution of two successive filamentary discharges in DBD. The top panels (a1)-(d1) display the electron den-sity distributions around the first discharge, while the bottom panels (a2)-(d2) correspond to the second dis-charge.

81

poster Simulations

Fluid modelling of a capacitively-coupled radio-frequency discharge in hydrogen excited by tailored voltage waveforms

J.M. Orlac’h1, T. Zhang2, T. Novikova2, V. Giovangigli3, E. Johnson2, P. Roca i Cabarrocas2

1Laboratoire EM2C, CNRS, Ecole CentraleSupelec, Université Paris-Saclay, France2Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), CNRS, Ecole Polytechnique, Université Pa-

ris-Saclay, France 3Centre de Mathématiques Appliquées (CMAP), CNRS, Ecole Polytechnique, Université Paris-Saclay, France

1. IntroductionTailored voltage waveforms in capacitively-coupled ra-

dio-frequency (RF-CCP) discharges are now seen as an in-teresting tool for the optimization of plasma enhanced chemical vapor deposition (PECVD) process, e.g. for pho-tovoltaic applications [1] [2].

In this work, we use a fluid model to simulate a hydrogen rf discharge excited by temporally asymmetric waveforms and compare our results to previous results from a hybrid PIC-MCC model and from experimental measurements [1].

2. ResultsWe have studied in particular the evolution of the DC

bias as a function of the waveform parameters, e.g. number of harmonics or phase shift. It is found that our fluid model is able to reproduce the values of the DC bias with a com-parable accuracy as a hybrid model [3].

Classical expressions for ion mobilities available in the literature are compared. However, none of them is per-fectly consistent with the experimental and hybrid model results, as can be seen in Fig. 1 for sawtooth waveforms.

The sensitivity of our results to charged species transport coefficients and ionization rate is also investigated. It is found that electron transport coefficients have a negligible influence on the value of the DC bias, while the latter is highly sensitive to ion mobilities and ionization rate, which are shown to be the main determinants of the DC bias. The

values of the DC bias for three different ion mobility pa-rameters are shown in Fig. 2. One can see that for a proper value of ion mobility, the experimentally measured DC bias can be reproduced with a comparable accuracy as with a hybrid model.

Our results show that fluid models can compete with hy-brid models, provided accurate transport coefficients and reaction rates are used.

References [1] B. Bruneau et al. Plasma Sources Sci. Technol., 25,

045019 (2016)[2] Kim et al., Nature Scientific Reports, 7, 40553 (2017)[3] J.-M. Orlac’h, T. Novikova, V. Giovangigli, E. John-

son, P. Roca i Cabarrocas, submitted to PlasmaSources Sci. Technol.

Fig. 2. Variation of the DC bias as a function of the phase shift for sawtooth waveforms. Sensitivity to ion mobility coefficient.

Fig. 1. Variation of the DC bias as a function of the num-ber of harmonics for sawtooth waveforms. Comparison of three different mobility models.

82

Simulations poster

Theoretical Backgrounds of the Apokamp-type Atmospheric Plasma Jet in the Electro-negative Gas Medium

V. Kozhevnikov1, A. Kozyrev1, A. Kokovin1, A. Sitnikov1

1Institute of High Current Electronics, Russian Academy of Sciences, Tomsk, Russia

1. Introduction

In 2016 the group of experimentalists led by Eduard Sos-nin in the Institute of High Current Electronics has been discovered a new phenomenon in gas discharge physics: an extended plasma jet developing perpendicular to the bend-ing point of the arc discharge channel between two elec-trodes [1], [2]. This phenomenon occurs if the discharge ignites between two electrodes, one - is under the high pulse-periodic potential, and the other has floating poten-tial, i.e. connected via a capacitor to a ground. The dis-charge has been entitled as “apokamp” (from Greek απó - “off” and καμπη - “bend”). As it was found, a single needle or a conical jet of 6–7 cm length being attached to the bend-ing point of the current channel (Fig. 1) represents an apo-kamp.

This unusual new type of gas discharge is observed at high (atmospheric) and medium pressures in gas mixtures containing a small portion of an electronegative gas, e.g. oxygen or chlorine. In inert gases, this phenomenon is not observed. It should be noted that depending on the param-eters of voltage pulses, the apokamp can be also repre-sented by several plasma jets developing perpendicular to the current channel from the points of its bending [3].

2. Theoretical model

We use the deterministic model ("two-moment model") of a multicomponent discharge plasma to describe self-sus-

tained periodic discharge in pure oxygen both in the inte-relectrode gap and in the surrounding space above the elec-trodes. To simplify the consideration a 2D-model is used instead of 3D, so the discharge between two plane elec-trodes with similar physical conditions as in the experiment was considered. The high-voltage potential is connected to the pulse voltage source through the 10 k ballast load. The floating potential electrode is connected to the ground through the 10 pF capacitance. The model also includes grounded electrode far beyond the discharge electrodes system.

We consider simplified plasma-chemical reactions and species sets for oxygen. Namely, the reduced formulation includes only electrons, neutral molecules O2, positive O2

+

and negative O2- single charged ions. The reactions are re-

stricted to three: electron impact ionization, attachment and ion-ion recombination.

3. Preliminary conclusions

From the experiment [3], it is known that the apokamp starts its formation after the power source supplies 400-600 periods of a high-voltage pulse to the gap. In order to skip this long transitional stage, in the simulation we set the in-itial non-uniform pre-ionization and the temperature field in the form of Gaussian “spots”. They approximately cor-respond to the experimental distributions at the time point of the jet start.

In this case, the formation of the visible apokamp part occurs in one pulse. Numerical calculations showed that the jet body has a complex spatially inhomogeneous struc-ture formed by the discrete dynamics of plasma bullets. The jet has a predominantly ion-ion composition, which causes its slow decay during the “dead time” of voltage generator i.e. up to 60 μs while the on-time is of 1.5-2.5 μs. The apokamp propagation speeds are close to the experi-mental values, as well as its visual appearance and the shape of the initiating discharge channel. References [1] E. A. Sosnin, V. S. Skakun, V. A. Panarin, D. S. Pech-

enitsin, V. F. Tarasenko, and E. K. Baksht, “Phenome-non of apokamp discharge,” JETP Letters, 103, 12, (2016).

[2] V. S. Skakun, V. A. Panarin, D. S. Pechenitsyn, E. A. Sosnin, and V. F. Tarasenko, Russian Physics Journal, 59, 5, (2016).

[3] E. A. Sosnin, V. A. Panarin, V. S. Skakun, E. K. Baksht, and V. F. Tarasenko, EPJ D, 71, 2, (2017).

Fig.1 Experimental appearance of the apokamp: 1 – float-

ing potential electrode, 2 – high-voltage electrode; 3 – dis-

charge channel; 4 - bright process; 5 – apokamp jet chan-

nel.

83

poster Simulations

Modelling for NH3 synthesis in dielectric barrier discharges with catalysts

K. van ‘t Veer1,2, F. Reniers2, A. Bogaerts1

1University of Antwerp, Department of Chemistry, Research Group PLASMANT, Universiteitsplein 1, 2610 Wilrijk-Antwerp, Belgium

2 Université Libre de Bruxelles, Faculty of Sciences, Analytical and Interfacial Chemistry Department, Avenue F. D. Roosevelt 50, B-1050 Brussels, Belgium

Nitrogen fixation based on non-thermal plasmas is gaining interest as a possible alternative for the current Haber-Bosch process, which is mainly suited for large scale and has a huge environmental footprint. Dielectric barrier discharges (DBDs) have been studied in particular for the formation of ammonia (NH3) from nitrogen and hydrogen (N2/H2) gas mixtures with various specific catalysts [1][1].

A zero-dimensional (0D) plasma kinetics model is used to study the formation of NH3 from N2/H2 mixtures in an atmospheric pressure packed bed DBD (PB DBD). Both gas phase and surface kinetics are taken into account explicitly. The present study aims to achieve a highly modular 0D model, in which detailed gas phase kinetics are coupled to surface reactions, whose rates are determined using transition state theory [2], allowing for the study of a wide range of catalysts in an accurate plasma environment. The model will be combined with experimental work, that involves both a simple planar DBD for diagnostics and a PB DBD, and make use of past experience with (higher dimensional) modelling of PB DBDs [3-4].

N2 and H2 conversions, NH3 yield and the specific reaction mechanisms are of interest, as well as numerical studies of the plasma conditions and the catalysts, in order to access the viability of N2 fixation using (PB) DBDs. In

addition, the kinetics of vibrational states of N2 and H2 are taken into account and their role will be investigated, based on previous accounts [2].

With the aim for an overall modular model, concerning the plasma conditions and the model chemistry, present efforts will also lead to studying other gas mixtures for nitrogen conversion in a DBD.

References [1] A. Bogaerts and E. C. Neyts, “Plasma Technology :An Emerging Technology for Energy Storage,” ACSEnergy Lett., vol. 3, no. 4, pp. 1013–1027, 2018.

[2] P. Mehta et al., “Overcoming ammonia synthesisscaling relations with plasma-enabled catalysis,” Nat.Catal., vol. 1, no. 4, pp. 269–275, 2018.

[3] A. B. Koen Van Laer, “Influence of Gap Size andDielectricConstant of the Packing Material on thePlasmaBehaviour in a Packed Bed DBDReactor: A FluidModelling Study,” Plasma Process Polym, vol. Volume14, no. Issue 4-5, pp. 1–11, 2017.

[4] W. Wang, H. H. Kim, K. Van Laer, and A. Bogaerts,“Streamer propagation in a packed bed plasma reactor forplasma catalysis applications,” Chem. Eng. J., vol. 334, no.November 2017, pp. 2467–2479, 2018.

84

Simulations poster

2D (axial-azimuthal) Particle-In-Cell benchmark for ExB discharges

T. Charoy1, A. Tavant1,2, A. Bourdon1, P. Chabert1

1Laboratoire de Physique des Plasmas, CNRS, Ecole polytechnique, Sorbonne Universités, Université Paris-Sud, Ob-servatoire de Paris, Université Paris-Saclay, PSL Research University, 91128 Palaiseau, France

2Safran Aircraft Engines, 27208 Vernon, France

Even though Hall-Effect Thrusters (HET) have been in-tensively studied for the past few decades [1,2], electron transport across the magnetic field is still not well under-stood. Recent studies have shown that Electron Cyclotron Drift Instabilities (ECDI) could be a main cause of the anomalous transport observed. To get more insights on this phenomenon, we modified a highly-parallelized 2D Parti-cle-In-Cell Monte Carlo Collision (PIC MCC) code, that was previously used to simulate the radial-azimuthal plane of a HET [3], in order to study the axial-azimuthal one.

Prior to use this code extensively, it was important to be sure of its correctness. Unit tests have been used to verify specific modules and we made sure it was complying to an 1D Helium benchmark [4]. However, we needed a simula-tion case that was closer to the ones we will study, i.e. an ExB discharge configuration. We chose a case simplified from a real HET, close to the one used in [5], where a fixed ionization source term is imposed, and the collision module is disabled. The first preliminary results were in good agreement as seen on figure 1 where results from 3 differ-ent groups (LAPLACE, CERFACS and LPP) are displayed. We have now 8 different groups who are simulating the case defined. After comparing the results given by these 8 different codes, we will converge to one of the first 2D PIC benchmark.

Figure 1: Axial evolution of ion density and axial elec-tric field (azimuthally and time averaged) obtained with 3

different codes on the same simulation case

However, even though some hypothesis have been made from a real HET configuration, the physics inherent to this case is still complex. It has been recently noticed [6] that some numerical parameters such as the number of macro-particles per cell, might affect the simulation results. A fo-cus on the code convergence depending on this number have been done, to make sure the chosen simulation case was reliable.

Moreover, we also used this simplified case to get more insights on specific numerical models previously used. For example, it is still unclear what boundary conditions for the cathode we should apply in a axial-azimuthal PIC code? It appeared that if we change the model or even only the tem-perature of the electrons emitted, it changes the discharge behavior. The length of the azimuthal (periodic) direction has also been varied to look at its impact on the instabilities propagating in that direction.

Acknowledgments This work has been partially funded by the Agence Natio-nale de la Recherche under the reference ANR-16-CHIN-0003-01 and Safran Aircraft Engines within the project POSEIDON. The authors were granted access to the HPC resources of CINES (under the allocation A0040510092 made by GENCI) and of CERFACS at Toulouse. References [1] J.C. Adam et al., Plasma Phys. Control. Fusion 50-

124041 (2008). [2] J.P. Boeuf, Journal of Applied Physics 121-011101

(2017). [3] V. Croes et al., Plasma Sources Sci. Technol. 26-034001

(2017). [4] M.M. Turner et al., Phys. Plasmas 20-013507 (2013) [5] J.P. Bœuf et al., Phys. Plasmas 25-061204 (2018) [6] S. Janhunen et al., Phys. Plasmas 25-011608 (2018)

85

poster Simulations

Electron kinetics in fast-pulsed discharges

A. Tejero-del-Caz1, V. Guerra1, D. Gonçalves1, M. Lino da Silva1, L. Marques2,N. Pinhão1, C. D. Pintassilgo1,3 and L. L. Alves1

P

1P Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, Lisboa, Portugal

P

2P Centro de Física da Universidade do Minho, Campus de Gualtar, Braga, Portugal

P

3P Faculdade de Engenharia da Universidade do Porto, Porto, Portugal

1. IntroductionPredictive tools for non-equilibrium low-temperature

plasmas (LTPs) should describe properly the kinetics of electrons, responsible for stimulating plasma reactivity. Here, we focus on plasmas produced in N2-O2 gaseous mixtures, aiming to deliver a KInetic Testbed for PLASMa Environmental and Biological Applications (KIT-PLASMEBA) [1], comprising the development of simula-tion tools and the critical assessment of collisional-radia-tive data.

In this framework, we have developed the LisbOn KI-netics Boltzmann solver (LoKI-B) [2], an open-source sim-ulation tool that solves a time and space independent form of the two-term electron Boltzmann equation (EBE), for non-magnetized non-equilibrium LTPs created from differ-ent gases or gas mixtures. The simulation tool gives a mi-croscopic description of the electron kinetics and calculates macroscopic quantities, such as electron rate coefficients and transport parameters, which are key for solving global models [3]. Indeed, LoKI-B is coupled to a Chemistry solver (LoKI-C), that receives these parameters as input data.

Recently, there has been increasing interest in non-equi-librium LTPs created by fast-pulsed discharges, because of their potential advantages in different technological appli-cations [4]. However, due to the lack of readily available time-dependent EBE solvers, several assumptions are usu-ally made to incorporate the electron kinetics into the cor-responding chemistry models: introducing effective source terms that account for the electron-impact creation of ex-cited species [5], or considering a quasi-stationary situation for electrons [6,7] solving a time-independent form of the EBE for the different (and time varying) values of the re-duced electric field, E/N.

2. ResultsWe have studied the temporal evolution of the electron

kinetics in a dry-air (80% N2 - 20% O2) pulsed-discharge, excited by a reduced-field with the form

𝐸/𝑁 = 100√𝑡

10−6 exp (−𝑡

10−6) (Td) . (1)

The simulations compare the results obtained when solving the time-dependent EBE with those of the quasi-stationary assumption. To carry out this analysis, we have extended the capabilities of LoKI-B, allowing for time-dependent

calculations of the EBE. Figure 1 shows the temporal evolution of the electron-

impact ionization rate coefficients for N2 and O2, along with the reduced-field pulse. For times below 10-6 s, the results of the quasi-stationary solution deviate considera-bly from those obtained with the time-dependent EBE, overestimating the electron rate coefficients. This effect is due to the instantaneous relaxation of the electron energy distribution function in the quasi-stationary approach.

Fig. 1 Electron ionisation rate coefficients for N2 (blue) and O2 (red), considering a time-dependent (solid) or a stationary (dashed) solution of the EBE. The black-solid line represents the reduced electric-field pulse, given by equation (1).

Acknowledgments This work was funded by Portuguese FCT – Fundação para a Cie cia e a Tecnologia, under projects UID/FIS/50010/2013 and PTDC/FISPLA/1243/2014 (KIT-PLASMEBA).

References [1] http://plasmakit.tecnico.ulisboa.pt[2] A. Tejero-del-Caz et al, Plasma Sources Sci. Technol., submit-

ted (2018).[3] L.L. Alves and A. Tejero-del-Caz, FLTP-Simulations (2019).[4] R. Brandenburg et al, Plasma Sources Sci. Technol. 26,

020201 (2017).[5] E.A.D. Carbone et al, Plasma Sources Sci. Technol. 25,

054004 (2016).[6] W. Wang et al, J. Phys. D 51, 204003 (2018).[7] M. Šimek, and Z. Bonaventura, J. Phys. D 51, 504004 (2018).

86

Simulations poster

DC magnetron discharge used for nanoparticle growth: comparison of particle-in-cell simulations with experimental measurements

L. Couëdel1,2, A. Chami2, C. Arnas2

1Physics and Engineering Physics department, University of Saskatchewan, Saskatoon, Saskatchewan, Canada,2CNRS, Aix-Marseille Université, PIIM UMR 7345, Marseille, France.

1. IntroductionThe formation of nanoparticles (NP) in glow discharges

has been for a long time a subject of high interest. In ra-dio-frequency (RF) discharges, NP growth and its effecton discharge parameters have been studies using reactive[1,2], and by sputtering of the electrodes [3]. In direct-current (DC) sputtering glow discharge, NP formation,dynamics and transport has also been studied using differ-ent cathode materials [4-8].

Since the mid-1990’s, RF and DC magnetron-sputteringaggregation sources are commonly used to produce metalnanoparticles [9, 10]. They normally consist of a high-pressure plasma chamber connected to a low pressure ex-pansion chamber in which a beam of NP can be filteredand collected. NPs are formed in the high-pressure side ofthe MS-AS by cathode sputtering. However, very fewstudies are devoted to the characterization of the mag-netron plasma in which NPs are growing and the link be-tween plasma parameters and NP growth dynamics.

2. Experimental setupThe experiments were performed in an argon DC unbal-

anced magnetron discharge having a 3” diameter tungstencathode (see Fig.1). The cathode was facing a groundedanode and the inter-electrode distance could be changedfrom 5 cm to 10 cm. Either two glass half-cylinders ortwo stainless steal half cylinders were used to confine theplasma. A 1 cm gap was kept between them for optical di-agnostics and radial Langmuir probe measurements. Anargon pressure between 10 Pa and 40 Pa (5 sccm gasflow) was set during the experiments. The discharge sys-tem was contained in a cylindrical vacuum chamber of 30cm diameter and 40 cm length. A regulated power supplywas used to bias the cathode. The discharge current waskept at a constant value (from 100 mA to 500 mA). Underthe chosen operating conditions, the cathode was sput-tered and tungsten NPs could be grown.

3. Particle-in-cell simulationsThe simulation were performed using the VSIM soft-

ware from TechX corporation [11]. 2D3V simulations incylindrical geometry were preformed. The simulation boxwas cut in 512×512 cells. Dirichlet boundary conditionswere used for the cathode (V=Vbias) and the anode (V=0V), For the side wall, Dirichlet boundary condition wereused to simulate the metallic cylinder (V=0 V). For theglass cylinder a dielectric was added to the simulation.

The nominal density was set to 1012 cm-3 and the particleweight was variable. A time step of 10-12 s was used andthe simulations were run for a few millions time steps un-til equilibrium was reached.

4. ResultsThe simulated potential and density profiles were com-

pared to experimental measurements. The influence of thesputtered atoms on the discharge properties were also in-vestigated. A particular attention is given to the influenceof the sputtered metal on the electron temperature. Resultsare correlated to NP growth dynamics.

References[1] M. Cavarroc, et al., Phys. Rev. Lett. 100, 045001

(2008).[2] I. Stefanovic, N. Sadeghi, and J. Winter, J. Phys. D:

Appl. Phys. 43, 152003 (2010).[3] D. Samsonov and J. Goree, Phys. Rev. E 59, 1047

(1999).[4] G. M. Jellum and D. B. Graves, J. Appl. Phys. 67,

6490 (1990).[5] C. Arnas, et al. , Phys. Plasmas 20, 013705 (2013).[6] Kishor Kumar K., L. Couëdel, and C. Arnas, Phys.

Plasmas 20, 043707 (2013).[7] L. Couëdel, Kishor Kumar K., and C. Arnas, Phys.

Plasmas 21, 123703 (2014).[8] S. Barbosa, et al., J. Phys. D: Appl. Phys. 49, p.

045203 (2016).[9] H. Haberland, et al., J. Vac. Sci.Technol. A 12, 29250

(1994).[10] T. Acsente,et al., Mat. Lett. 200, 121 (2017).[11] C. Nieter, and J. R. Cary, J. Comp. Phys. 196, 448

(2004)

Fig. 1: Schematic of the experimental set-up.

87

poster Simulations

Towards a benchmark for two-dimensional particle-in-cell simulation

Romain Lucken¹, Antoine Tavant¹², Anne Bourdon¹, Pascal Chabert¹

¹ LPP, CNRS, École polytechnique, UPMC Univ Paris 06, Univ. Paris-Sud, Observatoire de Paris, Université Paris-Saclay,Sorbonne Universités, PSL Research University, 91128 Palaiseau, France

² Safran Aircraft Engines, Electric Propulsion Unit, F-27208 Vernon, France

January 25th, 2019

Particle-in-cell (PIC) simulations have become a commontool for the modeling of low-density plasma discharges in1D and 2D. While the accuracy and the convergence of PICmodels in 1D have been investigated with a good level ofdetails [1], this is not the case yet for 2D systems. In thiswork, we simulate self-consistently a two-dimensionalinductively coupled plasma discharge in helium with theLPPic code. Current drive and power drive methods areimplemented and compared. The simulation setup featuresconductive walls and cross sections coming from the LXCatdatabase. All numerical parameters are provided withparticular care.The convergence with respect to the cell size, the time stepand the number of particles per cell is investigated and theplasma characteristics are compared with analyticalformulas coming from simplified fluid models [2]. Acknowledgments

This work was granted access to the HPC resources ofCINES under the allocation 2018-A0040510092 made byGENCI, and it was partially funded by CHEOPS project thathas received funding from the European Unions Horizon2020 research and innovation program under grantagreement No 730135.

[1] M. M. Turner, A. Derzsi, Z. Donko, D. Eremin, S. J.Kelly, and T. Mussenbrock. Simulation benchmarks for low-pressure plasmas: capacitive discharges. Phys. Plasmas,20(1):013507, 2012.[2] R. Lucken, V. Croes, T. Lafleur, J.-L. Raimbault, A.Bourdon, and P. Chabert. Edge-to-center plasma densityratios in two-dimensional plasma discharges. PlasmaSources Sci. Technol., 27:035004, 2018.

Figure 1: Plasma characteristics at steady-state

Table 1: Sample simulation parameters

88

Simulations poster

Control of the plasma jet dynamics by external electric fields

N. Yu. Babaeva, G. V. Naidis

Joint Institute for High Temperatrures, Russian Academy of Sciences, 125412 Moscow, Russia

1. Introduction Application of the additional electric field by using ex-ternal electrodes can provide a considerable level of con-trol over the dynamics of an atmospheric pressure plasma jets. In this way fluxes and reactive species arriving to the downstream surface can be also controlled. In this paper we study the possibilities of such control via application of uniform or non-uniform external electric fields. The numerical model used in this investigation is nonPDPSIM whose algorithms are discussed in paper [1]. 2. Control with the uniform external field

In the series of publications appeared in the middle of 90s [2-4] it was shown that the minimum value of a uni-form external (stability) field E0 required for a stable propa-gation of a streamer is 5 kV/cm for positive and 15 kV/cm for negative streamers. The propagation of the plasma jet can be considered as the propagation of a sequence of ionization waves (IWs) or streamers guided by a channel of noble gas. In this case, the value of E0 depends on many factors and it is difficult (if possible at all) to establish the value of the stability field for these IWs. This is partially due to the fact that one needs to account for the mixing of he-lium with surrounding air because the ionization effi-ciency maximizes in regions where the molar fraction of air has a certain value. In the present paper we show that it is still possible to manipulate the jet parameters by ap-plying the external uniform electric field. In Fig. 1a to 1d the decay length for the plasma bullet in a single shot pulse discharge is presented. The bullet is initiated inside the tube (single pin-electrode arrangement) by applying a voltage of -3 kV. The field from this voltage rapidly de-creases with the distance. After the exit of the tube the bullet propagates in the region of a weak uniform electric field. The distance of the bullet propagation increases with the increase of the external uniform field.

3. Control with external ring electrodes G. Naidis and J. Walsh [5] showed that application of voltage pulses to a ring electrode placed around the axis of jet propagation results in a decrease or increase (in dependence on polarity) of the bullet velocity. The results were obtained using a 1.5D computational model [5] and further confirmed by experiment [6]. In this paper the influence of positive and negative voltage pulses applied to an external ring electrode is studied computationally using 2D nonPDPSIM platform (Fig. 1e). The increase or decrease of the IW velocity and the electric field in its front is investigated as a function of polarity and magni-

tude of the external field and the moment of its applica-tion. 4. Acknowledgment

The authors are grateful to Prof. M. J. Kushner (Uni-versity of Michigan) for initiating their interest to the plasma jet topic. References [1] S. Norberg, E. Johnsen and M. J. Kushner, Plasma

Sources Sci. Technol. 24, 035026 (2015). [2] N. Babaeva and G. Naidis, IEEE Trans. Plasma Sci-

ence 25, 375 (1997). [3] N. Babaeva and G. Naidis, Physics Letters A 215, 187

(1996). [4] N. Allen and A. Ghaffar, J. Phys. D 28, 331 (1995). [5] G. Naidis and J. Walsh, J. Phys. D. 46, 095203 (2013). [6] P. Olszewski, E. Wagenaars, K. McKay, J. Bradley

and J. Walsh, Plasma Sources Sci. Technol. 23, 015010 (2014).

Fig.1. (a,b,c,d) Decay distance of a plasma bullet for different values of external uniform field E0. (e) Arrangement of external ring electrodes.

89

poster Simulations

A high performance, two-dimensional electrostatic particle-in-cell simulationcode with verification

M. M. Turner1 and H. J. Leggate1

1National Centre for Plasma Science and Technology and School of Physical Sciences, Dublin City University, Dublin

9, Ireland

1. Introduction

We discuss a two-dimensional particle-in-cell simula-

tion with Monte Carlo collisions intended for simulating

low-temperature plasma devices, including those featur-

ing magnetic fields. At this stage, this is not, of course, an

especially novel concept. However, this particular code

has two features that are distinctive relative to many other

similar codes, namely that we aim for high performance

on a wide variety of typical modern commodity computa-

tion platforms and we have attended carefully to a demon-

strating correctness of the code relative to a number of ex-

act solutions and benchmark cases.

2. High performance

The recent tendency in modern commodity computing

platforms using general purpose hardware has been to-

wards offering an increasing amount of parallelism at sev-

eral distinct levels. At the lowest level, there is vectorisa-

tion, essentially permitting simultaneous operations on

data in adjacent memory locations using extended proces-

sor instructions such as Intel’s SSE or AVX, or equiva-

lents from other manufacturers. In principle, these facili-

ties permit accelerations by factors like 4 or 8, although of

course less is usually observed in practice for various rea-

sons. Exploitation of these instructions requires carefully

organized data structures and algorithms. The next level

of parallelization is multi-threading, usually using a cross

platform programming interface such as OpenMP. On a

typical desk top workstation with two processors, there

might be available 16 or 24 threads. Again, an increase in

performance by this factor is possible in principle but dif-

ficult to realize in practice. Between vectorisation and

multi-threading, a performance enhancement by a factor

of 10-20 might be seen in actuality. This is quite suffi-

cient to enable many two-dimensional particle-in-cell

simulations to be executed on a personal work station

with reasonable convenience. However, certainly not all

computations of interest are in this category, and further

advances in computation speed involve the use of multi-

ple computing nodes in parallel, using a parallel program-

ming model such as MPI. MPI systems may offer thou-

sands of computing nodes, although as usual there will

difficulty in achieving good performance on so many. An

electrostatic particle-in-cell simulation is likely to be lim-

ited by the performance of parallel elliptic solvers to effi-

cient operation on a few hundred nodes, which is the case

with the present code. Evidently, a code that exploits par-

allelism at all these levels must be complex, in spite of the

basic simplicity of the particle-in-cell method. Conse-

quently, correctness of the code becomes an increasing

concern.

We have tested the present code against wide range of

exact solutions in the course of its development. These

exact solutions each test some subset of the codes features

(e.g., the field solver, the particle mover), and together

they cover practically all of the codes functionality.

3 Conclusions.

We will discuss the code in detail, and present examples

of tests against exact solutions.

90

Wednesday May 15th

Diagnostics & Simulations

Diagnostics invited

Advanced optical diagnostics and validation of computational approaches for the chemical kinetics in atmospheric pressure plasmas

Timo Gans1

1York Plasma Institute, Department of Physics, University of York, York, UK

1. Abstract Atmospheric pressure plasmas are versatile and efficient

sources for reactive species production at ambient room temperature. This enables the development of new plasma technologies for various environmental and healthcare ap-plications. The non-equilibrium chemical kinetics is initi-ated and determined by the electron dynamics. Due to the strongly collisional environment and associated short elec-tron energy relaxation times the electron dynamics can be tailored using multi-frequency power coupling techniques, enabling separate control of key parameters like electron density and electron mean energy. Reactive atomic species play key roles in the chemical kinetics and details strongly depend on the feed-gas composition. Measurements and predictive simulations of key reactive species are equally challenging due to the strongly collisional environment and their multi-scale nature in space and time. The most prom-ising approach is the exploitation of complementary ad-vantages in direct measurements combined with specifi-cally designed numerical simulations. Picosecond two-photon absorption laser induced fluorescence (TALIF) spectroscopy allows us to measure absolute densities of atomic oxygen (O), nitrogen (N) and hydrogen (H), even in chemical environments with complex reaction kinetics and associated collisional quenching processes, through di-rectly resolving the effective lifetime with sub-nanosecond resolution. This is particularly important in realistic situa-tions for technological applications with plasma operation and species penetration into ambient air. The picosecond TALIF measurements are compared with direct VUV syn-chrotron absorption spectroscopy under well-defined gas compositions showing very good agreement. Further in-sight into the chemical kinetics is obtained through addi-tional UV & IR absorption spectroscopy (OH, O3, CO2, CO) measurements and synergistic combination with multi-scale numerical simulations of the chemical kinetics. The presentation will focus on examples of He-O2-N2-H20 mixtures for bio-medical applications and He/Ar-CO2 mix-tures for CO2 conversion into value-added chemicals.

2. Acknowledgement

This work was supported by the EPSRC through grant EP/K018388/1. 3. Selected related publications [1] C. O'Neill, J. Waskoenig, T. Gans, Tailoring electron

energy distribution functions through energy confine-ment in dual radio-frequency driven atmospheric pres-sure plasmas, Applied Physics Letters, 2012, 101 (15), 154107.

[2] T. Murakami, K. Niemi, T. Gans, D. O’Connell, W. G. Graham, Afterglow chemistry of atmospheric-pressure helium–oxygen plasmas with humid air impurity, Plasma Sources Sci. Technol., 2014, 23, 025005.

[3] J. Golda, J. Held, B. Redeker, M. Konkowski, P. Beijer, A. Sobota, G. Kroesen, N. St. J. Braithwaite, S. Reuter, M. M. Turner, T. Gans, D. O’Connell and V. Schulz-von der Gathen, Concepts and characteristics of the ‘COST Reference Microplasma Jet’, J. Phys. D: Appl. Phys., 2016, 49, 084003.

[4] A. Wijaikhum, D. Schroeder, S. Schröter, A. R. Gibson, K. Niemi, J. Friderich, A. Greb, V. Schulz-von der Gathen, D. O’Connell and T. Gans, Absolute ozone densities in a radio-frequency driven atmospheric pressure plasma using two-beam UV-LED absorption spectroscopy and numerical simulations, Plasma Sources Sci. Technol., 2017, 26, 115004.

[5] J. Dedrick, S. Schröter, K. Niemi, A. Wijaikhum, E. Wagenaars, N. de Oliveira, L. Nahon, J. P. Booth, D. O’Connell and T. Gans, Controlled production of atomic oxygen and nitrogen in a pulsed radio-fre-quency atmospheric-pressure plasma, J. Phys. D: Appl. Phys., 2017, 50, 455204.

[6] S. Schröter, A. Wijaikhum, A. R. Gibson, A. West, H. Davies, N. Minesi, J. Dedrick, E. Wagenaars, N. de Oliveira, L. Nahon, M. J. Kushner, J. P. Booth, K. Niemi, T. Gans, D. O’Connell, Chemical kinetics in an atmospheric pressure helium plasma containing hu-midity, Phys. Chem. Chem. Phys., 2018, 20, 24263

[7] L. Bischoff, G. Hübner, I. Korolov, Z. Donkó, P. Hart-mann, T. Gans, J. Held, V. Schulz-von der Gathen, Y. Liu, T. Mussenbrock, J. Schulze, Experimental and computational investigations of electron dynamics in micro atmospheric pressure radio-frequency plasma jets operated in H2/N2 mixtures, Plasma Sources Sci. Technol., 2018, 27, 125009

[8] A.R. Gibson, Z. Donkó, L. Alelyani, L. Bischoff, G. Hübner, J. Bredin, S. Doyle, I. Korolov, K. Niemi, T. Mussenbrock, P. Hartmann, J.P. Dedrick, J. Schulze, T. Gans, D. O’Connell, Disrupting the spatio-temporal symmetry of the electron dynamics in atmospheric pressure plasmas by voltage waveform tailoring, Plasma Sources Sci. Technol., 2019, 28, 01LT01

93

invited Diagnostics

Diagnostic and modeling of fast pulsed discharges

T.L.Chng1, I.S. Orel1, Ch.Ding1, G.V. Pokrovskiy1, M.Alicherif1, S.A. Shcherbanev1,I.V. Adamovich2, N.A.Popov3, S. M.Starikovskaia1

1Laboratoire de Physique des Plasmas (CNRS, Ecole Polytechnique, Sorbonne University, University Paris-Sud, Observatoire de Paris, University Paris-Saclay), FR-91128Palaiseau Cedex, France

22Nonequilibrium Thermodynamics Laboratories, Department of Mechanical and Aerospace Engineering, Ohio State University, Columbus, OH 43210, USA

2Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991, Leninsky gory, Russia

1. BackgroundFast nanosecond (ns) pulsed discharges came into the

international scientific scene in the mid 1990’s [1], being later developed for high pressure applications such as plasma assisted combustion [2-4] and flow dynamics [5]. This talk will present a review of experimental and nu-merical methods developed during the last 10-15 years with the aim of studying ns discharges, and the results obtained for a wide range of pressures (0.1 mbar - 15 bar), mixture compositions and specific delivered energies.

2. Approaches to experiments and modelingElectrical diagnostics allows time resolved (synchro-

nized with an accuracy of 0.2 ns) discharge electrical cur-rent, reduced field and deposited energy. Measurements of electron density can be performed by electrical methods

and by Stark broadening; H, O, N lines are typically used. Electric field in the discharge can be measured with tens of picoseconds and hundreds of microns resolution using the second harmonic generation of the ps laser emission traversing the region of the electric field.

ICCD imaging with spectral filtering provides

information about discharge uniformity in space and typ-ical dimensions of the plasma.

Developing, in parallel, comprehensive experimental diagnostics and numerical modeling provide insight into the physics of nanosecond discharges. During the last few years, codes describing volumetric and surface nanosec-ond discharges taking into account non-local effects on the discharge front, detailed chemical kinetics in the dis-charge/afterglow, and following fast energy release and hydrodynamic perturbations are now available.

3. Results and perspectivesAt low pressures (Fig. 1), the ns plasma is uniform

while with pressure increase (Fig. 2) the discharge is more concentrated in space. At moderate pressures, the dis-

charge develops uniformly in space within a wide range of specific deposited energies: from 0.001 to 1-5 eV/particle. At and above atmospheric pressure, a few modes are possible, from a low current corona discharge to a filamentary discharge with a typical current density of 104-105 A/cm2.

Figure 2. Electrode system (the HV electrode is a 2 cm diameter disk in the center) and nSDBD discharge. Air, P=10 bar; U=+46 kV, ICCD gate is 1 ns.

Peculiarities of kinetics of the discharges with high reduced electric fields and high delivered energy will be discussed using examples of the classical fast ionization wave (FIW) discharge at low specific delivered energy, the capillary FIW discharge at moderate pressure and the nanosecond surface dielectric barrier discharge (nSDBD) at high pressure (Fig.2).

Acknowledgements This work was partially supported by LabEx Plas@Par,

French National Research Agency (ASPEN Project), French-Russian international laboratory LIA KaPPA (CNRS) and RFBR 17-52-16001. The support of Prof. Adamovich by the Ecole Polytechnique Gaspard Monge Visiting Professor Program is gratefully acknowledged.

References [1] LM Vasilyak, SV Kostyuchenko, NN Kudryavtsev,

IV Filyugin, Phys.-Uspekhi, 1994, 163, 263-286[2] A Starikovskiy, N Aleksandrov, Prog. Energy Combust.

Sci. 2013, 39, 61–110[3] IV Adamovich, I Choi, N Jiang, J-H Kim, S Keshav,

WR Lempert, EI Mintusov, M Nishihara, M Samimy,M Uddi PSST, 2009, 18, 034018

[4] Y Ju, W Sun, Prog. En. Comb. Sci. 2015, 48, 21–83[5] S B Leonov, I V Adamovich, V R Soloviev PSST,

2016, 25, 063001

Figure 1. Uniform ns discharge in air. P=25 mbar, U=5 kV, f=1 kHz

94

Diagnostics oral

Velocity distribution function of heavy particles in HiPIMS by evaluation of optical emission line profiles

J. Held1, A. Hecimovic2, A. von Keudell1, V. Schulz-von der Gathen1

1Experimental Physics II, Ruhr University Bochum, Germany

2Max Planck Institute for Plasma Physics, Garching, Germany

1. Introduction High power impulse magnetron sputtering (HiPIMS), is

a comparatively new deposition technology where a tradi-tional magnetron sputtering configuration is operated with short, high-voltage pulses. The plasma created in the mag-netic field trap is ionized to a high degree (90 %) and con-tains a large amount of sputtered atoms and ions [1]. HiP-IMS has been observed to create better coatings than tradi-tional direct current magnetron sputtering but at a reduced deposition rate [2,3]. This has been attributed to the return of ions to the target surface, which in turn is explained by strong electric fields present in the magnetic field trap [4].

This “return effect” can be investigated by comparing the movement of ions and neutrals close to the target sur-face. One way to obtain the velocity distribution function (VDF) of atoms and ions is by high-resolution optical emission spectroscopy. The profile of emission lines is broadened by the Doppler effect which can be used to de-termine the VDF.

2. Setup

Figure 1 shows the experimental setup. The plasma emission is observed through a chamber window either perpendicular or parallel (not shown) to the target surface. The spectrograph (Zeiss PGS 2) has a focal length of 2 m, a 1300 l/mm grating and is equipped with an iCCD camera (Andor iStar). Operating in the third diffraction order a spectral resolution of 1.5 pm (pixel-to-pixel) is obtained. The emission from a hollow cathode lamp is used as a ref-erence.

3. Results

Figure 2 shows the VDF of titanium neutrals and ions in the direction perpendicular to the target surface as deter-mined by deconvolution of optical emission lines which are observed simultaneously. The VDF of sputtered neutrals was compared to a simple model based on the Krook colli-sional operator.

The evolution of the VDF of both sputtered neutrals and ions during the discharge pulse was measured and com-pared. It was found, that the VDF and its evolution during the discharge pulse can mostly be explained by resonant charge exchange and Coulomb collisions. Under the inves-tigated discharge conditions, ion movement seems to be dominated by collisions and not by the electric field. The influence of strong electric fields which are supposed to be present in the HiPIMS target region could not be observed.

This work has been supported by the German Science Foundation (DFG) within the frame of the collaborative re-search center SFB-TR 87. References [1] J. Bohlmark et al. J. Vac. Sci. Technol. A 23 18–22

(2005). [2] J. Alami et al. J. Vac. Sci. Technol. A 23 278–80

(2005). [3] K. Sarakinos et al. J. Phys. D 40 2108 (2007). [4] N. Brenning et al. Plasma Sources Sci. Technol. 21

025005 (2012). [5] J Held et al. Plasma Sources Sci. Technol. 27 105012

(2018).

Fig.1 Experimental Setup [5].

Fig.2 VDF of sputtered titanium neutrals and ions at the end of the discharge pulse.

95

oral Simulations

Numerical and experimental study of radio frequency micro atmospheric pressure plasma jets driven by voltage waveform tailoring: effects on electron heating and generated species Y Liu1, I Korolov2, J Schulze2, T Hemke3, and T Mussenbrock1

1 Electrodynamics and Physical Electronics Group, Brandenburg University of Technology Cottbus-Senftenberg, Germany 2 Department of Electrical Engineering and Information Science, Ruhr-University Bochum, Germany 3 Institute of Theoretical Electrical Engineering, Ruhr-University Bochum, Germany A two dimensional fluid dynamics model is applied to simulate a micro atmospheric pressure plasma jet (μAPPJ) with a helium oxygen mixture driven by a single radio frequency as well as voltage waveform tailoring sources. An Ω- to Penning-mode transition of the electron heating dynamics has been observed for the single frequency discharges by both simulations and experiments. By using the peak or valley type tailored voltage waveforms, it is observed that the electron heating dynamics are strengthened when the sheath is collapsing, inconsistently with the low pressure discharge. Moreover, this variation of the electron heating dynamics plays a significant role on the generation of reactive neutral species, which are important for plasma surface processings. Financial support granted by the German Research Foundation in the frame of SFB 1316 (project A4) is gratefully acknowledged.

96

Diagnostics oral

Collision Energy Trasnfer LIF: a molecular probe for CO2 dissociation in a nanosecond pulsed discharge

L.M. Martini1,3*, M. Ceppelli1, M. Scotoni1, G. Dilecce 2,1, P. Tosi1,2

1Dipartimento di Fisica Università di Trento, Trento, Italy

2P.Las.Mi Lab NANOTEC – CNR, Bari, Italy 3present address: Department of Applied Physics - Eindhoven University of Technology, The Netherlands

*Contact e-mail: l.m.martini@tue.nl

1. Introduction Laser Induced Fluorescence (LIF), intended as a way to

measure the concentration of transient species, involves, as the observable, the fluorescence from an electronically excited state prepared by absorption of resonant laser light:

M + ℎ𝑣𝐿 → M∗ → M∗∗ + ℎ𝑣𝑓𝑙𝑢𝑜 (1) Collision energy transfers (CET) processes involving

M* and the background molecules have a heavy influence on the fluorescence outcome. This is generally considered as a shortcoming since quantitative use of LIF for the characterization of the transient species M requires a de-tailed knowledge of the collision frequencies. Reversing the point of view, this knowledge allows to use the laser prepared electronic state M* as a transient quantum sen-sor for the medium composition, that can be used in hos-tile environments characterized by rapid changes (sub-s scale) of temperature, pressure and gas composition. This is possible since the CET processes with the background molecules redistribute the energy initially deposited in a single rovibronic state. In this contribution we present the general aspects of LIF in a collisional environment, introduce the concept of CET- LIF [1], and describe its application to the measurement of the CO2 dissociation in a nanosecond repetitively pulsed (NRP) spark discharge, using OH(A) as a molecular sensor [2], and the collision rate coefficients for non-thermal rotational distributions (see the companion abstract [3]). 2. Methodology basics The classical LIF scheme for OH detection using the tran-sitions of the 3064 Å system is employed: OH(X, v′′ = 0) + ℎ𝑣𝐿 → OH(A, 1) → OH(X, 1) + ℎ𝑣(1,1)

↓ VET OH(A, 0) → OH(X, 0) + ℎ𝑣(0,0).

(2)

Fig. 1 shows a schematic drawing of the experimental apparatus used (refer to [2] for a detailed description). Under the assumption that the dissociation of CO2 follows the reaction:

2 ∙ CO2 → 2 ∙ CO + O2, (3) we can express the composition of the gas as a function of the CO2 conversion (𝛼):

𝛼 = 𝑚𝑜𝑙𝑒𝑠𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑𝐶𝑂2 𝑚𝑜𝑙𝑒𝑠𝑖𝑛𝑝𝑢𝑡𝑒𝑑

𝐶𝑂2 .⁄ (4)

Fig. 1 Schematic drawing of the experimental apparatus. FC: mass flow controllers, WFG: waveform generator, NPG: nano-second discharge power supply. Thanks to CET-LIF it is possible to draw a relation be-tween the measured fluorescence spectrum and the CO2 conversion. In addition, the gas temperature in the post-discharge is inferred from the excitation spectrum of close-spaced transitions involving different rotational level of the ground state of OH. 3. Results In the present contribution, we present our recent results on the gas temperature estimation in a CO2 NRP post-discharge. The temperature is observed to decrease from values close to 3000 K at 2 µs after the plasma pulse to values around 1400 K after 140 µs. We discuss the va-lidity of the CET-LIF method when applied to an envi-ronment where the change in gas temperature is consid-erable. Finally, CET-LIF technique is used to calculate CO2 conversion from the analysis of the OH fluorescence spectrum in the post-pulse of the NRP discharge. A com-parison with standard analytical techniques is presented. 4. References [1] L. M. Martini, N. Gatti, G. Dilecce, M. Scotoni and P. Tosi, 2018 Plasma Phys. Controlled Fusion 60 p. 014016 [2] L. M. Martini, S. Lovascio, G. Dilecce and P. Tosi, 2018 Plasma Chem. Plasma Process. 38 p.707-718 [3] M. Ceppelli, L. M. Martini, G. Dilecce, M. Scotoni and P. Tosi, 2019 This book of abstracts

97

oral Simulations

Kinetic Modeling and Simulation of the Planar Multipole Resonance Probe

Chunjie Wang1, Michael Friedrichs2, Jens Oberrath2, and Ralf Peter Brinkmann1

1Theoretical Electrical Engineering, Ruhr-University, Bochum, Germany2Institute of Product- and Process Innovation, Leuphana University Luneburg, Germany

∗Contact e-mail: Chunjie.Wang@rub.de

1. IntroductionActive Plasma Resonance Spectroscopy (APRS)

is an electric plasma diagnostic method: An RF signalis fed into the plasma via a probe and the spectrum ofthe response is analyzed using a mathematical model.The planar Multipole Resonance Probe (pMRP) is anAPRS design that can be integrated flatly into thechamber wall to minimize process perturbation [1-2].It consists of two semi-disc electrodes E± which areinsulated from each other and from the groundedchamber wall. A thin dielectric cover is optional.The RF signal V (t) = V cos(ωt) is applied to thetwo electrodes in an antisymmetric fashion.

possessing a comparable behaviour to the MRP and leading tothe design of the planar multipole resonance probe (pMRP).Here, a corresponding adaption of the mathematical model hasto be considered.

While the MRP can be used for an all-embracing charac-terization of the whole plasma volume, the pMRP representsa stationary measurement concept suitable for monitoringfunctions in industrial plasma processes. Placed directly intothe reactor wall, this wall-mounted sensor concept reduces theinfluence on the plasma to a minimum. The resulting completeassembly and the 3D-viewing of the pMRP itself within CSTMicrowave Studio is shown in Fig. 4 (a) and Fig. 4 (b),respectively.

(a) (b)

Fig. 4. Realized simulation models within CST Microwave Studio: (a) Reac-tor wall with flush mounted pMRP within a flange inside a plasma and (b) 3D-viewing of the pMRP based on two separated PCBs and coaxial feeding.

The pMRP is still fed by a coaxial cable and inserted intoa flange with a diameter of 17 mm at the reactor wall. Arequired vacuum bellow is neglected in the simulation model.The pMRP assembly consists of a double layer PCB includingsignal feeding, balun, and a stub at the front serving as a holderfor a second PCB. The single layer of the MRP can be realizedby a second double layer PCB with two printed plane half discscontaining a drilling in the middle, suitable for the stub of thefirst PCB. Hence, no multilayer assembly is necessary for theproduction of this sensor. An additional thin polyvinyl chloride(PVC) film used for the separation of plasma and probe isneglected in the drawing. The final assembly of the pMRPhas a total length of approximately 20 mm without the coaxialfeeding and a diameter at the second PCB of approximately8 mm with a metalized diameter of 6 mm for the half discs.

To avoid additional disrupting resonances, the pMRP ispositioned with an additional space to the inner flange wall.Here, the flange used for the simulations has to be dimen-sioned with a diameter between 11 mm and 17 mm in theobserved frequency range, to minimize additional resonances.Furthermore, the positioning tolerances are limited to a rangeof approximately ±0.5 mm in all directions for reliable results.Otherwise, additional disrupting resonances can be observed.

The resulting resonances for a variation of the plasmaelectron frequency —between 2.5 GHz and 3.1 GHz corre-sponding to a density range between 7.7 · 1016 m−3 and1.2 · 1017 m−3— with ν set to 0.015 ωpe can be seen inFig. 5.

Utilizing the pMRP, one dominant resonance peak isclearly observable, moving to higher frequencies for risingplasma electron frequencies, while the resonance amplitude

2.52.7

2.93.1

1 1.25 1.5 1.75 2

−32

−28

−24

−20

−16

−12

−8

−4

0

Amplitude

/dB

Frequency / GHzfpe

/ GHz

Fig. 5. Simulated reflection coefficient |S11| of the pMRP for a variation ofthe plasma electron frequency between 2.5 GHz and 3.1 GHz, correspondingto a variation of the plasma density.

is decreasing. Hence, the pMRP shows almost the samebehaviour as the MRP. Compared to the MRP, additionalresonances can be seen due to the placement of the setupitself within the reactor wall. If the resonance positions arecompared between MRP and pMRP, it can be seen that theobservable resonance frequency is nearly equal. The small shiftresults from the different dielectric properties of the deployedquartz glass compared to the PVC film. Furthermore, a higherresonance amplitude can be noticed for the pMRP for allsimulated plasma electron frequencies. Hence, the pMRP issuitable for an evaluation of its resonance behaviour for thedetermination of the plasma density.

Fig. 6 shows the simulated magnitude of the electrical fieldfor the MRP and the pMRP at the dominant mode for a plasmaelectron frequency of 2.3 GHz. Here, the electrical field ofthe MRP and the pMRP result in an observable resonancefrequency at 1.21 GHz and 1.22 GHz, respectively.

(a) (b)

Fig. 6. Magnitude of the electrical field for a plasma electron frequency offpe = 2.3 GHz: (a) Viewing of the dominant mode of the MRP coupled intothe plasma at a resonance frequency of Ωl=1 = 1.21 GHz and (b) Viewingof the dominant mode of the pMRP coupled into the plasma at a resonancefrequency of Ωl=1 = 1.22 GHz.

A good coupling into the plasma is clearly observablewithin the simulations. Furthermore, a spatially resolved pen-etration of the electrical field can be seen for both probes.

IV. REALIZATION AND MEASUREMENT

Based on the performed simulations, two prototypes of theprobes —the MRP and the pMRP— are realized as described

Fig. 1: Planar Multipole Resonance Probe

2. Kinetic Model of the pMRPA kinetic model describes the plasma dynamics

as coupled to the probe. The active region is smallerthan the scales λ (mean free path), λs (skin depth),and L (plasma size). The model is Cartesian 1d/3v,with z = 0 denoting the wall and z →∞ the plasma.Linear response theory applies: the electron velocitydistribution f and the potential φ are split into theirequilibrium values f = fM(z,~v) (i.e., Maxwellian)and φ = φ(z) and small perturbations δΦ and fM δf .The equilibrium is given by the Bohm model in formof a differential equation for the static potential φ:

−∂2φ

∂z2=

1√1− 2Φ

− exp(φ). (1)

The perturbation follows a Vlasov equation coupledto Poisson’s equation. In the spectral domain:

−iωδf + ~v ·∇δf − ~v ·∇δφ+∂φ

∂z

∂δf

∂vz= 0, (2)

−∇2δφ = −∫fM δf d3v. (3)

Solving (2) by integration over unperturbed orbits andsubstituting into (3) yields a linear integral equationfor δφ that can be solved numerically.

3. Response spectrum of the pMRPThe spectral response of the probe-plasma system

can be expressed by the complex admittance Y (ω),the ratio of the antisymmetric electrode current I andthe harmonic input signal V :

Y (ω) = −± iω1

V

∫

E±ε0∇φ·d2~f. (4)

The calculations are carried out for a probe withelectrode radius Rs = 3 mm. The dielectric cover isd = 0.3 mm and has a relative permittivity of εr = 4.It monitors a plasma with a density of ni = 1016 m−3

and an electron temperature Te = 4 eV. Fig. 2 showsthe real part of the complex admittance Y of the probeas a function of ω/ωpe. The resonance and its broad-ening by collision-less damping are clearly visible.This is one of the expected kinetic effects.

Fig. 2 Spectral response of the pMRP

4. SummaryIn this work, a kinetic model of the pMRP in

the collision-less low pressure regime was presented.The spectral response of the probe-plasma systemwar found by calculating the complex admittance.This new model covers kinetic effects and overcomesthe limitations of the cold plasma model.

5. AcknowledgmentSupport by the German Research Foundation via

project DFG 360750908 is gratefully acknowledged.

References[1] M. Friedrichs, J. Oberrath, EPJ 5, 7 (2018)[2] C. Schulz, I. Rolfes, IEEE SAS, 263 (2014)[3] J. Oberrath, R.P. Brinkmann, PSST 23, 045006

(2014)

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Thursday May 16th

Diagnostics

Diagnostics invited

Precise and sensitive absorption spectroscopy using Fourier transform and Ver-nier spectrometers based on optical frequency combs.

L. Rutkowski1,2, A. C. Johansson1, P. Maslowski3, J. Morville4, A. Foltynowicz1

1 Department of Physics, Umeå University, 901 87 Umeå, Sweden 2 Univ Rennes, CNRS, IPR (Institut de Physique de Rennes)-UMR 6251, F-35000 Rennes, France

3 Institute of Physics, Nicolaus Copernicus University in Toruń, ul. Grudziądzka 5, 87-100 Toruń, Poland 4 Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, Villeurbanne F-69622, France

1. Introduction

Optical frequency combs provide broad bandwidth, high spectral resolution and brightness. Because of these fea-tures, precision frequency comb spectroscopy is the tech-nique of choice when fast acquisition, high signal-to-noise ratio, multi-species detection or simultaneous measure-ment of entire absorption bands are required. Among the different spectrometers designed to benefit from these ad-vantages, Fourier transform mechanical spectrometers (FTS) [1-3] and Vernier spectrometers [4,5] have proven very effective and offer complementary advantages.

2. Comb-based Fourier transform spectroscopy Compared to Fourier transform spectroscopy using unco-

herent light sources such as lamps (FTIR), comb-based FTS offer higher signal-to-noise ratios in shorter acquisi-tion time. It also enable the measurement of high resolution spectra with compact interferometers when using the sub-nominal resolution method [1, 2].

The FTS nominal resolution is limited to the inverse of the maximum optical path difference of the acquired inter-ferogram. Using an optical frequency comb allows to over-come this limitation when the nominal resolution of the FTS is matched to the comb mode spacing, as depicted in Fig. 1. The instrumental line shape (ILS) affecting each comb mode is a sinc function whose zero-crossings are po-sitioned at the neighboring modes. Thus, the intensity of each spectral element is equal to the corresponding comb mode intensity, without any contribution from the neigh-boring modes, and the frequency is given by the comb mode frequency. The spectral sampling can be increased by interleaving spectra measured with different comb mode positions (see the different colors in Fig. 1). Using this technique high precision spectroscopy of CO2 in the near infrared was performed [3].

3. Vernier spectrometer in continuous regime Vernier spectroscopy is a cavity-enhanced technique

where the cavity resonances are used to analyze the comb

spectrum [4, 5]. It relies on a coupling of the frequency comb with a cavity whose free spectral range (i.e. the spac-ing of the resonances) is slightly mismatch from the comb mode spacing, as illustrated in Fig. 2. This creates a Moiré pattern in frequency domain and allows transmitting groups of modes, named Vernier orders, whose spectral width (i.e. the spectral resolution of the Vernier, V) is given by the frequency mismatch and the cavity finesse.

Using a standard grating at the cavity output to separate spatially the groups of mode, one of the Vernier order can be selected and its intensity measured as the central fre-quency of the order is swept across the comb spectrum by changing the cavity length. When at least 4 comb modes are comprised in the linewidth of the Vernier order, the Ver-nier is in the continuous regime, where the comb structure is entirely washed away. This makes this spectrometer im-mune to frequency-to-amplitude noise conversion, yet the frequency calibration provided by the comb is lost, requir-ing an external calibration.

4. Conclusion Comb-based FTS and Vernier spectrometer offer comple-

mentary advantages: FTS enables high frequency precision and provides absorption line profile with precision beyond the Voigt profile [3] while Vernier offers a smaller and ro-bust implementation, well suited for field application.

References [1] P. Masłowski, et al., Phys. Rev. A 93, 021802 (2016). [2] L. Rutkowski, et al., J. Quant. Spectrosc. Radiat. Transf. 204,

63 (2017). [3] A. C. Johansson, et al., CLEO: Science and Innovations, 2018,

San Jose, United States. OSA, paper STu3P.6. [4] L. Rutkowski, J. Morville. Opt. Letters, 39(23), 6664 (2014). [5] L. Rutkowski, J. Morville. J. Quant. Spectrosc. Radiat. Transf.,

187, 204 (2017).

Fig. 1. Sub-nominal resolution method pictured in the frequency domain. Comb modes: vertical bars; ILS: instrumental lineshape.

Fig. 2. Vernier filtering in continuous regime in frequency do-main. Red vertical bars: transmitted comb modes; dashed grey vertical bars: filtered comb modes; grey solid line: cavity trans-mission curve; blue solid line: Vernier transmission curve.

101

oral Diagnostics

Mid-infrared direct frequency comb spectroscopy of plasma processes

N. Lang1, A. D. F. Puth1, S.-J. Klose1, G. Kowzan2, S. Hamann1, J. Röpcke1, P. Masłowski2, J. H. van Helden1

1Leibniz Institute for Plasma Science and Technology (INP), Greifswald, Germany 2Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Toruń, Poland

1. IntroductionOver the last two decades, chemical sensing using mid-

infrared laser absorption spectroscopy (MIR-LAS) in the vibrational fingerprint region (3-20 µm) using quantum cascade lasers (QCLs) has become an established diagnos-tic tool in plasma science [1]. However, the investigation of complex plasmas is hampered by a lack of broadband tunability of the laser sources and the limited time resolu-tion of the techniques currently available to study processes such as plasma-surface interactions. Recent developments in laser physics allow overcoming the drawbacks of limited broadband capability and time resolution by using optical frequency combs (FCs) as light sources in MIR-LAS. A femtosecond MIR frequency comb enables the fast meas-urement of multiple species simultaneously, because it emits instantaneously a spectrum, which consists of tens of thousands of discrete, equally spaced frequency lines that are spread over hundreds of nanometers and are frequency stabilized to a precision of better than 10-12. Consequently, direct frequency comb spectroscopy (DFCS) in the MIR would essentially replace the need for simultaneous meas-urements using thousands of precise narrow lasers and hence has the potential to become the ultimate tool for pre-cision broadband spectroscopy. We report on MIR-DFCS as a novel plasma diagnostic applied to spectroscopic in-vestigations of plasma nitrocarburizing processes.

2. Preliminary resultsActive screen plasma nitrocarburizing (ASPNC) is an

advanced technology for the hardening of steel compo-nents using pulsed N2-H2 plasmas with an active screen made of solid carbon to produce carbon-containing species, which support the generation of anti-corrosive layers of high quality. To gain further insights in the treatment pro-cess of the materials, spectroscopic investigations are car-ried out in a specially designed and downscaled plasma ni-triding reactor, PLANIMOR, based on an industrial scale ASPNC reactor [2]. With the wide spectral coverage of broadband DFCS many molecular species can be detected simultaneously with high sensitivity and time-resolution yielding comprehensive data on their kinetics in the plasma and their interactions with a surface.

Our frequency comb has a repetition rate of 250 MHz and operates around 3.2 µm (2900-3500 cm-1). The broad-band transmission was analysed with a special kind of Fou-rier transform spectrometer (FTS), which has an optical path difference that is precisely matching the repetition rate

of the FC. Consequently, the spectra are free from distor-tions caused by the instrumental line shape, which usually limits the resolution of FTS [3]. The spectral coverage of the comb enables to record complete rovibrational bands of CH4, C2H2, C2H6, HCN, and NH3 in a single measurement with sub-nominal resolution. Furthermore, varying the rep-etition rate of the comb, the Doppler-limited line profiles of individual transitions could be analyzed with unmatched precision. An example of a spectrum obtained for a gas mixture of 10 sccm H2 and 10 sccm N2 at a power of the screen plasma of 100 W is shown in Fig. 1. We will discuss the influence of process parameters, such as pressure, screen plasma power, and gas mixture, on the concentra-tions of multiple key species.

References [1] J. Röpcke, P. B. Davies, S. Hamann, M. Hannemann,

N. Lang, J. H. van Helden, Applying quantum cascadelaser spectroscopy in plasma diagnostics, Photonics 3,45 (2016).

[2] S. Hamann, K. Börner, I. Burlacov, H.-J. Spies, J.Röpcke, Spectroscopic Investigations of Plasma Ni-triding and Nitrocar-burizing Processes Using an Ac-tive Screen: A Comparative Plasma Chemical Study ofTwo Reactor Types, Contr. Plasma Phys. , 689(2015).

[3] P. Maslowski, K. F. Lee, A. C. Johansson, A. Khoda-bakhsh, G. Kowzan, L. Rutkowski, A. A. Mills, C.Mohr, J. Jiang, M. E. Fermann, A. Foltynowicz, Sur-passing the path-limited resolution of Fourier-trans-form spectrometry with frequency combs, Phys. Rev. A , 021802 (2016).

Fig. 1 An example of a spectrum obtained for a gas mixture of 10 sccm H2 and 10 sccm N2 at a power of the screen plasma of 100 W. Upper part: measure-ment; lower part: simulation.

102

Diagnostics invited

Investigating CO2 plasma kinetics with in situ infrared absorption Fourier trans-formed spectroscopy and complementary diagnostics

O. Guaitella1, A.S. Morillo-Candas1, M. Grofulovic2, R. Engeln3, T. Silva2, V. Guerra2

1Laboratoire de Physique des Plasmas, Ecole Polytechnique-CNRS-Univ Paris-Sud-UPMC 91128Palaiseau, France

2IPFN, Instituto Superior Tecnico, Universidade de Lisboa 1049-001 Lisboa, Portugal 3Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

1. General

Many approaches are currently being explored to try to reduce the CO2 emissions responsible for global warming. The scale of the problem is so vast that only a combination of several technologies seems to be able to provide solu-tions. Among them, the use of cold plasma is certainly a very promising approach. Recycling of CO2 can be consid-ered in different gas mixtures for methanation (CO2+H2), or gas reforming (CO2+CH4) purposes for instance. The ac-tion of the plasma on the CO2 molecule can then be to dis-sociate it by direct electronic impact or by vibration pump-ing from the asymmetric mode, or simply to make the CO2 reactive with other species by sufficiently exciting the vi-bration levels. In order to design efficient plasma reactors for CO2 conversion it is therefore necessary to understand and control the energy transfers between vibration levels and the reactivity of excited molecules. It is also essential that the oxygen atoms resulting from the dissociation of CO2 do not recombine with CO, which is very detrimental to the energy efficiency of the process. To control the reac-tivity of molecules and radicals produced, CO2-containing plasmas are often coupled to catalytic surfaces whose im-pact on the plasma kinetics is often challenging to study.

2. Methodology

The purpose of our work was therefore first to study ex-perimentally and by 0D kinetic modeling the kinetics of a pure CO2 plasma [1]. For this purpose, the vibration tem-perature of the different CO2 and CO modes was measured by in situ time resolved Fourier Transformed InfraRed spectroscopy (FTIR) [2], but also with Raman scattering [3], and by TALIF and actinometry to determine the den-sity of oxygen atoms. In a second step, these different di-agnostics were used in the same plasma but with three dif-ferent types of experiments:

- in the presence of materials with a large specific surface area to study their impact on the CO2 plasma kinetics

- without gas flow (referred to as "closed reactor" in the following) to study the evolution of the gas mixture during a series of plasma pulses

- in the closed reactor but in the presence of O2-labelled isotopes (18O2) to study the exchange of oxygen atoms be-tween CO and CO2

3. Experimental setups The glow discharge is initiated in a glass tube of 2 cm

inner diameter and 23 cm in length. The infrared beam of the FTIR Bruker Vertex V70 passes through the plasma re-actor. Measurements with gas flow (7.4 sccm of pure CO2) in the presence of SiO2 glass fibers of large specific surface area were performed with 5 ms and 10 ms plasma pulses between the pulses. Measurements in closed reactor were performed with pulses from 0.5 to 50 ms and similar inter-pulse times.

4. Results

The vibrational excitation of CO and CO2 increases when the glass fibres material is inserted into the plasma reactor. This material considerably increases the surface in contact with plasma leading to a full recombination of the oxygen atoms. In doing so, the quenching of the vibration states in the gas phase is reduced, resulting in an increase in the vibration temperature.

Measurements in a closed reactor have made it possible to constrain the cross section for direct electron impact dis-sociation by analyzing the characteristic time of CO pro-duction during the first plasma pulses.

Isotope exchanges have made it possible to study the role of wall reactions and the importance of CO+O recombina-tion. References [1] Grofulović, M., Silva, T., Klarenaar, B. L. M., Morillo-

Candas, A. S., Guaitella, O., Engeln, R., Guerra, V. (2018). Kinetic study of CO2 plasmas under non-equi-librium conditions. II. Input of vibrational energy. Plasma Sources Science and Technology, 27(11), 115009

[2] Klarenaar, B. L. M., Morillo-Candas, A. S., Grofu-lović, M., Van de Sanden, R., Engeln, R., & Guaitella, O. (2018). Excitation and relaxation of the asymmetric stretch mode of CO 2 in a pulsed glow discharge. Plasma Sources Science and Technology

[3] Klarenaar, B. L. M., Grofulović, M., Morillo-Candas, A. S., van den Bekerom, D. C. M., Damen, M. A., Van De Sanden, M. C. M., Engeln, R. (2018). A rotational Raman study under non-thermal conditions in a pulsed CO2 glow discharge. Plasma Sources Science and Technology, 27(4), 045009.

103

Thursday May 16th

Simulations

Simulations invited

Time-Slicing in Multi-Physics Modeling: Using Hybrid Methods in Low Temperature Plasma Simulations to Address

Disparate Time Scales

Mark J. Kushner

University of Michigan, Dept. Electrical Engineering and Computer Science, Ann Arbor, MI 48109-2122 USA mjkush@umich.edu

Computational low temperature plasma physics has

many goals. For example, at one extreme is a special-ized simulation intended to address a particular physical phenomenon in a fundamental study of plasma transport. Petascale (exoscale?) computing using non-reactive gases with simulation times of days to weeks would be an ac-ceptable method of investigation. At the other extreme, industrial computing of the type used to design plasma processing equipment requires desktop solutions capable of addressing many ions, tens of species, hundreds of re-actions, surface chemistry, radiation transport, electro-magnetics and circuitry; and execute overnight (if not faster). Is a petascale solution for plasma transport that does not include radiation transport and photoelectron emission any more correct or less wrong than a desktop solution that does not fully resolve a sheath?

These are unanswerable questions because these two computational approaches have different metrics for goodness – they are addressing different problems. The more multi-physics you have in the fundamental simulation, the better the answer will be. The more fundamental the plasma kinetics you have in the mul-ti-physics simulation, the better the answer will be. Truth is somewhere in between – and that in between re-volves around resolving time scales.

The disparate time scales of multi-physics simulations is a continuing challenge in developing models intended to address applications from a fundamental perspective. For example, in a typical microelectronics processing reactor, the chemistry on surfaces in contact with the plasma requires many to tens of seconds to come into a steady state (if it ever comes into a steady state). Simul-taneously resolving plasma transport with sub-ns time steps tightly coupled with surface chemistry is computa-tionally intractable in the absence of exoscale computing. Some compromise is required when including surface chemistry in a fundamentals based plasma kinetics model.

A similar quandary is faced in including kinetic effects in multi-dimensional plasma transport in multi-physics models. The computational challenges of solving Boltzmann’s equation in 3-dimensions and time simulta-neously with transport equations for electrons, a dozen ions and tens of neutral species are well known. In the absence of highly specialized codes or exoscale compu-ting, some compromise is required to capture the domi-nant physics while having a simulation that takes a rea-

sonable amount of time. One compromise that attempts to delicately tread on

that middle ground is a combination of time-slicing and hybrid techniques. Time-slicing refers to separating physical processes that have different fundamental time scales into different modules. The simulation then ex-changes information between the modules on time scales appropriate for the physics of that module. The steady state is defined when time derivatives in all modules trend towards zero. Hybrid methods refer to, for example, separately solving Boltzmann’s equation and it moments to produce transport coefficients that increase the accura-cy of fluid approaches while not limiting the fluid to the same time constraints as the kinetics. The veracity of time-slicing and hybrid methods is in part determined by how tightly one couples the timescales and physical pro-cesses.

In this talk, challenges will be discussed for imple-menting time-slicing and hybrid techniques in mul-ti-physics plasma models. Some of the issues to be ad-dressed are: Are the expectations for multi-physics time-slicing and

hybrid techniques for V&V different than for pure transport codes?

How tightly should physics modules be coupled? How can transients be addressed in time-slicing? What is the minimum set of information that needs to

be sent between modules? Are time-slicing and hybrid-techniques archaic in a

world of exoscale computing? How can flexibility (e.g., addressing different plasma

sources) be expediently built into fundamental codes?

107

invited Simulations

“Passing XSAMS”: Towards a More Efficient Handling of Plasma Input Data

Jan van Dijk1

1Elementary Processes in Gas Discharges, Department of Applied Physics, Eindhoven University of Technology

1. Introduction

There is an increasing awareness in our plasma physics

community that the accuracy of simulation results de-

pends as much on the quality of the input data as on the

quality of the simulation tool itself. The realization of a

high-quality input data set is a multi-faceted problem. The

obvious and most important problem is that experimental

results are not always available, a fact that is mitigated by

the increased viability and reliability of first-principles

calculations [1]. A second issue is the efficient retrieval of

data that are available. Today, this task is easier than ever

thanks to websites such as NIST, VAMDC and LXCat [2].

2. Data Dissemination and Preprocessing Tools

That said, there is still a lot left to be desired. Firstly, in-

put data commonly need preprocessing. As an example,

interaction potentials must typically be integrated into dif-

ferential cross sections or collision integrals and the tools

for achieving such tasks are not generally available. Sec-

ondly, there is no proper method in place for the dissemi-

nation (publication and retrieval) of complete plasma-

chemical data sets. The result is that end-users who wish

to reproduce, verify or extend previous studies are usually

forced to manually re-create entire chemistry models: a

laborious and error-prone task. Finally, even when data

sets are made available electronically, this is usually done

in formats that are specific for the software that was used

for the simulation [3]. Custom parsers must then be devel-

oped to translate the data into the format that is required

by the targeted simulation tool, another source of poten-

tial errors and sure frustration.

3. “Passing XSAMS”

In this contribution we report on the intermediate re-

sults of research project “Passing XSAMS”, in which

Eindhoven University of Technology (TU/e), the eScience

Center Amsterdam and TU/e spin-off company Plasma

Matters B.V. are working on practical plasma data ex-

change formats and various data processing tools [4].

After a brief introduction of the JSON-based data for-

mat we will present our MagnumPI tool. This software of-

fers accurate and efficient methods for integrating a vari-

ety of inter-molecular model potentials or tabulated po-

tential data into differential cross sections, transport cross

sections or collision integrals. It builds on a combination

of the algorithms of Colonna et al. and Viehland et al. [5],

handles orbiting conditions and is able to handle the

tricky case of resonant charge exchange processes. The

free (C++) software can be deployed in traditional ways

(as a collection of a shared library and programs), or us-

ing web technologies.

The demonstration of the MagnumPI potential integra-

tor will be used to guide a more general yet practical dis-

cussion about the realization of a more efficient plasma

input data workflow.

[1] Bartschat, J. Phys. B 51(13) (2018)

[2] Pitchford et al. Plasma Process. Polym., 14 1600098.

[3] Koelman et al. Plasma Process. Polym., 14 1600155

[4] https://www.esciencecenter.nl/project/passing-xsams

[5] Viehland et al. The Journal of Chemical Physics 144

074306 (2016)

108

Simulations invited

Modeling streamers and related transient discharges

U. Ebert1,2

1CWI Amsterdam, 2TU Eindhoven, The Netherlands

The streamer process The late Dave Sentman, one of the fathers of the research on atmospheric sprite discharges, called streamers the ele-mentary particles of discharge physics. Streamers are the first nontrivial stage of electric breakdown of gases, driven by the self-consistent enhancement of the electric field at their tips; they are nonlinear structures, and very far from equilibrium. Streamers appear at many stages in nature and technology. In nature they appear in the streamer corona of lightning leaders, determining leader propagation and leader attach-ment to objects; they are directly visible as gigantic jets and sprite discharges above thunderstorms, and potential light-ning damage also has to be estimated when exploring other planets in the solar system. Streamers and leaders are de-termining processes in high voltage technology, in particu-lar, in switch gear for electricity nets where presently a re-placement of the extreme greenhouse gas SF6 is sought for, and they appear in plasma medicine, plasma agriculture, air purification, plasma assisted combustion and a plethora of other technological fields. Streamer modeling faces a whole list of challenges: 1. The past decades have seen many 1.5 D and 2D simu-

lations of streamers. A 2D calculation assuming cylin-drical geometry is appropriate for single streamers, but in almost all cases streamers come in large numbers and interact with each other or with objects like elec-trodes or dielectric bodies, requiring fully 3D compu-tations.

2. “Individual” electrons can determine the growth pro-cess by their rare appearance or by their high energy. Therefore common fluid approaches need to be modi-fied to include single particle dynamics in cases of streamer inception, streamer branching and electron run-away.

3. Modeling more than a couple of streamers in 3D with a fluid model is currently not yet achievable. However, streamer coronas can consist of ten thousands of streamers. To model them requires steps of model re-duction to simplified tree models (also known as so-called fractal models). More microphysics need to be included into currently available tree models.

4. Streamers trigger a chain of chemical reactions, even-tually they can heat the gas sufficiently to transit into leaders, and the evolving discharge can also interact with the gas motion. This requires proper chemical models and numerical or analytical means to deal with the widely varying time scales.

Goal of the talk: I will review the state of our 3D modeling of emergence, propagation, branching and interaction of streamers, and I will try to identify open questions. On the computational side much progress in the field has been made by Jannis Teunissen who joined my Multiscale Dynamics group again in a tenure track position in October 2018. Find our recent work on the topic on https://homepages.cwi.nl/~ebert/publications.shtml http://teunissen.net/wiki/doku.php?id=publications:start PS: Hot topics in lightning physics are the stepped propa-gation of negative lightning leaders and the mechanism how they emit terrestrial gamma-ray flashes, but probably out of scope of the present meeting.

109

Index

Cech, J., 53Simek, M., 6

Achard, J., 21Adamovich, I., 8, 9, 94Ahr, P., 27Ali Cherif, A., 60Alicherif, M., 94Alvarez Laguna, A., 72Alves, L. L., 75, 86Anesland, A., 45Arnas, C., 87Astafiev, A., 16Aubert, X., 21, 26Audonnet, S., 34Awakowicz, P., 74

Boke, M., 50Babaeva, N., 89Baude, R., 36Bauville, G., 21Becker, M., 11Benedikt, J., 50Biermann, H., 52Biskup, B., 50Bogaerts, A., 76, 84Bonaventura, Z., 56Bongers, W., 32Booth, J., 23, 73Bouef, J., 68Bougdira, J., 3Bourdon, A., 71, 72, 85, 88Bove, A., 38Bradley, M., 31Brandenburg, R., 41Briefi, S., 51Brinkmann, R. P., 35, 74, 78, 98Brochhagen, M., 50Bronold, F., 5, 79Bruggeman, P., 49

Bruggemann, P., 46Burlacov, I., 52Butterworth, T., 25

C. Ding, 59Cada, M., 24Camenen, Y., 36Carbone, E., 12, 30Cartry, G., 45Celik, S., 34Ceppelli, M., 54, 97Chabert, P., 71, 72, 85, 88Chami, A., 87Chang, H., 19Charoy, T., 68, 71, 85Chatterjee, A., 23, 73Chng, T., 8Chng, T. L., 94Chudjak, S., 18Cherigier-Kovacic, L., 13Couedel, L., 31, 87Cvelbar, U., 14Czarnetzki, U., 27–29

D’Isa, F., 12Dalke, A., 52Damen, M., 44David, P., 36de la Rosa, M., 48De Oliviera, N., 73De Pascale, O., 38de Poucques, L., 3Desecures, M., 3Dilecce, G., 38, 54, 97Ding, C., 94Dittmeyer, R., 62Dogariu, A., 10Dong, L., 81Donko, z., 70Dostal, L., 53

110

Doveil, F., 13Drag, C., 23, 73Duluard, C., 26Dussart, R., 15

Ebert, U., 109El Farsy, A., 3Engeln, R., 44, 103Eremin, D., 35, 69, 74Escarguel, A., 36

Fan, W., 81Fantz, U., 12, 51Fehske, H., 5, 79Fleury, M., 21Foltynowicz, A., 101Friedl, R., 51Friedrichs, M., 35, 98Frhler, C., 51

Gangwar, R., 22Gans, T., 37, 93Garcia-Caurel, E., 42Gazeli, K., 21Gianella, M., 37Giovangigli, V., 82Godyak, V., 57Goldberg, B., 8, 10Goncalves, D., 86Gong, J., 35Gonzalez-Fernandez, V., 36, 48Grutzmacher, K., 48Grofulovic, M., 103Grosse, K., 7Guaitella, O., 23, 42, 61, 73, 103Guerra, V., 61, 86, 103Gupta, S., 22

Hage, D., 44Hamann, S., 52Hartmann, P., 70He, Y., 77Hecimovic, A., 12, 95Heinß, J., 74Held, J., 7, 95Hemke, T., 96Hendrickx, H., 32Hennecke, A., 17

Hoder, T., 11, 56Hoffer, P., 6, 11, 56Huang, Q., 47Hubicka, Z., 24

Inada, Y., 43Ingels, R., 76Iseni, S., 15

Jahanbakhsh, S., 41Janda, M., 17, 80Jardali, F., 76Jelonnek, J., 62Jensen, M., 76Johansson, A., 101Johnson, E., 82Juhasz, Z., 70Jurov, A., 14

Kai, M., 7Kapran, A., 55Kasri, S., 21Kemaneci, E., 77Kerdjoudj, H., 34Khan, V., 55Kim, D., 19Kim, S., 19Klose, S., 37Kogut, D., 45Kokovin, A., 83Kondeti, V., 49Korolov, I., 96Kozhevnikov, V., 83Kozyrev, A., 83Krcma, F., 18Kroesen, G., 30Kudrna, P., 24, 33Kuhfeld, J., 28Kushner, M., 107Kusyn, L., 52, 56

Lutke Stetzkamp, C., 27Layet, J., 45Lazzaroni, C., 21Lefaucheux, P., 15Leggate, H., 90Lepikhin, N., 29Leys, C., 14

111

Limburg, A., 20Link, G., 62Lino da Silva, M., 86Liu, Y., 96Loffhagen, D., 11Lombardi, G., 21Lopaev, D., 73Lucken, R., 71, 88Luggenholscher, D., 28, 29

Magin, T., 72Manfred, K., 23, 37Marques, L., 86Martini, L., 44, 54, 97Maslowski, P., 101Massot, M., 72Merk, F., 51Michaud, R., 15Miles, R., 10Modic, M., 14Moreno, J., 31Morillo-Candas, A., 61, 103Mussenbrock, T., 96Myshkin, V., 55

Nahon, L., 73Naidis, G., 89Naphade, M., 8Navarrete, A., 62Navratil, Z., 53Nayak, G., 46Nijdam, S., 20Nikiforov, A., 14Norman, H., 37Novikova, T., 82

O’Connell, D., 37Oberrath, J., 35, 98Orel, I., 34, 94Orlac’h, J., 82Orr, K., 9

Pasquiers, S., 21Peeters, F., 32Pinchuk, M., 16Pinho, N., 86Pintassilgo, C., 86Pokrovskiy, G., 58, 94

Popov, N., 94Poye, A., 36Press, S., 37Prukner, V., 6Puth, A., 52Perez, C., 48

Rafalskyi, D., 45Rakhimova, T., 73Rasek, K., 5, 79Reniers, F., 84Reuter, S., 10Richtig, G., 37Riedel, F., 37Righart, T., 32Ritchie, G., 4, 23Roca i Cabarrocas, P., 82Rutkowski, L., 101Rpcke, J., 52

S. A. Shcherbanev, 59S. M. Starikovskaia, 59Sackers, M., 63Sadeghi, N., 30, 46Santos Sousa, J., 21Schmedt, C., 62Schmidt, J., 6Schmidt-Bleker, A., 37Schulz-von der Gathen, V., 15, 95Schulze, J., 96Scotoni, M., 54, 97Senesi, G., 38Shcherbanev, S., 94Shu, Z., 47Silva, A., 61Silva, T., 103Simek, M., 11Simeni, M., 9Simonin, A., 45Sitnikov, A., 83Slikboer, E., 42Sobota, A., 42Soldatov, S., 62Spies, H., 52Sremauocki, I., 14Sretenovic, G., 15Srivastava, R., 22Starikovskaia, S. M., 8, 34, 58, 60, 94

112

Stefanovic, I., 74Stepanova, O., 16Sahel, P., 53

Tahri, L., 45Tallaire, A., 21Tang, Y., 9Tavant, A., 71, 85, 88Tejero-del-Caz, A., 61, 75, 86Thiemann-Monje, S., 63Tichy, M., 24, 33, 55Tosi, P., 54, 97Tsankov, T., 27Tuharin, K., 33Turek, Z., 24Turner, M., 67, 90

Valenta, J., 53van ’t Veer, K., 84van de Sanden, M., 32van de Schans, M., 20van de Steeg, A., 25, 32van Dijk, J., 108van Helden, J., 37, 52, 102van Rooij, G., 25, 32van Veldhuizen, E., 30Voloshin, D., 73von Keudell, A., 7, 63, 95

Wang, C., 98Wilczek, S., 35Wolf, A., 32

Xiong, L., 47Xiong, Q., 47Xu, L., 74

You, S., 19

Zanaska, M., 33Zanaska, M., 24Zemnek, M., 53Zhang, T., 82Zhang, Y., 10Zyryanov, S., 73

113

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