Plasma pulsed irradiation preparation of lead coated photocathodes Robert Nietubyć on behalf of the Collaboration Narodowe Centrum Badań Jądrowych Świerk

Plasma pulsed irradiation preparation of lead coated photocathodes

Robert Nietubyćon behalf of the Collaboration

Narodowe Centrum Badań Jądrowych Świerk

National Centre for Nuclear Studies

http://www.ipj.gov.pl/

http://www.google.se/url?sa=i&rct=j&q=eucard2&source=images&cd=&cad=rja&docid=v1XVcoIzwtwryM&tbnid=xj0CbfzNL0wW0M:&ved=0CAUQjRw&url=https%3A%2F%2Fespace.cern.ch%2FEuCARD%2F2013%2Fannual-and-KickOff-meeting%2FPICTURE%2520LIBRARY%2FForms%2FDispForm.aspx%3FID%3D193%26RootFolder%3D%2FEuCARD%2F2013%2Fannual-and-KickOff-meeting%2FPICTURE%2520LIBRARY&ei=vF3cUZrxGYaPswbw9oD4BQ&bvm=bv.48705608,d.Yms&psig=AFQjCNGJoL9WJobB427V0JaHTrV5r0_8TA&ust=1373482786294101

Workpackage: WP12 Innovative RF technologiesTask: WP12.5 Photocathodes

Milestone

MS80 Demonstrated operation of improved deposition system M30Report on samples characterisation NCBJ

Deliverables

D12.8 Optimised procedure for microdorplets flattening with an UV laserM36Report NCBJ

D12.9 Pb/Nb plug photocathodes measurements and characterization. M42Report HZDR (+ HZB + DESY + NCBJ)

Pb photocathode deposition for improved performance of SRF guns. - deposition improvement, - post-deposition treatment, - Q and QE measurements

Eucard commitments

Eucard2 - 2nd Annual Meeting, 22.04.2015, Barcelona


Motivation

s.c. metals

Tc Pb La V Hg

Tc[K] 7.8 7.2 5.9 5.3 4.2

does not exist on Earth

too reactive

superconductor of the 1 st type

critical H 80 mT

critical T 7.2 K

work function 3.95 eV

High average power FEL Fully sc injectorThin lead film

J. Sm

edle

y

• low probable phenomena

• diluted samples

• special applications

• industrial applications

• 2.5 µJ at 266 nm gives 100 nC at

QE = 5·10-4, 25 W at 1st harmonic YAG

• 100 kHz – ranged repetition rate

• 10 A of average current

too low TK

The goal is to build a Nb injector with the superconducting cathode made of lead for use in CW or near-CW operated high average current accelerators

•Calculations showed reasonable QE•Experiment supported the calculation results•Vacuum arc deposited film showed the best performance

J. Smedley Eucard2 - 2nd Annual Meeting, 22.04.2015, Barcelona


A number of QE, RF quality measurements as well as injector operation have been performed till now and gave with reasonable results but still rather far from what we need for the cw and long pulse operations of XFEL. Particularly the coating was still not satisfactory:•uniformity•thickness•droplets and other roughness•adhesion

There is a contradiction between having a thick film and having a film without droplets because filtering reduces the plasma flow and deposition requires the time longer than the lifetime of the arc cathode

Imperfections until 2014



straight UHV arc coating followed with rod plasma recrystalisation Filtered arc depositon

2014, two ways

Eucard2 - 2nd Annual Meeting, 22.04.2015, BarcelonaEucard2 - 2nd Annual Meeting, 22.04.2015, Barcelona


Deposition system reconstruction to find better compromise between thickness and low micro-droplets population:

Post–deposition flattening:

plasma ion pulses irradiation

laser irradiation

annealing

• UHV non-filtered arc

• UHV filtered arc

• Evaporation

pre-deposition Nb substrate preparation

polishing BCP EP

Quest for film improvement



Coating method

Deposition rate

C, O and N contents (wt %) EDS results

Pb layers’ morphology and continuity;SEM observations

Evaporation 60 nm/min

Pb ≈ 90 %C ≈ 3 %O ≈ 3 % N ≈ 2 %

UHV filtered arc deposition

200 nm/min Pb ≈ 93 %C ≈ 2 %O ≈ 1 %

UHV non-filtered arc deposition + remelting in pulsed plasma ions

3000 nm/min. Pb ≈ 94 %C ≈ 2 %O ≈ 1 %

Semi-spherical extrusions of diameters < 5 µm density <50mm-2 ,

Spherical extrusions of diam. up to 40 µm density <50mm-2 ,

Numerous massive craters can be flattened at a cost of layer perforation and discontinuity

Results



as deposited after 2 pulses after 6 pulses

Plasma over-irradiated film



Question:Is it possible to melt and flatten the droplets with plasma pulses 1.5 J/cm2 , 1 μs ? How thick the Pb must be to avoid perforation?

Method: Time dependent measurements, 2D FEM calculation solving the heat transfer equation (FEMM)

Model:•Pb/Nb 10 μm•Various morphological features

Heat transfer calculations



18.04.23

1.0 µs, pulse end

1.1 µs

1.3 µs

1.6 µs

2.0 µs

2.5 µs

3.0 µs

Heat transfer movie

To be continued…


18.04.23

3.75 µs

7.0 µs

8.5 µs

10 µs

13 µs

19 µs

30 µs

Heat transfer movie


Melted material ranges in depthdroplet 10 µm * flat area 7 µm

Liquid state duration at 3 µmdroplet 6.5 µsflat area 6.5 µs

Liquid state duration at the surface droplet 13 µsflat area 8 µs

Boiling affects the material down to droplet 2 µmflat area 1 µm

during about 1 µs

*) droplet height is 5 µm

Conclusions from heat transfer calculations


8

6

4

2

0

-2

-4

-6

dept

h [µ

m]

12840t [ µs]

mel

ting

flat

surf

ace

vap

oris

atio

n fla

t su

rfac

e m

eltin

g dr

ople

t v

apor

isat

ion

drop

let

melting and vaporisation fronts

droplet

bulk


2000

1500

1000

500

T [

K]

20151050y [um]

flat surface small droplet

2.5s

7.0s

19s

flat

surf

ace

1.0s end of pulse

Interpretation

Material forming a droplet is hotter than the material under a flat surface during 19 µs

Material under the droplet is colder than that under the flat surface down to 15 µm

Plasma pulses applied to the film thicker than 10 µm can flatten the droplets and save the complete lead coverage



1 plasma pulse of 1.8 J/cm2

15 µm Pb underway…

Deposition and plasma pulsing

Too thin and thus perforated

8 µm Pb

UHV arc deposition

X 500 X 1000

12 µm Pb

1 plasma pulse of 1.5 J/cm2

2 pulses of 1.5 J/cm2

3 pulses of 1.5 J/cm2

X 1000 X 1000 X 2000

• Distinct areas of perforations size of which did not exceed 10 µm

• non-coated area amounting to 3-5% of the total layer surface.

20 µm20 µm

20 µm



20 µm Eucard2 - 2nd Annual Meeting, 22.04.2015, Barcelona

Current density from a thin layer Nb-Pb cathodes reached by different deposition methods

HZDR

Cathode preparation

Emax

[MV/m]

Imax

[nA/cm2]

Emin

[MVm]

Imin

[nA/cm2]

UHV filtered arc200 1600 30 140

UHV filtered arc + conditioning

200 400 30 106

UHV non-filtered arc + plasma pulses

200 294 30 18

Evaporated Pb layer 200 190 30 30

Evaporated and coditioned Pb layer

200 200 30 28

Breakdown current measurements




Measurement setupWill be used for samples diagnostics right after deposition:•Breakdown current •Quantum efficiency vs hvFurnished with•Xenon lamp 150 W•Czerny – Turner monochromator with diffraction grating 1200 lines/mm optimised for 250 nm•calibration photodiode•picoampermeter 10 pA – 1 µm

Diagnostic chamber in construction

MEASLINE, Janusz BudziochEucard2 - 2nd Annual Meeting, 22.04.2015, Barcelona


Plug with evaporated film

Niobium plug with Pb evaporated on the tip

Elec

tron

gun

with

the

plug

Vertical test stand at DESYD. Kostin, J. Sekutowicz

movable input antenna

cavity probe

Evaporated layer, smooth but not adherent and not tight




4. SRF Gun Cavity Vertical Cryostat Tests b. recent results4. SRF Gun Cavity Vertical Cryostat Tests b. recent results

1.6-cell SRF-gun with Pb-coated plug and uncoated plug ,T=1.8K1.6-cell SRF-gun with Pb-coated plug and uncoated plug ,T=1.8K

19

Degradation at 32 MV/m to red curve. After warm-up to 100K performance recovers to yellow curve.

D. Kostin, J. Sekutowicz, Last Pb Thin Film Cathode SRF Gun Tests, April 8th, 2015


Performed works which have not been described here:

Filtered arc coated layers were in parallel tested at HZDR for QE and at HZB for dark current.

Other related activities



•Pulsed plasma treatment is a promising way

•Diagnostics development

•Further photoinjector tests with plasma melted Pb film

Conclusions and prospects




NCBJ:R. Nietubyć

J. Lorkiewicz

R. Mirowski

M. Barlak

J. Witkowski

W. Kosińska

W. Grabowski

…………..

HZB:T. Kamps

R. Barday

…………..

DESY:J. Sekutowicz

D. Kostin

…………..

HZR:J. Teichert

R. Xiang

…………..

Collaboration


Workpackage: WP12 Innovative RF technologiesTask: WP12.5 Photocathodes

Milestone

MS80 Demonstrated operation of improved deposition system Demonstrated plasma pulse flattening of the unfiltered Pb/ Nb layerM30Report on samples characterisation NCBJ

Deliverables

D12.8 Optimised procedure for microdorplets flattening with an UV laser Optimised procedure for preparation of flat clean and adherent Pb/Nb photocathode filmsM36Report NCBJ

D12.9 Pb/Nb plug photocathodes measurements and characterization. M42Report HZDR (+ HZB + DESY + NCBJ)

Milestone and Deliverable modifications


Documents

Plasma pulsed irradiation preparation of lead coated photocathodes Robert Nietubyć on behalf of the Collaboration Narodowe Centrum Badań Jądrowych Świerk