Joint meeting of SEWG 05 and 07 of EU TF PWI, Ljubljana, Slovenia, 1.-2. October 2009 In-situ studies of H/D interaction with W, Ta and Cu Progress report

Joint meeting of SEWG 05 and 07 of EU TF PWI, Ljubljana, Slovenia, 1.-2. October 2009

In-situ studies of H/D interaction with W, Ta and Cu

Progress report on:

WP09-PWI-05/03a/MHEST – BS 0.35ppy+PS 0.15ppy“Reflection and re-emission of excited H2 and D2 molecules from high-Z surfaces”

Iztok Čadež, Sabina Markelj, Primož Pelicon, Zdravko Rupnik

Association EURATOM-MHESTJožef Stefan Institute, Ljubljana, Slovenia

EU TF PWI - SEWG High Z Materials and Liquid metals (05)


Outline

1. Introduction and objectives

2.Comments on previous results

3.Work performed in 2009

4.Work plan for the rest of 2009

5.Work proposed for 2010








Present WP09-PWI-05-03a/MHEST: “Reflection and re-emission of excited H2 and D2 molecules from high-Z surfaces“

is a continuation of two tasks from WP08,

PWI-08-TA-07/MHEST/BS/02 “In-situ ERDA studies of hydrogen and deuterium interaction with tungsten“, and

PWI-08-TA-10/MEHST/BS/01 “Production and reflexion of vibrationally excited hydrogen molecules at material surfaces (W)”

We also proposed continuation of this program for

WP2010: ”Interaction of excited H2 and D2 molecules with high-Z surfaces”


The general objective of our work is to quantitatively characterize production and relaxation of excited particles on high-Z metals, in particular vibrationally excited H2 and D2 molecules on W.

WP09-PWI-05-03a/MHEST: “Reflection and re-emission of excited H2 and D2 molecules from high-Z surfaces“

Task objectives: Detailed understanding of processes leading to emission of neutral vibrationally excited hydrogen molecules (H2 and D2) (VEH) from plasma facing components and reactor vacuum vessel wall.

• Milestone 1: Determination of vibrational distribution of emitted molecules from W and Ta – final report by the end of 2009.

• Milestone 2: Determination of H and D content on high Z metals subjected to the flow of hydrogen atoms and molecules.


Motivation and background

• Rich experience on production of vibrationally excited hydrogen molecules (VEH) by atom recombination on surfaces was gained during our previous work.

• Following this, we have constructed a unique experimental tool for vibrational spectroscopy of hydrogen molecules (different isotopologues) based on dissociative electron attachment. It is easy to operate and thus allow original experiments to be performed.

• Having at the same time in our laboratory Ion beam analytical (IBA) methods ERDA (Elastic Recoil Detection Analysis) and RBS (Rutherford Back Scattering) triggered us to join this two experimental sets of techniques and perform deeper studies of hydrogen recombination process on surfaces.

• Vibrational spectrometer allows us to do some specific studies of surface reactions but also of some volume collision processes.


Relevance to PWI studies

• Providing data (reaction rates, cross sections) for modelling codes (edge plasma, erosion and deposition, negative ion sources, neutral vapour cushion in transient phenomena (?)) on processes involving VEH – importance of these is well documented in some cases and feedback on particular needs would be most welcomed.

• Understanding energy exchange by neutrals between wall and edge plasma.

• Search for new phenomena involving VEH and impurity or seeding particles (e.g. production of VEH by thermal dissociation of hydrocarbon on hot tungsten, production of other excited particles at hot tungsten surface).








Hydrogen vibrational spectroscopy is based on the detection of negative ions produced by the dissociative electron attachment (DEA) in hydrogen through the “4 eV” resonance state:

AB (X 1g+,v) + e → AB- (X 2u

+) → A- + B

where A and B stands for any of hydrogen isotopes, H, D or T.

Method was first developed at LDMA, UPMC, Paris in 80-ties.

Vibrational distribution of studied target gas is determined by deconvolution of measured yield of low energy A- ions as a function of electron energy in 0 - 5 eV.

Experimental techniques/1 – vibrational spectrometer


Homogeneous magnetic field (B ≈ 60G) is used for guiding electron beam and for ion extraction.

Experimental set-up developed recently

Efficient zero-energy ion collection is achieved by combined action of weak electrostatic penetration field and guiding magnetic field.Sufficient mass selectivity to distinguish H- from D-.

Vibrational distribution of hydrogen molecules in interaction region is obtained from H- (D-) ion yield variation with electron beam energy in the range from 0 to 5 eV by

S. Markelj et al., Int. J. Mass Spectrom. 275 (2008) 64

By changing electrode polarity, the positive ions are detected what allows determination hydrogen atom concentration by the same set-up!


-1 0 1 2 3 4 50

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-1 0 1 2 3 4 50

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v =

0123456789

v =

012345678910111213

Electron energy [eV]

H- yi

eld

[cou

nts/

s]

Cold (v=0) H2(v) (2) Convoluted

(a)


D- y

ield

[cou

nts/

s]

D2(v) (2)

Convoluted

(b)

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,010-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

v=0 1 2 3 4 5 6 7 8 9 10 11 12 D2

v=0 1 2 3 4 5 6 7 8 H2

Rel

ativ

e po

pula

tion

Excitation energy [eV]

H2 - (1)

H2 - (2)

D2 - (1)

D2 - (2)

H2 - old

Tv=3700K

Tv=3400K

Pressure: 9x10-4 mbar. Impinging rate (H2 equivalent): 9.9x1017 cm-2s-1.H2 flow rate: 1.8x1017 molecule/s (0.4 sccm).Impinging rate of H-atoms on the sample (center): 1.7x1015 atoms/(cm2 s).

Vibrational distribution of hydrogen molecules created by atom recombination on W.

TV = 3700 ± 100 K for H2. TV = 3400 ± 100 K for D2. Rotational temperature is lower, 500K and 300K assumed for fitting for H2 and D2 respectively. Importance of impurities remains to be studied.

I. Čadež et al., J. Nucl. Mater. 390-391 (2009) 520 (PSI 2008)


Beam:

- 4.2 MeV 7Li2+ - very good separation of H and D surface peaks.-1.5 MeV 1H+ for few RBS measurements.

Geometry: Sample tilted 75°, RBS detector at 160°, ERDA detector at 30°.

ERDA particle filter: 11 µm Al foil

Dose calibration: mesh charge integrator (tungsten mesh, open area of 77.4 %).

Standard irradiation dose for single spectrum recording (on sample) – 4.4 C (1.4x1013 7Li atoms).

P. Pelicon et al., NIM B 227 (2005) 591

Experimental techniques/2 – Li-ERDA


Experimental development for in-situ studies of neutral hydrogen - metal interaction

Old set-up for in situ ERDA studies of hydrogen-material interaction

Sample is exposed to neutral, partially dissociated hydrogen atmosphere created by hot tungsten filament.

0,0

0,5

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r]

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p [oC

]

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Id

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1C; 06.04.13

0414Wa.opj

Inte

gral

sig

nal

H (surface) D (surface)

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gral

sig

nal

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1C

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p

S. Markelj et al., NIM B 259 (2007) 989








Experimental development for in-situ studies of neutral hydrogen - metal interaction

Hydrogen atom beam is directed to the sample mounted on the temperature controlled holder.

4.2 MeV 7Li2+ beam is hitting the surface at 15o. ERDA and RBS detector are positioned at 15o and 30o respectively with respect to the sample surface.

New set-up for in situ ERDA studies of hydrogen-material interaction was installed on reconstructed ERDA/RBS beam line of 2MV tandem accelerator during 2008

4.2MeV 7Li2+ beam

RBS detector

ERDA detectorHABS

Sample onthe heater


Studied sample material and sample holders

Two Tungsten samples were studied:- Tokamak grade obtained from IPP, Garching

(produced by Plansee) - PCWPCW- Hot rolled tungsten from Goodfellow - HRWHRW

Measurements were performed also with Ta and OFHC Cu and also with thin layer of hard amorphous hydrogenated carbon (a-C:H) (in collaboration with IPP Garching).

Two sample holders were constructed:- holder with ceramic heater (Boralec) 80oC – 1200oC (left)- holder with resistive heater and water cooling 10oC – 400oC (right)


ERDA measurements influence sample due to: - sample heating - projectile implantation- build-up of vacuum oil deposit

Surface roughness (by STM)

Power deposited in the target is 10 mW in 14 mm3 for 5nA of 4.2MeV 7Li2+ beam spread over 4mm x 4mm area.

Number of implanted 7Li during continuous 1 h irradiation: 5.6x1013 (nW = 6.3x1016 at/mm3)

HRW – RMS: 14nm PCW – RMS: 4nm


Hydrogen atom source

We use a commercial hydrogen atom source, HABS from MBE-Komponenten GmbH described in detail by Tschersich et al., J.Appl.Phys. 104 (2008) 034908.

Constant heating conditions were used during experiment:

- Heating current 13A; heating power 173W; capillary temperature 2000 K.

- Driving H2 and D2 pressure: typically 100-120-200 mTorr (13-16-27 Pa).

- Hydrogen dissociation rate 40%.

- From a-C:H erosion calibration measurements:

- Central H flux density at the sample: 1.6x1015 at/cm2s (124 mTorr driving P)

- Central D flux density at the sample: 1.0x1015 at/cm2s (143 mTorr driving P)


• Exposure of a-C:H film to hydrogen beam at angle 24o to the surface normal, distance capillary-sample 7.85 cm

• Sample temperature 550 K 20 K, erosion yield at this temperature Y=0.012 [Schluter et al., JNM 376 (2008), Schwartz-Selinger et al., J. Vac. Sci. Technol A 18 (2000)]

• Driving pressure 124 mTorr

Atom flux density on the sample

Hydrogen Atom Beam Source (HABS) - beam profile calibrationHydrogen Atom Beam Source (HABS) - beam profile calibration


Performed experiments from 23.9.2008 till 21.1.2009

HRW 23.9.-3.10.08.

22.-24.10.08.

PCW 14.-16.10.08.

6.-8.11.08.

21.-22.01.09.

Ta 7.-8.10.08.

Cu 10.-14.10.08.

3.-5.11.08.

Calibration: (kapton, a-C:H)

19.11.08.-20.1.09. (with break)


Measurements with PCWFirst heating and exposure to H2:

• Initial room temperature [H] ~20x1015cm-2.

• Heating to 1100oC and [H] reduced to ~5.

• After start of exposure to H with no heating (~85oC) first sudden jump of [H] observed (spec# 9,10) to 15-20. Then steady [H] increase with rate the rate of 5.6x1012at./cm2s.

• Clear displacement of W-edge RBS spectrum towards lower energy. indicating layer buildup to 170!

• New layer unstable and [H] decreases as soon as exposure stops and also under successive exposures to 120. However next morning same value of [H].

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cts]

cts Surface H concentration

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art/cm

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Preasure Baratron


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] H cts

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Concentration [10

15 part/cm2]

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081015spektri.opj

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Driving pressure [m

Torr]

Pressure Baratron

By assuming build-up of a carbon surface layer it was possible to reproduce well RBS edge displacement using SIMNRA modelling tool.


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emp

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cts H cts D

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s]0

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Surface concentration [10

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2]

081106a.opj

When sample is exposed to D-beam, sharp [D] rise is observed but not corresponding [H] decrease as was the case with HEC. By sample heating to 350-400oC and exposure to D beam it was possible to clean the surface.


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D2

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p [o C

]

Temperature TC1 Temperature THT Temperature TCH

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cts H cts D

Inte

gra

l sig

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l 050100150200250300350 concHcorr

concDcorr concC

Su

rface

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n [1

01

5 at./cm

2]

PCW

081107a.opj

Long term exposure to D-beam. Mainly rise of [D] is observed and only weak [H]. [C] increase is slower than for the case of exposure to H-atoms: only about 3x1012 Cat./cm2s as compared to 8x1012 Cat./cm2s. Possible reasons: different sample temperature, different atom flux, different atom velocities. Again, weak isotope exchange when H-beam is switched on.


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[mT

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Pressure BaratronH

2

081108a.opj

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In

tegr

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igna

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050100150200250300350

concHcorr concDcorr concC

Surface concentration [10

15 at./cm2]

Layer produced by long term exposure to D was stable next day and again very weak isotope exchange was observed when H-exposure was performed even at higher temperature.


After gaining experience with other samples (in particular with a-C:H) we performed final study of sample cleaning at higher temperature. By p-RBS it was possible to follow [C] decrease with temperature and then by exposure to H-beam. Final low values of [H] and [D] were confirmed by last measurements being Li-ERDA/RBS.


• Measurements (mainly by the end of 2008 but calibration and final PCW in January 2009)

• 7Li ERDA and p-RBS.

• Detailed data evaluation has been initiated.

• All measurement data are available for discussion to interested colleagues.

Work performed in 2009 until September 30th

Problem: Main reason for slow advance of our study on H2(v) production is observation that metal surface is strongly changing under H and/or D bombardment in HC contaminated environment. This produces non reproducible results but stimulates further work.


First chamber: 14 x 25 mm∅Filament: 0.∅ 2 mm WChannel: 4 x 10 mm∅Second chamber: 16 x 15 mm∅Exit Aperture: ∅ 6 mm Material: OFHC cupperTypical hydrogen flow: 6.7x10-3 mbar*lit/s

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H2

Q=6.65x10-3 mbar*lit/s

IRREC.

DISS.

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[mm]

Mean free path,

[ m

m ]

Vibrational distribution as well as a fraction of (still) present atoms in the beam are experimentally determined.

A simple source of VEHs has been developed for some particular experiments.

CxHy interaction with hot tungsten/1


CxHy interaction with hot tungsten/2

• Production of H2(v) molecules by thermal dissociation of C2H4 and C2H6 but not of CH4 on tungsten filament was observed and studied to some extent.

• Quantitative evaluation of data is under way.

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6

Ion

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/s]

090806a.opjIdis

[A]

H- signal@14eV

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bckg - H2O I

dis = 0 A

H2

x0.2

C2H

6


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/s]

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4

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ld [c

/s]

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H2(v)

H2(v)!

C2H

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Ion

yie

ld [c

/s]

H-/H2(v)!

H-/H2(v)!

H2(v)!

C2H

6

Ion

yie

ld [c

/s]


Idis

= 6 A

Tf = 2150 K

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090817a.opj

Tfil [

o C]

Idis

[A]

090817 - I serija 090817 - II serija 090818 - I serija 090818 - II serija CF-quartz(20.8.09) CF-quartz(20.8.09)/CORR

T[oC]; d = 0.25 mm

Polynomial Fit of PrSer820_C

Tfil/oC=248,55+406,826xIdis-22,81116xIdis

2








• Further experimental data evaluation.

• New experiments will be performed in the second half of 2009 with HABS equipped with newly acquired hydrogen purifying filter,

• All data evaluation will be performed before the end of 2009 and conclusions drawn.

Plan for the rest of year 2009








TA: PWI in a full-W deviceProposal: WP10-PWI-05-xx/MHST - BS

Interaction of excited H2 and D2 molecules with high-Z surfaces

• Studies of interaction of vibrationally excited hydrogen molecules from materials for PFC initiated in previous two years will be continued. Measurements will be performed with ITER-wall relevant materials, in particular W but some other high-Z metals such as Ta and Mo will be studied as well. We will upgrade vibrational spectrometer for hydrogen molecules (H2, HD, D2) by incorporating differential pumping of detector chamber and improving energy and mass selectivity. This instrument is unique of its kind thus allowing original studies not possible elsewhere.

• Surface reactions occurring under simultaneous bombardment by different neutral particles (atoms, cold and hot hydrogen molecules, impurity molecules) will be studied by IBA methods ERDA and RBS. Our recent experiments have revealed important synergistic effects of H(D) atoms, hydrocarbon impurities and possibly hot molecules on the build-up of carbon layers on W, Ta, Cu at room temperature. We plan to work on detailed understanding of formation and destruction of mixed layers at high-Z metals and possible role of seed impurities, in particular N2, will be studied.


- Energy resolution of the e-beam.

- Differential pumping of detector's chamber.

- Mass selectivity D vs. H.

- Adaptation for the possible use on other experiments.

Improvements of vibrational spectrometer


In our measurements we regularly observe a detector nose dependent on W temperature and in the presence of hydrogen – metastable atoms or molecules or UV photons?

Background detected from the hydrogen atomic source

- Activation energy: 3.05±0.15 eV0.00045 0.00050 0.00055 0.00060 0.00065 0.00070 0.00075

10-2

10-1

100

101

102

Noise

syg

nal [a

rb. u

nits

]

1 / T2 [ 1/K ]

SigMB calc. from parameters? Fit

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Solving an old and still open question (at least for us!) – emission of excited particles from hot W when exposed to hydrogen.


- TA proposed to SEWG Material migration on carbon erosion/deposition.

- Influence of surface created VEHs on optical emission from magnetized hydrogen plasma.

- MC modelling of gas cell containing atoms and VEHs (in collaboration with UNG).

Within our Association MHEST project the following activities are also planned in 2010:


Thank you for your attention!

Documents

Joint meeting of SEWG 05 and 07 of EU TF PWI, Ljubljana, Slovenia, 1.-2. October 2009 In-situ studies of H/D interaction with W, Ta and Cu Progress report