Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials,...

9th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 1

Jožef Stefan InstituteInstitut Jožef Stefan

Iztok Čadež, Sabina Markelj, Primož Pelicon and Zdravko RupnikJožef Stefan Institute, Association EURATOM-MHEST (SFA),

Ljubljana, Slovenia

Experimental Investigation of Interaction of Vibrationally Excited H2 and D2 with W and C

1. Introduction

2. Experimental methods- Vibrational spectroscopy of hydrogen molecules

- In situ measurement of hydrogen concentration depth profile by ERDA

3. Some results

4. Perspectives and plans for the near future



Vibrational/rotational temperatures in TEXTOR:H2 7140 K / 1078 KHD 5950 K / 932 KD2 5270 K / 850 K

From Brezinsek et al., 2003 (see also Brezinsek et al., Plasma Phys. and Control. Fus., 47 (2005) 615.

Optical spectroscopy of hydrogen molecules in fusion plasma

Diagnostic technique based on Fulcher band emission developed by U. Fanz et al.

Hydrogen molecules in edge plasma are ro-vibrationally excited

1. Introduction



B2-EIRENE simulation includes among others: MAR and MAD

H2 density field in Divertor:Neutral gas “cushion” is strongly reduced by H2 + H+ ion conversion

only H2(v=0) H2(v) included

H+ + H2(v) H + H2+, e + H2

+ H + H* H + H (MAR)H+ + H2(v) H + H2

+, e + H2+ H + H* H + H+ + e (MAD)

Kotov, Reiter in Sawada, FZJ-JSI meeting on PWI, 28-29 September 2005



From: Hollmann et al., 33rd EPS 2006,P4.172 (see also Hollmann et al., Plasma Phys. 13 (2006) 052510

From: Meulenbroeks et al., PRL, 76 (1996) 1840

Influence of hydrogen molecules produced by wall-recombination on properties of expanding thermal arc plasma

More recently, experiments at PISCES-A have shown high importance of hydrogen molecules for cross-field plasma cooling. Values of 4500 K, 700 K and 600K were determined for Tvib, Trot and Tkin respectively.

Plasma cooling by hydrogen molecules



Importance of vibrationally excited hydrogen molecules (different isotopologues) for PWI and edge plasma:

- “v” dependence of different binary CSs in plasma and therefore influence on phenomena such as plasma detachment in divertor and possibly on ELMs and disruptions.

- Possible role in processes at the wall (e.g. chemical erosion, retention)

- Energy exchange between the edge plasma and PFCs, castellated structures, vacuum wall etc.

Sources of VEH molecules: - edge plasma, - desorption, and- recombination at plasma facing components and at more remote surfaces



Processes of interest:

1. - Vibrational distribution of molecules released from surfaces due to thermal desorption and recombinative desorption for different surface conditions (temperature, composition, impurities). - Ratio of atomic to molecular species released from surface and its variation with surface parameters.

2. Interaction of vibrationally excited molecules with plasma-facing materials: - Change of vibrational distribution caused by interaction with surfaces, - Transfer of vibrational energy to the wall and its effects on erosion yields, and- Wall sticking probability for excited molecules.

3. Binary collisions (volume processes, spectroscopy).

Main goal:Providing quantitative data on processes with vibrationally excited hydrogen molecules needed for modelling edge plasma and PWI and search for specific phenomena with these molecules.

Isotope effect in above processes is of key importance!



Diagnostic technique 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.

2.1 Vibrational spectroscopy of hydrogen molecules

4 eV DEA characteristics:

Very strong rise of CS with v,R excitation (CSs up to 10-15 cm2 for high v) Vertical threshold production of low-energy ions and displacement of thresholds Pronounced isotope effect for low v Extensively studied theoretically (benchmark case) but only few experimental studies

2. Experimental methods



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

Experimental set-up

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

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



Source of HSource of H22(v) and D(v) and D22(v) (v) Hydrogen molecules dissociate on

tungsten filament Vibrationally excited molecules are

produced by recombination on the cold wall

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50.0

0.5

1.0

1.5

2.0

2.5

3.0v=0123456789

0123456789

v=0123456789

0123456789

Norm

alize

d sig

nal

Electron energy [eV]

hladen Id=3p8A KonvHlad Konv Id=3.8A

0 2 4 6 8 10 12 14 16 18

02468

1012141618

hladen Id=3p8A KonvHlad Konv Id=3.8A

Tv = 3600 K Tr = 700 K

Recombination of hydrogen atoms on metal surface can lead to very highly excited hydrogen molecules if atoms are only loosely bound to the surface – assumption of hot (not thermalized) atoms.



Hydrogen Exposure Cell (HEC)Hydrogen Exposure Cell (HEC)

Samples exposure to controlled neutral hydrogen atmosphere rate of dissociationrate of dissociation vibrational distribution of vibrational distribution of

moleculesmolecules Depth profile of H and D

determined by ERDA Main interest:

H and D depth profile in W, C and other materials

interaction of neutral particles (H2, H, D2, D) with materials

Beam: 4.2 MeV 7Li2+, Sample tilted 75°ERDA detector at 30°ERDA detector equipped with 11 µm Al foilDose controlled by mesh charge integrator (Tungsten mesh, open area of 77.4 %)

2.2 In situ measurement of hydrogen concentration depth profile by ERDA

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



Due to low solubility for tungsten hydrogen is mainly present at the surface. However, hydrogen content in the below surface bulk depends on the exposure history of the sample.

0 50 100 150 200 250

0

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60

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ela

stic

d

p fr

om d

(7Li

,p)

8Li

0413Wa,opj

Sig

nal

Channel

pre-exposure ( 13(a to e)) exposure to D ( 13(p to u))

ela

stic

p

incident ion: 7Li2+

incident energy: 4.2 MeVdose: 5x1C

Problems related to the present status of studies with HEC

1. Influence of the ERDA probing beam on the sample:- local sample heating - projectile implantation and creation of displacements- build-up of carbon deposit (dependent on the vacuum quality2. Cross section for elastic scattering of 7Li on D not found in the literature so that Rutherford CS had to be used for spectra evaluation. 3. Nuclear reaction d(7Li, p)8Li contributes to the signal but cross section for it was not found in the literature.

power deposited on the target under quoted beam conditions (5nA of 4.2MeV 7Li2+ over 4mm x 4mm):

10 mW in 14 mm3 ! Number of implanted 7Li during continuous 1 h irradiation:

5.6x1013 (nW = 6.3x1016 at/mm3)



-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,00

1

2

3

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5

6

H2;J=0;v=0123456789

No

rma

lize

d s

ign

al


Fsum/4.00A Fsum/3.80A Fsum/3.40A Fsum/3.00A Conv Id=3.80A Conv Id=4.0A

H2(v)/W

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

1E-7

1E-6

1E-5

1E-4

1E-3

0,01

0,1

1

0 1 2 3 4 5 6 7 8 9

Re

alti

ve p

op

ula

tion

Excitation energy [eV]

Zased Id=3.80A Zased Id=4.0A T=3750K

-1 0 1 2 3 40

10

20

30

40

0123456781012

D- y

ield

[s-1]


Fsum/3.40A D2 Fsum/3.80A D2 Fsum/4.20A D2 conv Id=3.80A

D2(v)/W

0,0 0,5 1,0 1,5 2,0 2,5 3,0

10-6

10-5

10-4

10-3

10-2

10-1

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Rel

ativ

e po

pula

tion

Exitation energy [eV]

zased Id=3.80A T=3300K

3.1 Production of vibrationally excited moleculesby recombination on W (PSI2008, P3-68)

H2 D2

3. Some results

W sample

Typical conditions in the source:

• H2 pressure: 0.9 μbar• Impinging rate – (H2): 1x10 18 cm -2 s -1

• H2 flow rate: 1.8x10 17 s -1

• Dissociation filament: Tf=1850 K, Sf=1.2 cm2

• H flux in the centre of the sample: j (H) = 1.7x10 15 cm -2 s -1.



Temperature dependence of H2(v) production on W

Surface of the sample is continuously exposed to the flow of thermal, partially dissociated hydrogen. In a stationary state the equilibrium is achieved and reemitted molecules are analysed by vibrational spectrometer. Reemitted atoms are also present in the gas beam but we can not distinguish them from the atoms comming from the filament without being adsorbed on the source inner wall or on the stample.

20 30 40 50 60 70 80 90 100 110

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-0,2 0,0 0,2 0,4 0,6 0,8 1,0

0

50

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Sig

na

l V=

3 [

c/s

]

t s [ oC ]

Sig

na

l v=

3 [

c/s

]

PBAR

[ mTorr ]

1 s per data point

Production of highly vibrationally excited molecules can only be explained by recombination of very loosely bound atoms at the surface. Density of such “hot” atoms depends on the surface temperature.



Almost negligible D- production

Cu H2

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,50,0

0,2

0,4

0,6

0,8

1,0

1,2

H2;J=0;v=0123456789

060928awD.opjElectron energy [eV]

No

rma

lize

d s

ign

al

B

Id=3A (1350oC)

Id=3,4A (1450oC) Konv Id=3A konv Id=3p4A

D2

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,00

2

4

6

8

100123456781012

D- y

ield

[s-1]

Electron energy

sum Id=3.4A sum Id=3.8A sum Id=4.2A conv

0 1 2 3 4 50,0

0,5

1,0

1,5

2,0

2,5

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3,5

4,00123456789

060515awD.opj

No

rma

lize

d S

ign

al

Electron Energy [eV]

Id=3A (1350 oC)

Id=3,4A (1450 oC)

Id=3,8A (1550 oC) Konv Id=3A Konv Id=3.4A konv_Id=3.8A

H2

D2

-0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,50

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01

2

3

4

5

6

7810

12

0123456781013

D - y

ield

[s-1

]

Electron energy [ev]

Id=2,8A (1300oC)

Id=3.4A (1450oC)

Id=4.2A (1650oC) conv Id=2,8A conv Id=3,4A conv Id=4,2A

Ta

Our results are confirm previous similar measurements for H2 and bring new data for D2.



1

10

100

1000

Time [min]

Ine

tgra

l sig

na

l

H surface D surface

1C

0 60 120 180 240 300 360 420 1500 1560 1620 1680 1740 1800

50

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110

D2p[m

To

rr] T

em

p [ oC

]

D2

temp

0,0

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1,0

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p

0,0

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1,0

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D2

p temp

p[m

To

rr]

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Te

mp

[ oC]

0 60

12

0

18

0

24

0

30

0

36

0

42

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48

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54

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Idis

[A

]

Time [min]

Id

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600

1C; 06.04.13

0414Wa.opj

Inte

gra

l sig

na

l

H (surface) D (surface)

By in situ ERDA studies using HEC we were able to get information on the state of the W surface under similar conditions as in the VEHM source. Besides getting new information on basic surface processes (chemisorbtion, adsorbtion, isotope exchange, diffusion to the bulk) our main interest is to well understand conditions that lead to the VEHM production.

ERDA experiments with HEC

Sample: rolled tungsten purchased from Goodfellow. Typical conditions: P 1 – 3 x 10-3 mbar; Tf 1700K (for Id=3A); molecule impinging rate1.9 x 1018 D2 mol/(cm2 s) @ 2.8 x 10-3 mbar.H concentration: 1.5 x 1016 at/cm2



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 cupperTyp. hydrogen flow: 6.7x10-3 mbar*lit/s

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with EA

Z [ mm]

Pre

ssu

re [ b

ar

]

H2

Q=6.65x10-3 mbar*lit/s

IRREC.

DISS.

10

100

1000

10000

[mm]

Me

an

free

pa

th,

[ mm

]

3.2 Search for a possible role of H2(v) in chemical erosion of carbon (PSI2008, P1-06 (In collaboration with IPP, Garching – Thomas Schwarz-Selinger and Wolfgang Jacob).

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

We developed a simple source of VEHMs for this and some othere experiments.



Samples : 17 x 17 mm2 Sample temperature: 295 K

Samples : 17 x 17 mm2 Sample temperature: 418 KZ = 54 mm

Samples : 24 x 24 mm2 Sample temperature: 423 KZ = 17 mm

Measured erosion of a-C:H layer can be explained by the presence of atoms in the VEHM source. Some unexplained modification of layer structure was observed at R.T.



4. Perspectives and plans for future

4.1 Detailed study of temperature dependence of vibrational distribution of H2 and D2 molecules reemitted from tungsten

- New sample holder – better control of cooling and heating.

- Study of temperature dependence of surface concentration by ERDA.

- Parallel study of vibrational distribution by DTVE-B. - Controlled sample exposure to H and D atoms from the source with a hot W capillary – an alternative approach to HEC.

This is an important issue for complicated structures such as for cooled castelated tiles and targets where molecular hydrogen reemission can be very important – relatively big and cold recombining surface.



Initial experiments with hydrogen atomic source

H2;J=0;v=0123456789

0 1 2 3 4 50

20

40

60

80

100

Ie Ts=1780 K

Ie Ts=2030 K


I [nA

]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Ts=1780 K Ts=2030 K Conv Ts=1780 K Conv Ts=2030 K

Norm

alized yield

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1v=0 1 2 3 4 5 6 7 8 9

Re

lativ

e p

op

ula

tion

Excitation energy [eV]

F Ts=1780 K Ts=2030 K Tv=3500 K Tv=3100 K

Vibrationally excited H2 created at hydrogen atom recombination on the walls of detection system.

Mass spectrometry by detection system in the positive ion mode –Atom and molecular ionization by electrons.

40 60 80 100 120 140 160 1801

10

100

1000

10000

Sig

na

l [co

unts

/s]

Hc [V]

H2; HABS ON

D2; HABS ON

D2; HABS OFF

No gas, HABS OFF+/H2O

D+

2H+

2 and D+

H+

Ee= 16.5 eV

HABS from Dr. Eberl MBE-Komponenten GmbH; http://www.mbe-components.com/



- The search for H2(v) role in interaction with a-C:H will be continued.

- The source has to be improved in order to eliminate the presence of atomic hydrogen in the beam and to make more reproducible vibrational temperature on the excite.

- Source is planed to be used also for experiments in plasma and atomic physics.

4.2 Experiments with the source of VEHMs



Background detected from the H2v source

with the detection system

In our measurements we regularly observe a detector nose dependent on W temperature and presence of hydrogen – metastable particles or high energy photons.

Background detected from the hydrogen atomic source

- Activation energy: 3.05±0.15 eV

0.00045 0.00050 0.00055 0.00060 0.00065 0.00070 0.0007510-2

10-1

100

101

102

Noi

se s

ygna

l [ar

b. u

nits

]

1 / T2 [ 1/K ]

SigMB calc. from parameters? Fit

1300 1400 1500 1600 1700 1800 1900 2000 21000

20

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Noi

se s

ygna

l [ar

b. u

nits

]

T2 [K]

Sig MB Fit

0 2 4 6 8 10 12 14 160

10

20

30

40

080513a.opj

BCKG

[c/s

]

PIG

[10-6 Torr]

Varriation of gas inlet Fit

Fit: Y =-0,42501+1,07027E6 X+6,11412E10 X^2

Tfil=1900 K

1200 1300 1400 1500 1600 1700 1800 1900 20000

2

4

6

8

10

12

14

16

Sign

al [c

ount

s /s

]

Tfil[K]

Variation of fillament temperature

4.3 An open question – emission of excited particles from hot W exposed to hydrogen.



4.4 Studies of hydrogen permeation through Pd and Ta

We are interested in hydrogen permeation through metal membranes in particular in vibrational temperature of molecules produced on the vacuum side by recombination of atoms brought to the surface by diffusion through the bulk. Pd is the first case that we studied but measurements with Ta are also foreseen.

In previous studies by REMPI it was shown that rotational temperature of H2 (Schröter et al., J.Vac.Sci.Technol., A9 (1991) 1712) and D2 (Wetzig et al., Phys.Rev. B 63 (2001) 205412) molecules created by recombination following hydrogen atom permeation through Pd is lower than the membrane surface temperature (400 – 800K) but that v=1 vibrational state is populated more than predicted from the membrane temperature. Similar result was obtained when the Pd surface was contaminated by 0.5 ML of S (Rutkowski et al. Phys.Rev. B 66 (2002) 115405).



20 40 60 80 100 120 140 160 1800

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sig

na

l [ c

ou

nts

/ch

]

Channel

22oC

25-70oC

75-90oC

90-100oC

100-210oC

212-222oC

230-240oC

240-280oC

295-298oC

060210Pd.OPJ

0 50 100 150 200 2500

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30

0 50 100 150 200 2500

10

20

30

0 50 100 150 200 2500

10

20

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Channel

Sig

nal [

coun

ts/C

h]

A SodB2E SodF2I

060220a.OPJ

Channel

Sig

nal [

coun

ts/C

h] A SodF2I SMinN O

Channel

Sig

nal [

coun

ts/C

h]

SAA2AF AG AH SAI2AM

H/Pd

D/Pd

By ERDAVibrational spectroscopy by DTVE-B

Studies of hydrogen permeation through Pd (and Ta)

Markelj et al., NIM B, 2007

We have not observed convincing vibrational excitation of H2 molecules created by recombination after permeation through Pd and PdAg alloy and for next attempt HR electron gun will be used.

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50.0

0.2

0.4

0.6

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1.0

1.2

0123456789

Tv: 1100 Tri: 400 Tr0: 300

v1: 0.45%V2: 0.003%

No

rma

lize

d s

ign

al


015 dif-n cold Conv 023 perm-n perm conv

Tmemb

=515 K

0 2 4 6 8 10 12 14 16 18 200

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012345678 012345678

H-

yie

ld [

s-1

]

000/10 003 006 007 008/7.5 009 014

051109a.opj

spec.# t [oC] p [Torr]0 20 6,2(-8)3 168-176 3,0-3,2(-6)6 241-246 5,6-5,7(-6)7 246-248 6,0(-6)8 249-257 6,0-6,1(-6)9 257-259 6,0-5,3(-6)14 96-86 7,2-5,3(-7)

H-

yie

ld [

s-1

]

Electron energy [ eV ]

000/10 003 006 007 008/7.5 009 014

051109a.opj

Up to about 40 at% of H in Pd



Summary- New experimental tools have been developed which allow some complementary studies in the field of hydrogen interaction with materials.

- Vibrational temperature of molecules created by recombination on surface was measured for the samples (in particular W) exposed to the thermal beam of partially dissociated hydrogen and deuterium. Evidence of formation of excited particles and/or energetic photons through interaction of hydrogen with hot W.

- Time evolution of H and D concentration on W was studied under similar conditions by ERDA.

- A study of possible role of VEHMs in chemical erosion of graphite is started – either direct C abstraction or modification of layer properties are of interest.

- Detailed study of temperature dependence of VEHM production and of the surface H and D concentration on W is starting.

- Influence of surface produced VEHMs on plasma, vibrational spectroscopy of molecules released after permeation, possible state resolved TDS and few more interesting possibilities are keeping our enthusiasm high.

Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials,...

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Transcript of Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials,...