1. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 1 Iztok Čadež, Sabina Markelj, Primož Pelicon and Zdravko Rupnik Jožef Stefan Institute, Association EURATOM-MHEST (SFA), Ljubljana, Slovenia Experimental Investigation of Interaction of Vibrationally Excited H 2 and D 2 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

2. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 2 Vibrational/rotational temperatures in TEXTOR: H 2 7140 K / 1078 K HD5950 K / 932 K D 2 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

3. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 3 B2-EIRENE simulation includes among others: MAR and MAD H 2 density field in Divertor: Neutral gas “cushion” is strongly reduced by H 2 + H + ion conversion only H 2 (v=0) H 2 (v) included H + + H 2 (v)  H + H 2 +, e + H 2 +  H + H*  H + H (MAR) H + + H 2 (v)  H + H 2 +, e + H 2 +  H + H*  H + H + + e (MAD) Kotov, Reiter in Sawada, FZJ-JSI meeting on PWI, 28-29 September 2005

4. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 4 From: Hollmann et al., 33 rd 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 T vib, T rot and T kin respectively. Plasma cooling by hydrogen molecules

5. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 5 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

6. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 6 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!

7. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 7 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 1  g +,v) + e → AB - (X 2  u + ) → 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 cm 2 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

8. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 8 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.

9. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 9 Source of H 2 (v) and D 2 (v) Hydrogen molecules dissociate on tungsten filament Vibrationally excited molecules are produced by recombination on the cold wall 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.

10. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 10 Hydrogen Exposure Cell (HEC)  Samples exposure to controlled neutral hydrogen atmosphere  rate of dissociation  vibrational distribution of molecules  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 (H 2, H, D 2, D) with materials Beam: 4.2 MeV 7 Li 2+, Sample tilted 75° ERDA detector at 30° ERDA detector equipped with 11 µm Al foil Dose 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

11. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 11 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. 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 quality 2. Cross section for elastic scattering of 7 Li on D not found in the literature so that Rutherford CS had to be used for spectra evaluation. 3. Nuclear reaction d( 7 Li, p) 8 Li 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 7 Li 2+ over 4mm x 4mm): 10 mW in 14 mm 3 ! Number of implanted 7 Li during continuous 1 h irradiation: 5.6x10 13 (n W = 6.3x10 16 at/mm 3 )

12. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 12 3.1 Production of vibrationally excited molecules by recombination on W (PSI2008, P3-68) H2H2 D2D2 3. Some results W sample Typical conditions in the source: H 2 pressure: 0.9 μbar Impinging rate – (H 2 ): 1x10 18 cm -2 s -1 H 2 flow rate: 1.8x10 17 s -1 Dissociation filament: T f =1850 K, S f =1.2 cm 2 H flux in the centre of the sample: j (H) = 1.7x10 15 cm -2 s -1.

13. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 13 Temperature dependence of H 2 (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. 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.

14. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 14 Almost negligible D - production Cu H2H2 D2D2 H2H2 D2D2 Ta Our results are confirm previous similar measurements for H 2 and bring new data for D 2.

15. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 15 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; T f 1700K (for I d =3A); molecule impinging rate 1.9 x 10 18 D 2 mol/(cm 2 s) @ 2.8 x 10 -3 mbar. H concentration: 1.5 x 10 16 at/cm 2

16. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 16 First chamber: ∅ 14 x 25 mm Filament: ∅ 0.2 mm W Channel: ∅ 4 x 10 mm Second chamber: ∅ 16 x 15 mm Exit Aperture: ∅ 6 mm Material: OFHC cupper Typ. hydrogen flow: 6.7x10 -3 mbar*lit/s 3.2 Search for a possible role of H 2 (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.

17. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 17 Samples : 17 x 17 mm 2 Sample temperature: 295 K Samples : 17 x 17 mm 2 Sample temperature: 418 K Z = 54 mm Samples : 24 x 24 mm 2 Sample temperature: 423 K Z = 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.

18. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 18 4. Perspectives and plans for future 4.1 Detailed study of temperature dependence of vibrational distribution of H 2 and D 2 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.

19. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 19 Initial experiments with hydrogen atomic source Vibrationally excited H 2 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. HABS from Dr. Eberl MBE-Komponenten GmbH; http://www.mbe-components.com/

20. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 20 - The search for H 2 (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

21. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 21 Background detected from the H 2 v 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 4.3 An open question – emission of excited particles from hot W exposed to hydrogen.

22. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 22 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 H 2 (Schröter et al., J.Vac.Sci.Technol., A9 (1991) 1712) and D 2 (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).

23. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 23 H/Pd D/Pd By ERDA Vibrational 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 H 2 molecules created by recombination after permeation through Pd and PdAg alloy and for next attempt HR electron gun will be used. Up to about 40 at% of H in Pd

24. Jožef Stefan InstituteInstitut Jožef Stefan 9 th Workshop on Hydrogen in Fusion Reactor Materials, Salamanca, Spain, June 2-3, 2008 24 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.