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Iztok Čadež, Sabina Markelj, Primož Pelicon and Zdravko Rupnik Jožef Stefan Institute, Association EURATOM-MHEST (SF PowerPoint Presentation
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Experimental Investigation of Interaction of Vibrationally Excited H 2 and D 2 with W and C. Iztok Čadež, Sabina Markelj, Primož Pelicon and Zdravko Rupnik Jožef Stefan Institute, Association EURATOM-MHEST (SFA), Ljubljana, Slovenia. Introduction 2. Experimental methods

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

Iztok Čadež, Sabina Markelj, Primož Pelicon and Zdravko Rupnik

Jožef Stefan Institute, Association EURATOM-MHEST (SFA),

Ljubljana, Slovenia

  • 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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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1. Introduction

Optical spectroscopy of hydrogen molecules in fusion plasma

Vibrational/rotational temperatures in TEXTOR:

H2 7140 K / 1078 K

HD 5950 K / 932 K

D2 5270 K / 850 K

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

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

Hydrogen molecules in edge plasma are ro-vibrationally excited

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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B2-EIRENE simulation includes among others: MAR and MAD

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)

H2 density field in Divertor:

Neutral gas “cushion” is strongly reduced by H2 + H+ ion conversion

only H2(v=0)

H2(v) included

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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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Plasma cooling by hydrogen molecules

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.

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

From: Hollmann et al., 33rd EPS 2006,

P4.172 (see also Hollmann et al., Plasma Phys. 13 (2006) 052510

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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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.

  • Processes of interest:
  • - 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).

Isotope effect in above processes is of key importance!

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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2. Experimental methods

2.1 Vibrational spectroscopy of hydrogen molecules

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 1Sg+,v) + e → AB- (X 2Su+) → A- + B

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

  • 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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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Experimental set-up

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

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

Sufficient mass selectivity.

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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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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.

Source of H2(v) and D2(v)

  • Hydrogen molecules dissociate on tungsten filament
  • Vibrationally excited molecules are produced by recombination on the cold wall

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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2.2In situ measurement of hydrogen concentration depth profile by ERDA

Beam: 4.2 MeV 7Li2+, 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 %)

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 (H2, H, D2, D) with materials

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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide11

Problems related to the present status ofstudies 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 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.

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.

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)

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide12

W sample

3. Some results

3.1 Production of vibrationally excited molecules

by recombination on W (PSI2008, P3-68)

H2

D2

  • Typical conditions in the source:
  • H2 pressure: 0.9 μbar
  • Impinging rate – (H2): 1x1018 cm-2s-1
  • H2 flow rate: 1.8x1017 s-1
  • Dissociation filament: Tf=1850 K,
  • Sf=1.2 cm2
  • H flux in the centre of the sample:
  • j (H) = 1.7x1015 cm-2s-1.

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide13

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.

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.

Temperature dependence of H2(v) production on W

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide14

Ta

Cu

H2

D2

H2

D2

Almost negligible D- production

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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide15

ERDA experiments with HEC

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.

Sample: rolled tungsten purchased from Goodfellow.

Typical conditions: P 1 – 3 x 10-3 mbar; Tf 1700K (for Id=3A); molecule impinging rate

1.9 x 1018 D2 mol/(cm2 s) @ 2.8 x 10-3 mbar.

H concentration: 1.5 x 1016 at/cm2

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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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).

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

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

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

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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Samples : 17 x 17 mm2

Sample temperature: 418 K

Z = 54 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.

Samples : 17 x 17 mm2

Sample temperature: 295 K

Samples : 24 x 24 mm2

Sample temperature: 423 K

Z = 17 mm

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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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.

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

initial experiments with hydrogen atomic source
Initial experiments with hydrogen atomic source

Vibrationally excited H2created 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/

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide20

4.2Experiments with the source of VEHMs

  • 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.

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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

Background detected from the hydrogen atomic source

- Activation energy:3.05±0.15 eV

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 H2v source

with the detection system

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide22

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).

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

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Studies of hydrogen permeation through Pd (and Ta)

Vibrational spectroscopy by DTVE-B

By ERDA

D/Pd

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.

H/Pd

Up to about 40 at% of H in Pd

Markelj et al., NIM B, 2007

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008

slide24

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.

9th Workshop on Hydrogenin Fusion Reactor Materials,Salamanca, Spain, June 2-3, 2008