Insitu studies of HD interaction with W, Ta and Cu

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

• Progress report on
• WP09-PWI-05/03a/MHEST BS 0.35ppyPS 0.15ppy
• Reflection and re-emission of excited H2 and D2
  molecules from high-Z surfaces
• Iztok Cade, Sabina Markelj, Primo Pelicon,
  Zdravko Rupnik
• Association EURATOM-MHEST
• Joef Stefan Institute, Ljubljana, Slovenia

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

• Introduction and objectives
• Comments on previous results
• Work performed in 2009
• Work plan for the rest of 2009
• Work proposed for 2010

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• Introduction and objectives
• Comments on previous results
• Work performed in 2009
• Work plan for the rest of 2009
• Work proposed for 2010

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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
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• 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.

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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.

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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).

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• Introduction and objectives
• Comments on previous results
• Work performed in 2009
• Work plan for the rest of 2009
• Work proposed for 2010

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Experimental techniques/1 vibrational
spectrometer
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 1Sg,v) e ? AB- (X 2Su) ? 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.
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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
Homogeneous magnetic field (B 60G) is used for
guiding electron beam and for ion extraction.
By changing electrode polarity, the positive ions
are detected what allows determination hydrogen
atom concentration by the same set-up!
S. Markelj et al., Int. J. Mass Spectrom. 275
(2008) 64
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Vibrational distribution of hydrogen molecules
created by atom recombination on W.
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).
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. Cade et al., J. Nucl. Mater. 390-391 (2009)
520 (PSI 2008)
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Experimental techniques/2 Li-ERDA

• 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 mC (1.4x1013 7Li
  atoms).

P. Pelicon et al., NIM B 227 (2005) 591
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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.
S. Markelj et al., NIM B 259 (2007) 989
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• Introduction and objectives
• Comments on previous results
• Work performed in 2009
• Work plan for the rest of 2009
• Work proposed for 2010

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Experimental development for in-situ studies of
neutral hydrogen - metal interaction
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
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.
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Studied sample material and sample holders
Two Tungsten samples were studied - Tokamak
grade obtained from IPP, Garching
(produced by Plansee) - PCW - Hot rolled
tungsten from Goodfellow - HRW
Measurements were performed also with Ta and OFHC
Cu and also with thin layer of hard amorphous
hydrogenated carbon (a-CH) (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)
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• ERDA measurements influence sample due to
• sample heating
• projectile implantation
• build-up of vacuum oil deposit

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)
Surface roughness (by STM)
HRW RMS 14nm
PCW RMS 4nm
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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-CH 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)

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Hydrogen Atom Beam Source (HABS) - beam profile
calibration

• Exposure of a-CH 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 Y0.012 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
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Performed experiments from 23.9.2008 till
21.1.2009
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Measurements with PCW

• First 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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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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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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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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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.
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After gaining experience with other samples (in
particular with a-CH) 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.
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Work performed in 2009 until September 30th

• 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.

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.
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CxHy interaction with hot tungsten/1
A simple source of VEHs has been developed for
some particular experiments.
First chamber Ø 14 x 25 mm Filament Ø 0.2 mm
W Channel Ø 4 x 10 mm Second chamber Ø 16 x 15
mm Exit Aperture Ø 6 mm Material OFHC
cupper Typical hydrogen flow 6.7x10-3 mbarlit/s
Vibrational distribution as well as a fraction of
(still) present atoms in the beam are
experimentally determined.
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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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• Introduction and objectives
• Comments on previous results
• Work performed in 2009
• Work plan for the rest of 2009
• Work proposed for 2010

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Plan for the rest of year 2009

• 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.

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• Introduction and objectives
• Comments on previous results
• Work performed in 2009
• Work plan for the rest of 2009
• Work proposed for 2010

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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.

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Improvements of vibrational spectrometer

• 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.

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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.
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 3.050.15 eV
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Within our Association MHEST project the
following activities are also planned in 2010

• 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).

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• Thank you for your attention!