Measurements of hydrocarbon deposition in NSTX

1
Measurements of hydrocarbon deposition in NSTX
C. H. Skinnera, H. W. Kugela, W. R.Wamplerb for
the NSTX team.
aPrinceton Plasma Physics Laboratory, USA bSandia
National Laboratory
Columbia U Comp-X General Atomics INEL Johns
Hopkins U LANL LLNL Lodestar MIT Nova
Photonics NYU ORNL PPPL PSI SNL UC Davis UC
Irvine UCLA UCSD U Maryland U New Mexico U
Rochester U Washington U Wisconsin Culham Sci
Ctr Hiroshima U HIST Kyushu Tokai U Niigata
U Tsukuba U U Tokyo JAERI Ioffe
Inst TRINITI KBSI KAIST ENEA, Frascati CEA,
Cadarache IPP, Jülich IPP, Garching U Quebec
Hydrocarbon deposition has been recorded on two
quartz microbalances installed 0.77 m outside the
last closed flux surface of the National
Spherical Torus Experiment. This configuration
mimics a typical diagnostic window or mirror. The
deposits were analyzed ex-situ by Auger electron
spectroscopy, x-ray photoelectron spectroscopy
and ion beam analysis and found to be dominantly
carbon, oxygen and deuterium. A rear facing
quartz crystal recorded deposition of lower
sticking probability molecules at 10 of the rate
of the front facing one. Time resolved
measurements over a 4-week period with 497
discharges, recorded 123 nm of deposition,
however 67 nm of material loss occurred at 7
discharges. The net deposited mass of 13.5
microgram/cm2 matched the ion beam analysis
results within the 10 experimental uncertainty.
Of the total deposition only 22 nm (18) could be
clearly identified within 2 sec of 71 discharges,
a large fraction of remaining 101 nm occurred at
other times.
45th Annual Meeting of the Division of Plasma
Physics, October 27 - 31, 2003, Albuquerque, New
Mexico
2
Motivation
Deposition on TFTR graphite tile

• Deposition in next step devices will scale up two
  orders-of-magnitude with pulse length (bigger
  change than plasma parameters).
• Codeposition of tritium with carbon will prevent
  use of carbon PFCs in ITER unless credible
  tritium removal techniques are demonstrated on
  tokamaks.
• Deposition is serious threat to survival of first
  mirrors.
• Recycling behaviour will change in areas of net
  deposition.
• Condition of plasma facing surfaces generally
  remains hidden and undiagnosed.

50 microns
S Willms and W Reisiwig LANL
Deposition on mirrors
Experimentally measured solid points and
calculated lines with corresponding open markers
spectral dependencies of effective reflectance
for clean SS and for SS with carbon coating of
thickness shown on each curve. Voitsenya et al.,
Rev. Sci. Instrum. 72 (2001) 480.
3
NSTX Deposition on Wall Coupons
gas injectors
Factor of 5 Variation in Deposition
Toroidally Factor of 6 to 9 Variation in
Deposition Poloidally
Deposition from plasma operations September 2000
to August 2001 including 9 boronizations
4
NSTX Deposition Monitor

• Principle quartz crystal oscillates at 6 MHz,
  exact frequency depends on mass and on
  temperature - not affected by EMI .
• Deposition inferred from change in frequency
  (measured to 1Å, 1Hz)
• Detector is located in 4 tube outside main
  plasma chamber 77 cm from last closed flux
  surface
• Detector configuration mimics typical diagnostic
  windows and mirrors, - samples neutrals
  deposited on NSTX wall away from plasma.
• Gate valve permits sample retrieval for surface
  analysis without machine vent
• Data obtained January/February 03.

5
Installation on NSTX
Plasma facing crystal
detector inside
Shutter
Si coupon
Backward facing crystal
Bay K
Thermo- couples
plasma -gt
4 Gate Valve
oscillator
crystal _at_ R 231 cm 77 cm from last closed
flux surface 33 cm below midplane
Dep Mon T/C Signals, water cooling, air for
shutter
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Quartz crystal microbalance

• Infincon XTM/2 with bakable crystal
• Specification
• Detector resolution 1 Å ( one monolayer).
• Accuracy 0.5
• Bakeable to 450C - water cooled during
  deposition monitoring.
• Built in shutter on plasma facing detector.
• Thermocouples on both detectors record
  temperature.

Si witness coupon
Rear facing crystal
Gold coated quartz crystal
Thermocouple
Shutter (folded back)
Gold coated quartz crystal
Realtime readout
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Ex-situ Surface Analysis
Auger electron energy unique to each element
SEM scanning electron microscopy AES Auger
electron spectroscopy
Atomic concentration of elements in percent
(excluding hydrogen isotopes and helium)
normalized to 100 as measured by x-ray
photoelectron spectroscopy.
internal relaxation
secondary electron ejected
incident 10 keV electron
Inelastically scattered electron
3 keV Ar beam sputters surface to give depth
profile.
XPS X-ray photoelectron spectroscopy
photo electron energy related to binding energy
unique to each element
x-ray
Analysis by
Note AES, XPS does not detect hydrogen or helium.
Quartz 5 front facing
Ion beam analysis
Carbon density from proton scattering
incident 1.7 MeV proton
Deuterium density from d(3He,p)4He nuclear
reaction
incident 0.7 MeV 3He
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Deposition on forward backward facing crystals
Plasma facing quartz crystal
XPS elemental analysis
SEM
AES shows depth of layer to be 400 Å (assuming
sputter rate same as diamond-like carbon.) XPS
shows the surface of the samples contained mostly
carbon (as C-(C,H) and C-O) and oxygen. Quartz
Xtal 5 (plasma facing) also contained boron (as
B3 and possibly B-N) and nitrogen (as C-N and
NOx).
Carbon
Counts
Oxygen
Binding Energy (eV)
Back facing quartz crystal
XPS elemental analysis
SEM
AES shows depth of layer to be 45 Å (assuming
sputter rate same as diamond-like carbon.) The
surface contained mostly carbon (as C-(C,H) and
C-O) and oxygen also contained nitrogen (as C-N
and R4-N) and gold (as Au). The general
lineshape indicates the presence of an overlayer
on a gold substrate. The lineshape to the left
of the carbon peak indicates that carbon is in
this overlayer. Difference between plasma facing
and backward facing deposition related to
sticking coefficients.
Carbon
Oxygen
Counts
Binding Energy (eV)
Analysis by
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Ion beam analysis matches microbalance
Comparison of mass of deuterium and carbon in
deposits measured by Ion Beam Analysis (IBA), and
total mass measured by quartz microbalance of
samples exposed to plasma discharges.
D/C ratio 0.1 Quartz 5 front facing
13.5 13.3
The thickness is based on an assumed density of
1.6 g/cm3.
Excellent agreement between mass measured by
proton scattering and by microbalance
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Time resolved deposition
Deposition over period January 10th - February
14th 2003
Deposition over three hours, discharges 110150 -
110165
110156
for density 1.6 g/cm3
110155
No detailed correlation of enhanced deposition
with diagnosed plasma parameters
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Similar discharges - different deposition
Most discharges show low but cumulative deposition
Stored Energy, RF power, Strike point position,
Outer gap, tile temperature and gas pressure
etc... very similar.
Angstrom
Seconds
Some very similar discharges showed high
deposition
Angstrom
Pressure (torr)
Outer Strike Point Position
Seconds
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Material loss on some discharges
109987
109988
109987 (no loss)
109988 (loss)
109994
109988 (no loss)
Material loss over discharges 109988 - 109993
seconds
Decrease at t 2 seconds after the initiation of
the discharge.
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Material loss on some discharges
109988 (with material loss) 109987, 109994
(without loss).
Pressure (torr)
Outer Strike Point Position
No clear correlation to stored energy, NB power,
strike point position, outer gap, tile
temperature and gas pressure etc...
14
Deposition after discharge
110106
Delayed deposition following CHI discharge
109588.
Sequence of small increments of deposition, most
(but not all) occur 2-30 s after discharges
110106 110118 shown by P. Step rises in
deposition not coincident with a discharge appear
to be due small temperature rises.
seconds
Delayed deposition
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Summary

• NSTX shows net deposition on wall, heaviest at
  midplane and near gas injectors
• Quartz microbalance measured deposition 77 cm
  from plasma in diagnostic mirror-like geometry.
• 29 µg/cm2 deposition accumulated over 4 weeks and
  497 discharges with 16 µg/cm2 material loss over
  7 discharges.
• Deposited layers are mostly carbon with some
  oxygen and deuterium.
• Deposition recorded on surface facing away from
  plasma from low sticking probability radicals at
  10 rate of plasma facing surface.
• Mass of deposited layer independently confirmed
  by ion beam analysis.
• No detailed correlation of higher deposition
  discharges or material loss with diagnosed plasma
  parameters.
• Indications that about only half of deposition
  occurs promptly after discharges.
• Evaporation of polymer-like films between
  discharges has been previously postulated1.
• If confirmed this has important consequences for
  tritium retention and strongly supports use of
  shutters to minimise exposure of diagnostic first
  windows and mirrors.

1 A. von Keudell, C. Hopf, T. Schwarz-Selinger,
W. Jacob, Nucl. Fus, 39 (1999) 1451. suggested a
potential mechanism for deposition It is also
conceivable that, due to the relatively high
surface temperature in JET (gt500K), polymer-like
films produced during plasma interaction
evaporate after each discharge and are deposited
on the cold louvers in pulse pauses as well.