Effects of tungsten surface condition

1
Effects of tungsten surface condition on carbon
deposition
Y. Ueda, M. Fukumoto, A. Yamawaki, Y. Soga, Y.
Ohtsuka (Osaka U.) S. Brezinsek, T. Hirai, A.
Kirschner, A. Kreter, A. Litnovsky, V. Philipps,
A. Pospieszczyk, B. Schweer, G. Sergienko
(FZJ) T. Tanabe (Kyushu U.), K.Sugiyama
(Max-Planck-nstitute) K. Ohya (Tokushima U.), N.
Ohno (Nagoya U.) the TEXTOR team
18th International Conference on Plasma Surface
Interaction May 26-30, 2008 Beatriz Hotel,
Toledo, Spain
2
Topics in this talk

• Roughness effects on C deposition on W and C
• Pre-irradiation effects of W on C deposition
• High density He plasma
• H C mixed ion beam
• C deposition on W at elevated temperatures
• T 300 ºC, 550 ºC, 850 ºC

3
Background and purpose of this study

• Use of CFC in ITER DT phase
• CFC T retention problem greatly reduces DT
  shots number
• Tungsten several concerns Melting, high DBTT,
  Helium embrittlement
• Importance of Tungsten and Carbon material mixing
  
• Plasma facing wall, in gaps, (remote area)
• D(T) C mixed ion irradiation to tungsten
• Many basic studies have been done CD?W
• Complicated processes Chemical erosion, C
  diffusion in WC , RES
• Issues Actual surface condition, Mechanism
  based modeling
• Purpose of this study
• Effects of surface roughness on C deposition
• Effects of pre-treatment (He plasma exposure, HC
  ion irradiation)
• C deposition at elevated temperature
• Detailed study on the mechanism of C and W mixing

4
Erosion and deposition of carbon basics

• Carbon deposition is more pronounced on graphite
• Reflection coefficient is lower than that on W
• R0.6 (50eV C to W)
• R10-4(50 eV C to C)
• Carbon ML is easily re-sputtered by reflected H
  from W substrate

Difference in reflection
Enhancement of sputtering of surface C
A. Kreter, et al., Plasma Phys. Control. Fusion
48 (2006) 1401
5
Evolution of deposition/erosion (EDDY code)

• D C4 mixed ion irradiation to tungsten
• Simulated by EDDY code
• D 96, C 4
• As deposition proceeds, Yc and Rc drastically
  decrease.

Thickness change
Reflection of C Rc
Sputtering of C YC
6
13CH4 puff exp. with graphite limiter (TEXTOR)

• C deposition on graphite test limiter (TEXTOR
  exp.)
• Deposition Efficiency a
• Deposited 13C /injected 13CH4
• C on unpolished C (Ra 1 µm)
• a 9
• C on polished C (Ra 0.1 µm)
• a 1.7
• Surface roughness significantly affects C
  deposition
• Similar or larger than substrate effects (W or
  graphite)

Unpolished Ra 1 µm
a 9
Polished Ra 0.1 µm
a 1.7
Ohmic discharge
A. Kreter, et al., submitted (2008)
7
Experimental conditions for this study

• Effects of surface roughness on C deposition
• Tungsten
• Roughness Ra 9 nm, 18 nm, 180 nm
• Graphite (fine grained graphite)
• Roughness Ra 70 nm, 350 nm, 700 nm
• C deposition on pre-treated tungsten
• High density He plasma exposure
• Nano-structure formed
• H C ion beam pre-irradiation
• C surface concentration 60, 40, 10
• C deposition on heated tungsten
• Temperature range
• 300 ºC ITER wall
• 550 ºC Chemical Sputtering peak
• 850 ºC Thermal diffusion RES

8
Experimental setup for test limiter exposure

• Roof limiter system
• Samples on graphite roof limiter
• Position 46 cm (LCFS) 47.5 cm
• Base temperature 300 ºC
• Standard ohmic plasma
• Ip 350 kA, ne 2.5 x 1019 m-3
• Bt 2.25 T, Ohmic Power 0.3 MW
• Edge plasma Parameter (r 48cm)
• Te 40 eV, ne 2.5 x 1018 m-3

IR thermometer
TEXTOR
ALT-II limiter
0.8
55
Te
59 mm
ne
60 mm
LCFS
LCFS
Ion drift side
0.1
35
46
46
48
48
cm
cm
9
Postmortem analysis (NRA, SIMS , XPS)

• Profilometer
• Surface roughness measurement
• Stylus type (10 µm radius of curvature)
  (DEKTAK)
• NRA (Nuclear Reaction Analysis)
• Analysis beam 2.5 MeV 3He
• Protons produced by D(3He, p)4He 12C(3He, p)14N
  nuclear reactions were detected.
• SIMS (Secondary Ion Mass Spectroscopy)
• XPS (X ray Photoelectron Spectroscopy)
• Colorimetry
• Thickness of C deposition layer estimated by color

10
Setup for study on surface roughness effects

• Pure W samples
• Ra9 nm, 22 nm, 180 nm
• Difference in surface polishing
• Graphite (fine grained)
• Ra70 nm, 350 nm, 700 nm
• Experimental conditions
• 37 shots of OH discharge
• Radial position of 46 cm.
• Deposition mechanism
• Higher carbon density deeper into SOL
• Lower Te deeper into SOL
• Edge effects
• C deposition on W edge adjacent to graphite

Ion drift side
Ra180 nm
Ra9 nm
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C deposition and D retention on W

• C deposition
• Roughness enhances C deposition
• Ra180 nm Long tail
• Sharpe boundary between erosion and deposition
• D retention
• similar to C deposition
• no surface retention in erosion zone
• D/C 0.10.15

NRA measurement
Graphite
W
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D retention (C deposition) on graphite

• C deposition on graphite
• D retention was mainly in C deposition layer
• D/C const in deposition layer
• D retention C deposition
• Characteristics of C deposition on
  graphite
• Roughness enhanced C deposition also on graphite
• No sharp transition between erosion and
  deposition
• different from W

NRA
Measured position
13
C deposition on pre-treated tungsten HC
mixed ion beam pre-irradiation

• (1) (3) HC ion beam pre-irradiation
• 1 keV H3 C
• Fluence 5 x 1024 m-2
• 0.9 C in ion beam ?Surface C 60
• 0.3?40, 0.1?10

Before TEXTOR plasma exposure
W
W
W
(1) C 0.1 in ion beam
(3) C 0.9 in ion beam
(2) C 0.3 in ion beam
C
C
C
O
O
O
Atomic concentration of each pre-irradiated W
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C deposition on pre-treated tungsten He
plasma pre-exposure

• He plasma pre-exposure
• High density pure He plasma exposure in NAGDIS-II
  (Nagoya U.)
• Black surface after 1h exposure at 1300 ºC (flux
  1023 m-2s-1)
• Sudden change of surface color
• He bubble and nanostructure formation
• Surface structure removed before TEXTOR plasma
  exposure
• Loosely bound nano-structure was wiped out
  mechanically
• Roughness of He exposed W
• Roughness 15 nm (after exp.)
• Small pits could be missing due to stylus type
  measurement

M. Baldwin et al., I-20, PSI18
T1600 K
W surface in this work
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C deposition on pre-treated W
After
Before

• HC pre-irradiated W
• C deposition speed relates to surface C
  concentration
• only 10 initial C affects deposition
• No deposition on pure W (0C)
• Ra 10 nm for each W
• He pre-exposed W
• Enhancement of C deposition
• C profile long tail
• increase in deposition area
• large enhancement of deposition despite small
  roughness (15 nm)

46 shots (Ohmic plasma) r 46 cm (same as LCFS)
He pre-exposure
60
40
10
Carbon deposition
0
HC pre-irradiation
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Explanation of roughness effect on deposition

• Roughness (0.01-1 µm) ltlt Ion Lamor radius
  (0.1-1mm)
• D ion flux and C ion flux did not change locally
• local shading effect of D ions may not occur
• Some of sputtered or reflected particles
  redeposited immediately.
• Trapping rate depends on the morphology
• He roughened surface was very fine and
  complicated structure
• He induced roughness could have high trapping
  rate (C deposition)

He roughened W surface
M. Kunster et al., Nucl. Instrum. Meth.B145
(1998)320.
17
Partially heated limiter exp. for C deposition on
W
770 ? 930 ºC
520 ? 600 ºC
280 ?340 ºC
240?290 ºC
EXP-A
EXP-B
A
Heated
A
Heated
non-Heated
non-Heated
Deposition by edge plasma exposure No deposition
on the heated sample.
Deposition by edge plasma exposure Deposition due
to gas puff (CO) No deposition on the heated
sample.
CO gas desorbed above 700 ºC
A-A cross section
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Partially heated limiter exp. (heated W 520 ºC)
0 mm

• non-heated W (240 ºC280 ºC)
• Beltlike C deposition (asymmetry)
• D retention only on C deposition
• D/C ratio 0.3
• consistent with previous results
• Alimov, et. al. Physica Scripta T108 (2004) 46.
• Heated W (520 ºC600 ºC)
• no C deposition
• no near surface D retention
• near peak T of chemical sputtering
• Bulk diffusion and trapping (permeation) could
  occur in erosion area for both W
• Issue Role of WC mixed layer on D retention and
  permeation

56 mm
Heated 520?600 ºC
non-heated 240?290 ºC
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Partially heated limiter exp. (heated W 770 ºC)

• non-heated W (280 ºC340 ºC)
• Beltlike C deposition (asymmetry)
• Dense deposition by CO gas puff
• D/C ratio 0.25
• Heated W (770 ºC930 ºC)
• No C deposition
• No deposition of C originated from CO gas
• indication of bulk diffusion in some area

non-heated 280?340 ºC
Heated 770?930 ºC
NRA
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C depth profiles (heated W 770 ºC)

• A small amount of C only near the surface (SIMS
  1)
• No C in the bulk
• Diffusion length is consistent with previous
  diffusion results (SIMS 2)
• C concentration near surface 30 (XPS)
• Diffusion length
• Experiment 45 nm
• Estimation 37 nm ( )
• K.Schmid et al., J. N. M. 302 (2002) 96.
• Concentration dependent diffusion
• D 4 x 10-20 m-2s-1 (1030 K)
• C diffusion mainly between shots

30
(a)
75 nm
(b)
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2D carbon distribution
2D Carbon surface density (NRA)

• In area A (heated W)
• No C observed near CO gas puff
• In area B (heated W)
• C diffusion in bulk W

non-heated sample
Heated sample

• Ion energy could cause this difference
• C in plasma highly charged ( 4), thermalized
• impact energy E 580 eV (TeTi40 eV)
• C or CO from CO gas singly charged, not
  thermalized
• impact energy E 120 eV (Te40 eV, Ti0 eV)
• Ion range less than a few ML
• Implantation ? segregation ? sputtering,
  sublimation
• more study needed

22
Summary

• Roughness effect on C deposition
• Roughness significantly affects C deposition for
  both W and graphite substrates
• Increase in amount of C deposition
• Extension of C deposition area
• significant for large Ra (engineering surface
  Ra180 nm W)
• Dependence on surface morphology
• significant deposition on He exposed W surface
  despite low Ra (15 nm)
• Carbon deposition at elevated temperature
• Carbon deposition hardly occurred at least above
  520 ºC under TEXTOR edge plasma conditions
• C behavior at elevated temperatures (850 ºC)
  depends on incident carbon energy ? Sophisticated
  modeling needed
• C deposition on W C mixed layer
• Increase in C deposition with C concentration in
  tungsten (up to 60C) in substrates.
• Only 10 of C in W enhance C deposition
• Its effect is less than roughness effect