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
2Topics 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
3Background 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 : C+DW
Complicated processes : Chemical erosion, C diffusion in W+C , 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, H&C ion irradiation) C deposition at elevated temperature
Detailed study on the mechanism of C and W mixing
4Erosion and deposition of carbon : basics
Carbon deposition is more pronounced on graphite Reflection coefficient is
lower than that on W R~0.6 (50eV C to W)
R~10-4(50 eV C to C)
Carbon ML is easily re-sputtered by reflected H from W substrate
A. Kreter, et al., Plasma Phys. Control. Fusion 48 (2006) 1401
Difference in reflection
Enhancement of sputtering of surface C
5Evolution 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
Sputtering of C :YC
Reflection of C : Rc
613CH4 puff exp. with graphite limiter (TEXTOR)
C deposition on graphite test limiter (TEXTOR exp.) Deposition Efficiency
Deposited 13C /injected 13CH4
C on unpolished C (Ra ~ 1 µm)
~9% C on polished C (Ra ~ 0.1 µm)
~1.7%
Surface roughness significantly affects C deposition Similar or larger than substrate
effects (W or graphite)PolishedRa ~0.1 µm
A. Kreter, et al., submitted (2008)
UnpolishedRa ~1 µm
Ohmic discharge
~9%
~1.7%
7Experimental 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
8Experimental setup for test limiter exposure
IR thermometer 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
60 mm
59 m
m
Ion drift side
TEXTOR
Te ne
35
55
46 48 46 48
ALT-II limiter
cm cm0.1
0.8
LCFS LCFS
9Postmortem 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
10Setup for study on surface roughness effects
Pure W samples Ra~9 nm, ~22 nm, ~180 nm Difference in surface polishing
Graphite (fine grained) Ra~70 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
Ra~180 nm
Ra~9 nm
Ion drift side
11C deposition and D retention on W
C deposition Roughness enhances C
deposition Ra~180 nm : Long tail Sharpe boundary between
erosion and deposition D retention
similar to C deposition no surface retention in
erosion zone D/C = 0.1~0.15
NRA measurementW Graphite
12D 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
W Graphite
Measured position
13C deposition on pre-treated tungsten H&C mixed ion beam pre-irradiation
(1) – (3) H+C 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%
(1) C : ~0.1%in ion beam
(2) C : ~0.3%in ion beam
(3) C : ~0.9%in ion beam
Atomic concentration of each pre-irradiated W
W
CO
W W
C CO O
Before TEXTOR plasma exposure
14C 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
Before TEXTOR exposure
W surface in this work
T~1600 K
15
10
8
6
4
2
0
C a
real
den
sity
(x1
01
7 c
m-2
)
49.549.048.548.047.547.046.546.0
Radius (cm)
He roughened W Surface C ~60%: W Surface C ~40%: W Surface C ~10%: W Reference W
C deposition on pre-treated W
Carbon deposition
H+C pre-irradiated W C deposition speed relates to
surface C concentration only 10% initial C affects
deposition No deposition on pure W (0%C)
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)
Before After
He pre-exposure
H+C pre-irradiation
60%40%
10%
0%
46 shots (Ohmic plasma)r = 46 cm (same as LCFS)
16Explanation of roughness effect on deposition
He roughened W surface
Roughness (0.01-1 µm) << 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)
M. Kunster et al., Nucl. Instrum. Meth.B145 (1998)320.
17Partially heated limiter exp. for C deposition on W
770 930 ºC
Deposition by edge plasma exposure
Deposition due to “gas puff” (CO)
No deposition on the heated sample.
520 600 ºC
Deposition by edge plasma exposure
No deposition on the heated sample.
280 340 ºC240290 ºC
A
A’
A-A’ cross sectionCO gas : desorbed above ~700 ºC
EXP-A EXP-B
HeatedHeatednon-Heated
non-Heated
18
Partially heated limiter exp. (heated W : 520 ºC) non-heated W (240 ºC~280 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 ºC~600 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
Heated520600 ºC
non-heated240290 ºC
0 mm
56 mm
19
Heated770930 ºC
non-heated280340 ºC
non-heated W (280 ºC~340 ºC) Beltlike C deposition (asymmetry) Dense deposition by CO gas puff D/C ratio ~ 0.25
Heated W (770 ºC~930 ºC) No C deposition
No deposition of C originated from CO gas
indication of bulk diffusion in some area
Partially heated limiter exp. (heated W : 770 ºC)
NRA
SIMS 2SIMS 1
20C 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% (X
PS) 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(a)
(b) ~ 75 nm
Dtx 2
~30%
212D carbon distribution
2D Carbon surface density (NRA)
Ion energy could cause this difference C in plasma : highly charged (~ +4), thermalized
impact energy E ~ 580 eV (Te~Ti~40 eV)
C+ or CO+ from CO gas : singly charged, not thermalized impact energy E ~120 eV (Te~40 eV, Ti~0 eV) Ion range ~ less than a few ML Implantation segregation sputtering, sublimation more study needed
Heated sample non-heated sample
In area A (heated W) No C observed near
CO gas puff In area B (heated W)
C diffusion in bulk W
22Summary
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 : Ra~180 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
60%C) in substrates. Only 10% of C in W enhance C deposition Its effect is less than roughness effect