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 : C+DW
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

4. Erosion 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
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
0.8
ALT-II limiter 55 Te ne
59 mm
60 mm
LCFS
LCFS
Ion drift side
0.1 35 46 cm 48 46 cm 48

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
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
Ion drift side
Ra~180 nm
Ra~9 nm

11. C deposition and D retention on W
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 measurement W
Graphite

12. D retention (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

13. C 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%
Before TEXTOR plasma exposure W W W
(1) C : ~0.1%
in ion beam
(2) C : ~0.3%
in ion beam
(3) C : ~0.9%
in ion beam C C C O O O
Atomic concentration of each pre-irradiated W

14. C deposition on pre-treated tungsten 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
T~1600 K
W surface in this work

15. C deposition on pre-treated W
Before
After
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)
46 shots (Ohmic plasma)
r = 46 cm (same as LCFS)
He pre-exposure
60%
40%
10%
Carbon deposition 0%
H+C pre-irradiation

16. Explanation of roughness effect on deposition
Roughness ( 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.
He roughened W surface

17. Partially heated limiter exp. for C deposition on W
EXP-A
EXP-B A
Heated A’
non-Heated
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

18. Partially heated limiter exp. (heated W : 520 ºC)
0 mm
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
56 mm
Heated
520600 ºC
non-heated
240290 ºC

19. Partially heated limiter exp. (heated W : 770 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
Heated
770930 ºC
non-heated
280340 ºC
NRA
SIMS 2
SIMS 1

20. 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 m-2s-1 (1030 K)
C diffusion mainly between shots
~30%
(a)
~ 75 nm
(b)

21. 2D carbon distribution In area A (heated W) In area B (heated W)
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
Heated sample
non-heated sample
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

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 : 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