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

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

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

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

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

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）

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

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

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

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

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

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

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

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

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

Explanation of roughness effect on deposition 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. He roughened W surface

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

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

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

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)

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

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