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Effects of tungsten surface condition on carbon deposition

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)

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Effects of tungsten surface condition on carbon deposition

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  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 ~300ºC, ~550ºC, ~850ºC

  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 • ~300ºC : ~ITER wall • ~550ºC : ~Chemical Sputtering peak • ~850ºC : 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 : ~300ºC • 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 • 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 • 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 measurement Graphite W

  12. D retention (C deposition) on graphite W 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 tungstenH&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 (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

  14. C deposition on pre-treated tungstenHe plasma pre-exposure Before TEXTOR exposure • He plasma pre-exposure • High density pure He plasma exposure in NAGDIS-II (Nagoya U.) • Black surface after ~1h exposureat 1300ºC (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 T~1600 K W surface in this work

  15. C deposition on pre-treated W After Before • 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 (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) 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’ non-Heated Heated non-Heated Depositionby edge plasma exposure No depositionon the heated sample. Depositionby edge plasma exposure Depositiondue to “gas puff” (CO) No deposition on the heated sample. CO gas : desorbed above ~700ºC A-A’ cross section

  18. Partially heated limiter exp. (heated W : 520 ºC) 0 mm • non-heated W (240ºC~280ºC) • 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 (520ºC~600ºC) • 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 non-heated 240290 ºC Heated 520600 ºC

  19. SIMS 2 SIMS 1 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 non-heated 280340 ºC Heated 770930 ºC NRA

  20. C depth profiles (heated W : 770ºC) • 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)

  21. 2D carbon distribution • In area A (heated W) • No C observed near CO gas puff • In area B (heated W) • C diffusion in bulk W 2D Carbon surface density (NRA) non-heated sample 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

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