Erosion/redeposition analysis of CMOD Molybdenum divertor and NSTX Liquid Lithium Divertor J.N. Brooks, J.P. Allain Purdue University PFC Meeting MIT, July 8-10, 2009
CMOD Mo tile divertor erosion/redeposition analysis [ w/ D. Whyte, R. Ochoukov (MIT) ] W.R. Wampler et al. J. Nuc.Mat. 266-269(1999)217. Puzzling results for Mo tile erosion--high net sputtering erosion in apparent contradiction of some models. REDEP code package (rigorous) analysis of outer divertor conducted. {New probe diagnostic being implemented-mit.} Sheath BPH-3D code applied to very near tangential (~0.6 ) magnetic field geometry. D, B, Mo on Mo, sputtering erosion/transport analyzed, for 600, 800, 1000, 1100 KA shots, and for OH and RF phases. Uses TRIM-SP sputter yield and velocity distribution simulations. RF induced sheath effect studied. 2
Predicted Mo erosion along CMOD divertor {With preliminary N e, D + flux model, preliminary numerical tile-averaging model} WBC C-MOD outer divertor analysis. 1/28/09 8 plasma conditions (4 currents x (OH phase +RF phase), 1259 s total, 1% B + D sputtering and self-sputtering, (no B near-surface recycle), TRIM-SP pure-mo sputter simulation data. ~1e6 histories. WBC C-MOD outer divertor analysis. 1/28/09 8 plasma conditions (4 currents x (OH phase +RF phase), 1259 s total, 1% B + D sputtering and self-sputtering, (no B near-surface recycle), TRIM-SP pure-mo sputter simulation data. ~1e6 histories. 2200 150 Erosion, nm 2000 1800 1600 1400 1200 1000 800 600 no rf sheath w/rf sheath Erosion, nm 100 50 0-50 w/rf sheath 400 200-100 no rf sheath 0 0.55 0.54 0.53 0.52 0.51 0.5 0.49 0.48 0.47 0.46 Distance below midplane, m -150 0.55 0.54 0.53 0.52 0.51 0.5 0.49 0.48 0.47 0.46 Distance below midplane, m Gross erosion Net erosion 3
Code/data comparison- CMOD divertor Net Erosion DATA (post-exposure tile study-wampler et al.) WBC* Reasonable code/data agreement * {with Preliminary N e, D + Flux Model, preliminary numerical tile averaging model} 4
Code/data comparison-cmod divertor Gross Erosion Poor code/data agreement (higher code values) 5
REDEP/WBC NSTX Liquid Lithium Divertor Analysis [with D. Stotler,, R. Maingi et al.] Goals-determine: Surface temperature limit set by lithium sputtering/runaway and/or evaporation Lithium density in plasma edge/sol; core plasma contamination potential Flux of sputtered lithium to carbon surfaces and D-pumping capability. 6
REDEP/WBC LLD Analysis REDEP/WBC code simulation of NSTX Liquid Lithium Divertor (LLD). Full kinetic, sub-gyro-orbit analysis. (100,000 sputtered histories per simulation). Plasma parameters from UEDGE/DEGAS solution, 0.65 D + reflection coefficient [D. Stotler, R. Maingi, et al. (2008)]. Includes LLD surface temperature profile, at t = 2.01020 seconds [L. Zakharov]. (T max = 281 C). Incident particles: D + ions, Li ions. (C on Li under analysis) Energy-dependent and surface-temperature-dependent sputter yields for D, Li, incidence, from TRIM-SP code runs (J.P. Allain), for D containing Li; 45 incidence. Charged/neutral lithum sputtered fraction model (Allain) used (~2/3 charged, ~1/3 neutral). Reference WBC model used for recycle/resputter of sputtered Li + (net atomic Li sputtered ~ ½ of total gross (charged + neutral). 7
REDEP/WBC LLD Analysis continued Sputtered Li atom velocity distribution functions used in WBC per TRIM-SP results. Li I density-dependent and Te-dependent electron impact ionization rate coefficients from ADAS, S.D. Loch et al., Atomic Data and Nuclear Data Tables 92(2006)813. Sheath: BPHI-3D code run for NSTX conditions (Brooks/Ochuockov) Dual-structure (Debye sheath + magnetic sheath) found not to be present (due to weaker magnetic field, less oblique B-field incidence, viz. 5-10 NSTX vs. 1-2. for ITER, CMOD, etc.). Debye-sheath-only model therefore used in WBC. Locally-varying sheath potential eφ = 3kTe. 8
BPHI-3D Code- NSTX Sheath Analysis at Liquid Lithium Divertor No magnetic sheath predicted; Debye sheath only 9
REDEP/WBC code package-- --computation of sputtered particle transport 3-D, fully kinetic, Monte Carlo, treats multiple (~100) processes: Sputtering of plasma facing surface from D-T, He, self-sputtering, etc. Atom launched with given energy, azimuthal angle, elevation angle Elastic collisions between atom and near-surface plasma Electron impact ionization of atom impurity ion Ionization of impurity ion to higher charge states Charge-exchange of ion with D 0 etc. Recombination (usually low) q(e +VxB) Lorentz force motion of impurity ion Ion collisions with plasma Anomalous diffusion (e.g., Bohm) Convective force motion of ion Transport of atom/ion to core plasma, and/or to surfaces Upon hitting surface: redeposited ion can stick, reflect, or self-sputter Tritium co-deposition at surface, with redeposited material Chemical sputtering of carbon; atomic & hydrocarbon A&M processes 10
Interesting erosion/redeposition physics for sputtered lithium in the NSTX low-recycle plasma regime studied Large sputtered-atom ionization mean free path, order of 10 cm Large Li +1 gyroradius ( ~5 mm), due to relatively low B field Low collisionality, due to high T e, low N e Kinetic, sub-gyro orbit analysis required (i.e. WBC code) 11
WBC Simulation of LLD sputtered lithium transport: 50 trajectories shown; 2-D view UEDGE/NSTX GRID Long mean free paths seen for ionization; subsequent long, complex, ion transport 12
WBC Simulation of LLD sputtered lithium transport: 50 trajectories shown; 3-D view x = distance along divertor (strike point at zero), y=distance along toroidal field 13
Gross and net lithium erosion rates along LLD Lithium erosion rate, nm/s 11 10 9 8 7 6 5 4 3 2 1 WBC Analysis: NSTX Liquid-Lithium Divertor (w/uedge R=0.65 Solution, @2.01 s) Gross Net 0-0.05-0.03-0.01 0.01 0.03 0.05 0.07 Distance along divertor, X, m 14
Liquid Lithium Divertor: Li Transport Results Results, run of 5/5/09 (PRELIMINARY) Sputtering is OK. Peak gross rate = 10.3 nm/s, peak net rate = 5.7 nm/s. Average effective D + sputter yield = 0.07 50.3% of sputtered (neutral) lithium is ionized within the computation zone (LLD and associated near-surface grid). 49.7% of sputtered lithium escapes. Of the zone-ionized material, 91% is redeposited on LLD, 9% escapes. Fate of escaping lithium: ~13% (of total sputtered) goes towards core plasma (not followed further), ~24% goes to outside (higher major radius) of LLD, ~17% goes to inside of LLD. Thus, roughly 50% of sputtered Li would likely impinge on the various carbon surfaces. Total sputtered Li atom current = 1.38 x10 20 s -1. About 9% is from selfsputtering, rest from D + sputtering. 15
LLD redeposited lithium ion parameters Parameter (average across divertor) Charge state Energy Value 1.083 429 ev Transit time Angle (to surface normal) 41 µs 21 deg 16
Conclusions CMOD Molybdenum divertor Analysis Analysis completed for complex, ~1200 sec campaign, with 8 plasma conditions. Acceptable code/data comparison for net Mo erosion, not good for gross erosionpuzzling result. RF sheath significant but not a major effect. Gross rate data via Mo photon emission being re-analyzed (Whyte et al.), WBC analysis will continue as needed. NSTX Liquid Lithium Divertor Analysis Results are encouraging- --Moderate lithium sputtering; no runaway --About 50% of the lithium is transported to carbon surfaces --LLD could apparently handle much higher heat flux Several model enhancements being implemented Higher temperature case analysis, at ~5.0 seconds Carbon sputtering of lithium and C/Li material mixing; MD modeling of Li-D-C system (w/ P. Krstic ORNL) Higher power case-will need further UEDGE analysis 17