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

J.N Brooks, PFC/MIT 7/09 2 CMOD Mo tile divertor erosion/redeposition analysis CMOD Mo tile divertor erosion/redeposition analysis [ w/ D. Whyte, R. Ochoukov (MIT) ] 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. W.R. Wampler et al. J. Nuc.Mat (1999)217.

J.N Brooks, PFC/MIT 7/09 3 Erosion, nm Predicted Mo erosion along CMOD divertor Gross erosion Net erosion

J.N Brooks, PFC/MIT 7/09 4 DATA (post-exposure tile study-Wampler et al.) Code/data comparison- CMOD divertor Net Erosion Reasonable code/data agreement WBC

J.N Brooks, PFC/MIT 7/09 5 Code/data comparison-CMOD divertor Gross Erosion Poor code/data agreement (higher code values)

J.N Brooks, PFC/MIT 7/09 6 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. REDEP/WBC NSTX Liquid Lithium Divertor Analysis [with D. Stotler, R. Maingi et al.]

J.N Brooks, PFC/MIT 7/09 7 REDEP/WBC LLD Analysis

J.N Brooks, PFC/MIT 7/09 8 REDEP/WBC LLD Analysis continued

J.N Brooks, PFC/MIT 7/09 9 BPHI-3D Code- NSTX Sheath Analysis at Liquid Lithium Divertor No magnetic sheath predicted; Debye sheath only

J.N Brooks, PFC/MIT 7/ 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 REDEP/WBC code package--computation of sputtered particle transport

J.N Brooks, PFC/MIT 7/09 11 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)

12 WBC Simulation of LLD sputtered lithium transport: 50 trajectories shown; 2-D view Long mean free paths seen for ionization; subsequent long, complex, ion transport UEDGE/NSTX GRID

13 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

J.N Brooks, PFC/MIT 7/09 14 Gross and net lithium erosion rates along LLD

J.N Brooks, PFC/MIT 7/09 15 Liquid Lithium Divertor: Li Transport Results

J.N Brooks, PFC/MIT 7/09 16 Parameter (average across divertor)Value Charge state1.083 Energy429 eV Transit time 41  s Angle (to surface normal)21 deg LLD redeposited lithium ion parameters

J.N Brooks, PFC/MIT 7/09 17 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 erosion- puzzling 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 Conclusions