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Erosion/redeposition analysis of CMOD Molybdenum divertor and NSTX Liquid Lithium Divertor

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) ].

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Erosion/redeposition analysis of CMOD Molybdenum divertor and NSTX Liquid Lithium Divertor

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  1. 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

  2. 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. 266-269(1999)217.

  3. Predicted Mo erosion along CMOD divertor Erosion, nm Erosion, nm Net erosion Gross erosion

  4. Code/data comparison- CMOD divertor Net Erosion DATA (post-exposure tile study-Wampler et al.) WBC • Reasonable code/data agreement

  5. Code/data comparison-CMOD divertor Gross Erosion • Poor code/data agreement (higher code values)

  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.]

  7. REDEP/WBC LLD Analysis

  8. REDEP/WBC LLD Analysis continued

  9. BPHI-3D Code- NSTX Sheath Analysis at Liquid Lithium Divertor • No magnetic sheath predicted; Debye sheath only

  10. 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 D0 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

  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 Te, low Ne •  Kinetic, sub-gyro orbit analysis required (i.e. WBC code)

  12. 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

  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

  14. Gross and net lithium erosion rates along LLD

  15. Liquid Lithium Divertor: Li Transport Results

  16. LLD redeposited lithium ion parameters

  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

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