Plasma Material Interactions (PMI) Thrust for Enhancing Modeling & Predictive Computations

1. Plasma Material Interactions (PMI) Thrust for Enhancing Modeling & Predictive Computations J.N. Brooks1, J.P. Allain1, T.D. Rognlien2 1Purdue University 2LLNL ReNeW Meeting, UCLA, March 4-6, 2009

2. Thrust for Enhancing PWI Modeling & Predictive Computations • GOAL: Increase our predictive Plasma Wall Interaction (PWI) analysis and modeling capability; help plug gaps-- help identify workable surface materials, PFC designs, plasma operating parameters, define research needs. • Modest effort: 10 M$ (~ 2 M$/yr for 5 yrs; ~5 FTE’s/yr increase) w/follow-on after the initial 5 yr work. • Modest enhanced effort in plasma/material interaction predictive modeling & code validation. • Areas: Edge/SOL plasma with turbulence, sputtering erosion/redeposition, transient plasma effects on PFC’s. Analysis of present devices, ITER, start of PWI DEMO analysis. • Use: (1) Code Package OMEGA, (2) HEIGHTS Code Package, (3) other codes as feasible. • Perform code/data validation via US and international fusion device operation. • The models/codes generally exist: this thrust would fund increased use. • Would interact with thrusts/efforts to increase operating time, new device construction, supercomputer applications (e.g., Fusion Simulation Project), transient plasma control, core plasma theory/modeling, and similar relevant areas.

3. Plasma/Material Interactions Plasma/material interactions (PMI) is probably the single most critical technology issue for fusion. Concerns: (1) Plasma facing component lifetime (2) Core plasma impurity contamination (3) Tritium inventory/operational requirements Critical Issues: • Sputtering erosion and impurity transport • Plasma transient erosion (Edge Localized Modes (ELM’s), disruptions, runaway electrons.) • Plasma contamination due to erosion • Tritium co-deposition in eroded/redeposited material, and mitigation Important Issues: • Dust-formation and transport; safety • For tungsten-He, D-T, bubble formation and effects • Hydrogen isotope and helium trapping, reflection, etc.

4. Existing US PMI Modeling Capability

5. Current modeling challenges sputtering erosion/redeposition: • Convective (“blob”) transport: edge plasma solutions and 3-D time dependent impurity generation/transport. • Mixed materials: Be/W, Be/C, Be/C/W, etc. (sputter yields, tritium trapping, thermo-mechanical properties). • Self-consistent core/edge/SOL plasma + plasma-material-interactions. • Petascale Computing. Visualization • Liquid lithium surface analysis (e.g. NSTX). • Present device modeling & code validation.

6. Sputtering erosion/redeposition analysis for ITER Package-OMEGA* • UEDGE/DEGAS: D-T ion and neutral fluxes to divertor, wall; scrape-off layer plasma parameters (fluid/ Monte-Carlo) • TRIM-SP, ITMC: sputter yields (binary collision, single and mixed-material) • WBC: sputtered atom/ion transport in scrape off layer (3-D, full-kinetic, Monte Carlo) • REDEP/WBCpackage: divertor, wall erosion/redeposition analysis (impurity transport, atomic and molecular processes, sheath, sputter yields, tritium codeposition, etc.) • W-MIX, MD: mixed-material surface evolution, molecular-dynamic sputter yields • BPHI-3D: sheath analysis (3-D kinetic) • Data (where available) * Omnibus Modeling of Erosion Generalized Analysis [1-4] (1-4) J.N. Brooks, J.P. Allain, T.D. Rognlien, Phys. Plasmas 13(2006)122502., T.D. Rognlien, R.H. Bulmer, M.E. Rensick, J.N. Brooks, J. Nuc. Mat. 363-365(2007)658, J.N. Brooks, J.P. Allain PSI-18, J. Nuc. Mat. to be pub., J.N. Brooks, J.P. Allain, R.P. Doerner, A. Hassanein, R. Nygren, T.D. Rognlien, D.G. Whyte, “Plasma-surface interaction issues of an all metal ITER, Nuclear Fusion 49(2009)035007.

7. UEDGE/DEGAS plasma edge/scrapeoff layer solution for ITER (T. Rognlien et al.) • 2-D fluid equations solved for background plasma. • Flux-limited, self consistent neutrals model, solved simultaneously. • DEGAS-2 Monte Carlo code computes detailed charge exchange (CX) energy spectrum to the wall. • ITER base case: 100 MW input from core to edge. • Anomalous radial diffusion coefficients, D = 0.3 m2/s, Xei = 1 m2/s • Convective (“blob”) plasma transport modeled as a time-averaged, radially-varying convective component of the radial fluid velocity; Vc = 70 m/s max.

8. UEDGE/DEGAS ITER boundary plasma simulation Major difference in edge plasma solution with convective flow with convection higher density at wall First wall ~700 m2 Baffle ~ 100 m2 Divertor ~ 50 m2 D-T flux to the wall no convection (diffusion only) ~X50 wall flux with convection

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 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

10. ITER: Transport summary of sputtered outer first wall material; WBC code, 106 histories/run. Plasma with convection, reference impurity convection model. • Be core plasma contamination: Acceptable (~2%) • W core plasma contamination: Negligible • Most material redeposits on wall or baffle • {Key additional required work: convective transport model upgrades/use; detailed spatial resolution, inner wall analysis, wall sheath effects, rf sheath effects}

11. OMEGA / W-MIX-code results: Net beryllium growth rate on ITER base tungsten divertor (and gross rate from wall-to-divertor transport) • No beryllium growth over most of the surface (remains tungsten)-- • due to intense re-sputtering/reflection • Be growth occurs at/near strike point • Thermal effects (evaporation) do not affect results (but transient heating needs evaluation)

12. Be/W Alloy Formation-Assessment • The surface temperature is high enough (750 °C) to promote significant alloy growth, if at all, only in the region extending roughly 2 cm on either side of the strike point. • While this is encouraging, the extrapolation of relevant laboratory results to ITER is highly dependent on assumptions of exposure conditions, and obviously on plasma conditions, and is difficult to reliably predict at this time.

13. Tritium codeposition • For an ITER tungsten divertor and outer first wall; --- Codeposition is negligible due to minimal sputter erosion/redeposition and low T/W trap ratios. • For a beryllium wall: --Two rough methods are used here to update the T/Be codeposition estimate, both based on scaled laboratory data {1,2]a, The first uses a constant value of (D+T)/Be in codeposits of 0.08 at 200 C [1]. The second uses a scaling law developed [2] for (D+T)/Be in codeposits under varying codeposition temperature and flux conditions. • The codeposited tritium calculated using either approach is quite similar, 1.5–1.8 gT/400s-shot due to beryllium outer-wall erosion. {Key additional required work: convective transport model upgrades/use; detailed spatial resolution, inner wall analysis, wall sheath effects, improved T/Be codeposition temperature and flux-dependent models/use} [1] R.A. Causey, D.S. Walsh, J. Nucl. Mater. 245(1998)424. [2] G. De Temmerman et al. (PISCES data), Nucl. Fusion 48(2008)07008.

14. Plasma Transient Erosion • Edge Localized Modes (ELMs) • Plasma disruptions • Vertical Displacement Events • Runaway electrons

15. ELM Analysis (Hassanein et al.)Plasma-Target Interaction Physics Models in code package HEIGHTS

16. HEIGHTS results: ITER tungsten divertor ELM response as a function of ELM energy fraction; Q0 = 127 MJ released at midplane, ELM duration 1 ms • Melting occurs for high energy fractions (with considerable material loss)

17. Thrust Summary • We propose an initial 5 year funding increase to help remedy the existing gap situation in erosion/redeposition and transient plasma effects on PFC’s. • This would be used for a coordinated program to analyze existing and future devices and improve modeling capability. We would also interact strongly with our world fusion program colleagues. • A follow on program continuing this effort as well using new facilities would be defined. • Output/Goals • Improved understanding of present device PWI issues, • Improved predictions of plasma facing component performance and required plasma operating limits for ITER and beyond, • Help define needed plasma/material interaction R&D. • Tasks • Erosion/redeposition and transient effects code packages enhanced analysis (e.g., NSTX, CMOD, JET, ITER, some DEMO effort) and model upgrades. • Validation efforts-experiments, code/data comparison, on existing US and world-fusion program devices and off-line test stands (e.g. beam accelerators, linear plasma devices etc.). {Validation-on new devices if/when available.}