Supported by US DOE contracts DE-AC02-09CH11466 Dick Majeski Princeton Plasma Physics Lab with H. Ji, T. Kozub, A. Khodak, E. Merino, M. Zarnstorff Concepts for fast flowing liquid lithium walls and divertors

Liquid lithium PFCs offer alternative aproaches to reactor engineering and physics issues  Confinement and edge physics: –Lithium PFCs improve confinement –Solid and liquid lithium PFCs produce low core contamination –Lithium sputtering characteristics allow a hot, low density edge  Smaller reactor scale size, neutral beam fueling, higher burnup fraction  Engineering: –Renewable liquid surface –Neutron interactions only important for substrate –Convective heat removal (fast flow) permits use of low thermal conductivity substrates (steels) »Localized heat exchanger to remove plasma heat »Cycle coolant through hotter blanket to recover thermodynamic efficiency  Control of in-vessel tritium inventory, PFC no longer needs BOTH neutron AND plasma tolerance, allows low-pressure cooling loop  Requires significant technology development  Requires wider deployment of lithium PFCs in confinement devices

Lithium PFCs improve confinement Energy Confinement Time (ms) Pre-discharge lithium evaporation (mg) R. Maingi, et al., PRL 107 (2011)  Confinement improves in LTX  Any lithium coating improves performance relative to bare high-Z wall  Improvements in coating quality produce performance improvements  Global parameters improve in NSTX –H 98y2 increases from ~0.9   H 98y2 up to 2 observed –No core Li accumulation  ELM frequency declines to zero  Edge transport declines NSTX LTX

LTX: all impurities, including lithium very low, even with liquid lithium walls at 270 °C during gas fueling emissivity peak Li 2+  Lithium core concentration < 0.5% - Estimate from concentration at peak in Li 2+ emissivity - O < 0.05%, Carbon <0.1%  No significant core impurity accumulation for Z > lithium –Total Z eff from O, C, Li ~ 1.04  No other impurity lines detected  Lithium concentration even lower in NSTX -- <0.1%  NSTX is diverted  LTX is wall-limited

Flat electron temperature profile develops in LTX if edge gas load is removed  T e profile initially hollow, with strong fueling  Peaked profile develops  T e profile evolves to flat or hollow, to LCFS  Longer discharges with new OH programming  All fueling (from centerstack) terminated at 462 msec ~3-4 msec required to clear gas from duct LCFS  Edge electron temperature increases to 200 – 250 eV 464 msec 467 msec471 msec

Hot plasma edge is compatible with lithium PFCs  Self-sputtering of Li on D-treated Li also drops with energy: –24.5% at 700 eV –15.8% at 1 keV  Probability of direct reflection of incident H from lithium PFC also drops to 500 eV  Lithium sputtering decreases with energy above 200 eV –Li sputtering yield for D incident on deuterated Li, calculations and IIAX measurements (Allain and Ruzic, Nucl. Fusion 42(2002)202). 45° incidence.  At 700 eV the yield is 9% –Yield rises to slightly above 10%, just above the melting point –Yield is similar for H, D, T  Liquid not structurally damaged by high energy ions

Liquid lithium wall concept  Approach: recirculate liquid lithium within the TF volume –Flow speed: 10 – 20 m/sec  Sufficient for MW/m 2 divertor heat load  Integrates wall with divertor  Droplet or turbulent flow divertor –Further improve power handling  Inductive drive for pumping  J x B restraining currents in free-surface liquid lithium PFC ➱ High current, very low voltage ➱ Requires modest resistive electrical isolation between components  Modest level of thermal isolation to maintain lithium surface below blanket temperature

Low field side access for heating, diagnostics Inductively driven flow in HFS return ducts Optimal cross section of return ducts ➱ Permit low field side NBI ➱ RF launchers and other fueling High field side – axisymmetric, free surface flow Low field side – partial poloidal flow (axisymmetric, free surface) Lithium reservoir incorporates heat exchange system

Flowing lithium divertor concept under development for possible implementation in NSTX-U  Nearer-term divertor test may be feasible in NSTX-U –Recirculating, electromagnetically driven and restrained flow  Smaller scale –Smaller lithium inventory –Startup, operation, shutdown may be feasible within timescale of NSTX-U toroidal field pulse

Two approaches to tritium removal under study  Precipitation: –Solubility of hydrogenics in liquid lithium is <0.05 At. % at 200 C –For a total PFC inventory of 1-2 metric tonnes, 1-2 kg of tritium corresponds to 0.1 – 0.2% atomic –Approach: cool lithium PFC inventory in batches to 190 – 200C »Lithium deuteride, tritide precipitates out as a solid »Remove by filtration  Distillation: –Heat lithium stream (1-2 liters/minute) via electron beam »In this example, a 300 kW beam – similar to a modest e-beam welder – is required –Condense the lithium vapor, pump the liberated T,D –Multiple stages can be employed

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