Concept for fast flowing liquid lithium walls and divertors Dick Majeski Princeton Plasma Physics Lab with H. Ji, A. Khodak, T. Kozub, E. Merino, M. Zarnstorff Supported by US DOE contract DE-AC02-09CH11466
Liquid lithium PFCs offer a possible solution for reactor engineering issues ◆ Engineering features of liquid lithium: – Renewable liquid surface – Neutron interactions only important for supporting substrate » Liquids not damaged by neutrons, fast particles – 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 Ø Potential for control of in-vessel tritium inventory Ø PFC no longer needs BOTH neutron AND plasma tolerance Ø Admits low-pressure cooling ◆ Requires significant technology development
Liquid lithium PFCs offer alternative aproaches to physics issues ◆ Confinement and edge physics: – Lithium PFCs shown to improve confinement – Solid and liquid lithium PFCs produce low core contamination – Lithium PFC compatible with a hot, low density edge Ø Smaller reactor scale size Ø Neutral beam fueling Ø Higher burnup fraction ◆ Development requires wider deployment of lithium PFCs in confinement devices
Lithium PFCs improve confinement Good performance demonstrated with full liquid lithium wall ◆ Global parameters improve in NSTX NSTX Energy Confinement Time (ms) – H 98y2 increases from ~0.9 à 1.3-1.4 Ø H 98y2 up to 2 observed – Core Li accumulation <1% Pre-discharge lithium evaporation (mg) 6 2 m 2 liquid Cold R. Maingi, et al., PRL 107 (2011) 145004 � lithium shells 5 LTX Exp. τ E (ms) Confinement improves in LTX 4 ◆ u Any lithium coating improves 3 performance relative to bare high-Z wall 2 4 m 2 u Improvements in coating quality liquid lithium 1 produce performance improvements Passivated PFC – 80% of LCFS lithium - Core Li concentration 1-3% 0 0 1 2 3 4 τ E,ITER-98P(y,2) (ms)
Flat electron temperature profile develops in LTX if edge gas load is removed All fueling (from centerstack) terminated at 462 ◆ msec ~3-4 msec required to clear gas from duct Lithium PFCs eliminate recycled neutrals ◆ Shot 1504291543 t = 466.9 ms Shot 1504291634 t = 471.2 ms Shot 1504291045 t = 464.0 ms 300 300 300 467 msec LCFS 471 msec LCFS 464 msec LCFS 200 200 200 T e [eV] T e [eV] T e [eV] 100 100 100 0 0 0 0.40 0.45 0.50 0.55 0.60 0.65 0.40 0.45 0.50 0.55 0.60 0.65 0.40 0.45 0.50 0.55 0.60 0.65 R [m] R [m] R [m] ◆ T e profile initially ◆ Peaked profile ◆ T e profile evolves hollow, with strong develops to flat or hollow, to fueling LCFS Edge electron temperature increases to 200 – 250 eV ◆
Hotter plasma edge is compatible with lithium PFCs Lithium sputtering peaks at ~ 200 ◆ eV impact energy – 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 slightly for liquids to ~ 10%, just above the melting point – Yield is similar for H, D, T Liquid not structurally damaged ◆ by high energy ions Self-sputtering of Li on D-treated Li also drops with energy: ◆ – 24.5% at 700 eV – 15.8% at 1 keV u Probability of direct reflection of incident H from lithium PFC also drops to <10% for incident ion energy >500 eV
Liquid lithium wall concept Recirculate liquid lithium within the TF ◆ volume – Flow speed: 10 – 20 m/sec Ø 20 - 30 MW/m 2 divertor heat load Integrates first wall with divertor ◆ Allows droplet or turbulent flow divertor ◆ – Further improve power handling J x B poloidal current to restrain free-surface ◆ liquid lithium PFC – Require 100 mA/cm 2 to balance gravity in a 5T toroidal field Modest level of thermal isolation to maintain ◆ lithium surface below blanket temperature Fluid is returned to the torus top by inductive ◆ pumping (J × B force again) Power requirements for tokamak with 1-2 ◆ meter major radius appear modest – Require detailed analysis of duct flow
Low field side access for heating, diagnostics High field side – Inductively driven axisymmetric, free flow in return surface flow ducts feeding HFS Low field side – partial poloidal flow (axisymmetric, free surface) Small cross section for return ducts Lithium reservoir ➱ Permit low incorporates heat field side NBI exchange system. ➱ RF launchers Liquid salt? and other fueling Required in-vessel liquid lithium inventory 500 – 2,000 kg ➪ dominated by LFS system
Flowing lithium divertor concept would reduce required lithium inventory Free surface Flow-forming flow nozzle Reservoir (cooled) Nonaxisymmetric return ducts Nearer-term divertor test feasible in NSTX-U ◆ – Recirculating, electromagnetically driven and restrained flow – But: drag introduced by divertor fields Smaller scale ◆ » Lithium inventory ~20 kg for example of NSTX-U implementation – Startup, operation, shutdown may be feasible within timescale of NSTX-U toroidal field pulse
Two approaches to tritium removal under study Precipitation (M. Ono): ◆ – Solubility of hydrogenics in liquid lithium is 0.044 At. % at 200 °C » Order of magnitude increase at 300 °C – For a total PFC inventory of 0.5-2 metric tonnes, 0.5-2 kg of tritium corresponds to ~ 0.2% atomic – Approach: cool lithium PFC inventory to 190 – 200 °C » 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
Near-term plans ◆ Constructing ANSYS model for recirculating flow – Estimate current, power requirements to drive return flow in ducts » Transition to axisymmetric in-duct flow – Thermal transfer in reservoir – Model both wall and divertor systems ◆ Engineering studies for toroidal test stand – Toroidal field ~0.5T – Low aspect ratio coil set – Test free surface flow, recirculation in galinstan – Add normal (divertor) field components ◆ Combine test stand studies with renewed numerical modeling effort for free-surface flows
Summary ◆ Confinement: – Lithium PFCs offer improved confinement, low core impurity levels – Access to a hot edge for enhanced performance – ELM suppression in H-mode ◆ Engineering: – Renewable surface » Not damaged by fast particles, neutrons – “Self-cooling” PFC possible » Plasma heat removed with the liquid metal » Allows localized heat exchange; use of liquid salts » Recover thermodynamic efficiency by routing coolant through hot blanket – Approaches to T,D removal appear feasible ◆ Testing in confinement devices promising ◆ Technological development lags far behind
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