with A. Fisher, E. Gilson, J. Hinojosa, M. - PowerPoint Presentation

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with A. Fisher, E. Gilson, J. Hinojosa, M. Hvasta, H. Ji, E. Kolemen, T. Kozub, R. Maingi, and M. Ono (PPPL)andN. Morley (UCLA), H. Stone (Princeton U.)

Supported by US DOE contract DE-AC02-09CH11466

Dick Majeski

Development of flowing lithium walls for fusion systemsSlide2

OutlineSlow vs. fast flow for liquid lithium divertorsHeat removalScrape-off layer in a low recycling lithium tokamakProducing fast, free surface liquid metal flowsDevelopment of flowing LM PFCs: a proposal for a toroidal test standAn approach to D,T removal from lithiumFlowing LM systems at UCLA, PPPL, Princeton U.ConclusionsSlide3

Capillary (slow) flowThin liquid metal film restrained by surface tensionFlow rate sufficient for replacementHeat removed through supporting substrateSignificant heat dispersal via evaporation, radiationExamples: FTU capillary porous system (CPS), NSTX-U pre-filled tilesFast flowThicker (mm ~ cm) scale liquid lithium layerPumped and restrained by J×B forcesHeat removed advectively with the fluidFluid cooled in heat exchangerHigh pressure (helium) system, or low pressure (liquid salt)Flow rate: a few to ~ 20 m/secTEMHD-driven flow (University of Illinois) in an intermediate regimeTwo approaches to flow in a liquid lithium divertorSlide4

2× increase in heat load requires 4× increase in velocityITER example: (Pα + Paux) = 173 MW exhausted at 20 MW/m2Assume equal, radially constant, load on inner and outer divertorRequired flow velocity: 9.3 m/sec Lithium flow rate determined by divertor heat loads

Flow speed requirement determined by:Divertor heat load profileScrape-off layer (SOL) widthLimit surface temperature rise ∆Tsurface < 200 ºC

𝛋 = thermal conductivity, lithium = 85 W /mºC 𝜌m =mass density=0.5 g/cm3,Cp= specific heat capacity=3.56 J/gºCSlide5

Scrape-off width determined by radial gradients in temperature and density, outside the last closed flux surfacePdivertor ~ nSOL × TSOLLow recycling: isothermal Ti, Te (Krasheninnikov & Zakharov PoP, 2003)Temperature SOL scale length effectively infiniteDensity scale length determined by ion orbit width (Larmor radius in Bpol)Low recycling: edge Ti ~ core Ti But: SOL expected to be much wider with lithium PFCsIon orbits an order of magnitude wider than for high recycling SOLAlso: 80-90% of SOL ions mirror trapped

Ion loss rate to divertor much slowerDensity SOL scale length also affected by radial transport➪ Strongly broadened divertor power deposition profileImplications for capillary flow, TEMHD

BroadOuterSOLHFSedge

LTX SOL Slide6

Lithium temperature limit determined by evaporation

Sputtering strongly reduced with low recycling lithium PFCs Greatly reduced ion flux to the divertorPower conserved as recycling is reducedIon flux ~ (Tedge, recycling/Tedge, nonrecycling)Sputtering yield/ion drops at high ion energyEvaporative flux determines impurity influxBroad SOL with lithium PFCs screens impuritiesNeed to re-evaluate old (400 ºC) estimates of surface temperature limit LTX operated with full lithium wall at 270 ºCLimited by vacuum conditions, not lithium influx

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Flow speed requirements for liquid lithium PFCs in a reactor require new modeling of SOL, experimentsLTX (1-2% core lithium)

ReactorSlide7

Flow path in a tokamak reactor will be poloidalTransverse to toroidal magnetic fieldNonaxisymmetric flow generates Hartmann currents ➪ MHD dragBut axisymmetric flows cannot produce Hartmann currentsR. Woolley, APEX program, L. Zakharov, and Neil Morley, S. Smolentsev, D. Gao, Fus. Eng. Design 63-64(2002)343.No MHD drag if flow is along a flux surfacePoloidal current driven in the liquid metal can restrain surface J×Btoroidal forces support the liquid against gravity Require 100 mA/cm2 to balance gravity in 5T toroidal field10’s of kA total current at very low voltageTokamak-relevant fast flow can be demonstrated on a toroidal, magnetized test standExample: MTOR, now configured as a quarter-torus – QTOR - at UCLADevelopment of fast flow technology doesn’t require a tokamak plasmaSlide8

Focusing near term development on divertors

(Very) conceptual reactorRecirculate liquid lithium within the TF volumeFlow speed: 10 – 20 m/sec20 - 40 MW/m2 divertor heat loadJ x B poloidal current to restrain free-surface liquid lithium PFCLiquid lithium is circulated by J×B pump

Divertor concepts can be implemented in present day fusion experiments

EAST, NSTX-USlide9

Flowing liquid metal torus (FLIT)Test axisymmetric LM flows in toroidal fieldsGallium-Indium-Tin (Galinstan) firstDevelopment on QTOR (UCLA)Liquid lithium tests to follow1 T, R0 = 0.53 m, a = 0.3 m, b = 0.6 m10 sec pulse at 1 TAxisymmetricfree surface flow

J x B pumping to high-field side nozzleNew proposal: toroidal test stand for flowing liquid metal divertor, PFC developmentSlide10

Horizontal cut showing JxB pumping system20 kA per pumpLithium would reduce current requirementConcept employs axisymmetric flow, toroidally distributed JxB pumpingPump ductJxB pump electrodesSlide11

Removal of D,T from the liquid lithium PFC is essentialLiquid lithium inventory required for reactor implementation can vary greatly (ST-based examples)Inventory of 50 kg or less feasible for divertor-only system100 – 200 kg of lithium for divertor + high field side flowFor a total lithium inventory of 100 kg, 100 g of tritium corresponds to ~ 0.2% atomicHigher concentrations will precipitate LiD, LiTRemoval of D, T at atomic concentrations of a few tenths of a percentMuch higher concentrations than in a breeding blanketSlide12

Cooling + centrifugal separation of LiD, LiT is attractive approachSeparation via cascaded centrifuges Centrifuges would operate at ~190°C

Enriched slurry of LiD, LiT removed continuously at peripheryAlternatives:Yttria gettering of D,T (IFMIF)DistillationSolubility of hydrogen in lithium falls rapidly with temperature0.3 At. % at 300 °C ➪ 0.044% at 200 °

C.LiD, LiT will be formed Density of LiD, LiT twice liquid lithiumM. Ono et al., to be presented at 2016 FEC, KyotoSlide13

MTOR magnetFull torusR = 80 cma = 40 cmBinboard = 0.5T1/R falloffAxisymmetryQuarter torusBinboard = 1T1/R falloffExisting PbLi, Ga-In-Sn and Hg loops are used for closed channel and free surface experimentsSlide14

BOB magnetQTOR magnetBOB Magnet1m x 0.15m x 0.15m gap Up to 2T steady stateOpen top magnet gap for optical accessNew flexible orientation with respect to gravity Flow loops on both sidesCurrently being used for closed channel, heated PbLi blanket flow experimentsSlide15

Thermal Camera on top of the flow

Flow channel experiments in the Liquid Metal eXperiment (LMX): Current in LM can enhance the heat transport hot cold Force causes mixing in flow, improving heat flux to the bottom and sides of the channel. Heat source to the leftIR camera view from aboveSlide16

Current densitySimulations: LM heat transfer modified by JxB force Experimental setup:Lorentz force:

Enhance heat transferHeat transfer:

Use electric current and magnetic field to control LM temperature and heat fluxBy

Jx

Fz

-150 -75 0 75 150

Flow channel geometrySlide17

Flow along a curved substrate can stabilize a fluid film

➪nonconductive fluidCurved substratehi

𝛿 = hi/R ≪ 1iparameters: Define modified Bond number:(stability/instability boundary)

Howard Stone, Complex Fluids Group, Princeton UniversityRef: Trinh et al. Phys. Fluids 2014

ρSlide18

Flow requirements for liquid metal PFCs set by heat removalFlow rates ~10 m/sec may be needed for the divertorBut broader SOL power flow in a low recycling tokamak must be investigated Axisymmetric flows in a toroidal magnetic field can be investigated in test stands, without plasmaUCLA has a quarter-torus (QTOR) availableA 1 T toroidal test stand for flowing lithium divertor development has been proposed at PPPLPlasma physics/confinement knowledge gap for liquid lithium PFCs is being filledExperiments on NSTX, LTXTechnology development for fast flow badly neededConclusions