liquid metal divertor options for fnsf - ucla

FNSF/PFC session 4 August 2010

PRINCETON PLASMA PHYSICS LABORATORY

PPPL

Liquid metal divertor options for FNSF

Dick Majeski, PPPL



PPPL

Engineering features of liquid metal divertors

  Continuously renewed as new fluid enters the system   Neutron damage not a concern for liquid metals

–  Liquid metal is only subject to PMI –  Substrate is only subject to neutron damage

  PMI limited to sputtering + evaporation –  No significant erosion/redeposition issue for liquids

  Several design approaches –  Fast flowing jets, open-channel systems –  Slowly flowing systems with capillary restraint (porous refractory

metals)   Liquids commonly considered: lithium, gallium, tin   BUT – engineering of liquid metal systems is still at an early

stage of development –  No attempt at a full flowing liquid metal divertor design since the

ALPS program investigated candidates for NSTX



PPPL

Lithium and high recycling liquids (gallium, tin) have different fusion applications

  Low recycling lithium walls shown to increase energy confinement –  Smaller scale size, driven tokamaks/spherical tokamaks possible

»  Exploratory reactors with high tritium burnup fraction »  Fusion-fission hybrids » Neutron sources at high wall loading (>10 MW/m2) for material

testing   But: implementation of lithium walls in a nuclear tokamak designed for

high recycling walls carries few advantages –  For the FNSF divertor, a reasonable choice may be a high recycling

liquid metal – gallium or tin   If modest improvements in confinement or density control are desirable,

a tin-lithium eutectic might be a reasonable choice –  Self-cooled gallium, tin, Sn-Li jet –  J x B driven flows –  Evaporatively cooled porous system



PPPL

Evaporation limits the operating temperature range for liquid metals

Gallium-1100 C

Tin-1300 C

  But - 450 °C lithium limiter operation in CDX-U did not result in a lithium plasma –  Zeffective remained close to 1

  Only experimental data available

Lithium~450 C

SnLi

 Total flux (sputtering + evaporation) imposes a temperature limit (Rognlien)

– 500 – 600 °C for SnLi

– 800 – 900 °C for Sn  Operating limit for lithium is much lower

– <450 °C in a conventional divertor – No estimate has been performed for the Super-X geometry



PPPL

Flow rates for a liquid metal divertor   Flow rate is set by limits in erosion, temperature or liquid D-T inventory

–  Temperature, hydrogenic inventory limits most restrictive

  Capillary or thin-film systems rely on cooling from behind the liquid substrate. –  Flow rate of liquid not determined by heat removal –  For lithium, flow rate is determined by requirement that liquid be

removed before LiD(T) forms, precipitates   For gallium, tin thin-films, flow rate determined by erosion replacement

  Required flow rate is high for all “self-cooled” concepts (thermal limit) –  Jets or fast open-channel flow (e.g. J x B driven flows) –  Flow rate determined by heat flux, flow path –  Typical flow rates: 5-10 m/sec for 10 MW/m2 power flux (lithium)

»  Flow path: 10s of cm at most »  Estimate assumes only heat conduction, not convection »  Power limits higher for gallium, tin



PPPL

Gallium jet experiment in ISTTOK R.B.Gomes et al., Fus. Eng. Des. 83 (2008) p. 102

Gallium jet

Discharge behavior is similar with

gallium jet and graphite limiters



PPPL

High power handling tests of lithium systems

  Two approaches developed for very high power handling   Both approaches successful at exceeding conduction-limited power

density limits –  First approach (Red Star, Russian Federation) uses evaporation

of lithium in a porous mesh target »  Employs heat of evaporation »  Evaporating lithium provides vapor shielding of target

–  Second approach employs naturally generated (convective) flows in free surface liquid lithium for redistribution of heat (PPPL)

  Both approaches have issues for application in a tokamak –  Lithium influx with evaporative technique may be prohibitive –  High magnetic field may suppress self induced flows

  But both techniques have demonstrated heat handling capability in excess of 50 MW/m2



PPPL

Evaporative cooling could provide very high transient heat handling capability

  Evaporative flux must be confined to divertor region –  Candidate for a Super-X divertor target?

t, o C 100 500 900 1300 1700

Q,

MW/m2

Hg

Na

Li

Ga

10-1

100

101

102

M.N. Ivanovskiy et al., Evaporationand Condensation of Metals, Moscow,Atomizdat, 1976



PPPL

The organization of works in Russia on Lithium Capillary-Pore Systems problem

ROSATOM Federal State Unitary Enterprise “Red Star”

Very high power handling demonstrated - >50 MW/m2 (25 MW/m2 steady-state) ~60 MW/m2, 300 sec. demonstrated with a 3 mm liquid lithium film on CDX-U

Liquid Lithium Limiter on FTU

Beam spot

IR image of e-beam heated lithium tray limiter in CDX-U



PPPL

Summary Liquid metals for the FNSF divertor

  Liquid metal PFCs are at an early stage of development   Implementations of lithium in tokamaks primarily for wall conditioning

–  Very few experiments with liquid lithium as a PFC »  First-generation experiments started in NSTX, HT-7 »  Second-generation tests starting in LTX

  Few (two!) experiments with gallium in tokamaks; none with tin   Liquid metal PFC development may offer

–  In-situ renewal of PFCs –  Self-repairing walls (disruptions, ELMs) –  Separability of neutron damage, PMI issues –  Improved power handling –  Access to new tokamak confinement regimes (lithium)

»  But lithium is less attractive than gallium or tin as a LM PFC in “conventional” reactor designs

liquid metal divertor options for fnsf - ucla

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