Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

Liquid Metal Plasma Facing Components in burning Plasma Reactors
as discussed in the preceding sections, refractories and other solid materials require substantial
development for use as a PFc in a fusion reactor. operational limits on solid PFcs, at present, constitute
one of the most significant restrictions on design space for the intermediate device, demo
and follow-on fusion reactors. however, in addition to solids, there are several candidates for liquid
metal-based PFcs, including gallium, tin, lithium, and tin-lithium eutectics. among these,
lithium and probably the tin-lithium eutectic should provide a low recycling surface, while other
liquid metals are high recycling. a flowing liquid metal PFc would have limited residence time
(~tens of seconds) in a fusion reactor before removal and recirculation. hence, erosion, helium and
neutron damage, and tritium retention are not significant issues (provided that low recycling liquid
metals, such as lithium, can be adequately purged of tritium before recirculation). Plasma material
interaction issues (sputtering, evaporation) are now limited to the liquid metal PFc, whereas
the solid substrate supporting the liquid only sees the neutron damage. The separability of Pmi
and neutron damage considerably simplifies material qualification for reactors. The possibility
of using thin layers of liquid permits intensely cooled systems, with the plasma-exposed surface
closely coupled to the underlying coolant (either helium or a flowing liquid metal).
however, liquid metal PFc development is in an early stage. There are very few, relatively small,
liquid metal PFc test facilities in the Us. only a few liquid metal systems have been tested in tokamaks,
with a focus on lithium as a tool to reduce hydrogen recycling and high-z impurities. The
implementation of liquid metal PFcs in tokamaks has been predominantly in static or evaporative
systems (majeski 2006). additional tokamak devices (FtU and t-11m) have demonstrated
withstanding heat loads above 5 mW/m 2 (vertkov 2007) using capillary porous systems (cPs) for
liquid lithium PFcs. much higher power loading (>50 mW/m 2 ) has been demonstrated in evaporatively
cooled test stands, but not in tokamaks. The use of fast flowing liquid metal jets, for example,
has been tested in only one or two very small devices.
Prominent issues for both high and low recycling liquid metals include the entire problem of introducing
the liquid metal to, and removing it from, the reactor, and inducing stable flow to transport
the fluid from inlet to outlet. magnetohydrodynamic (mhd) effects caused by the excitation
of electrical currents in the liquid metal PFc must not cause macroscopic influx of the liquid metal
into the plasma. sputtering and evaporation must be kept to acceptable levels including temperature-enhanced
erosion, and this dictates the temperature limit for the coolant. heat removal
must be effective below these temperature limits and be compatible with the power conversion
system. coverage of the underlying substrate by the liquid metal, in the case of slow flow, must be
complete, since the substrate will not be designed for exposure to plasma. For jets or open-surface
channel flow, splashing and surface variations must be eliminated. For capillary systems, clogging
and nonuniform coverage must be avoided. The design of inlet manifolds and fluid collection
systems is a challenge for either type of system. tritium migration through the liquid metal
into underlying coolant channels must be investigated; since different liquid metals have differing
affinities for hydrogen, this work is specific to each candidate liquid metal and eutectic. Finally,
for lithium, the physics consequences of low recycling walls for tokamak equilibria must be
thoroughly explored, since the consequences for reactor design can be considerable. This last issue
closely and explicitly links liquid metal Pmi and the fusion core.
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