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

central to edge temperature can be maintained. The property of itG turbulence does not appear
to be universal. in numerical simulations using the Gene code, the W7-X and ncsX stellarator
designs have different properties from tokamaks. With appropriate design, it may be possible to
maintain good confinement with a cool plasma edge, which would eliminate elms and permit detached
divertor operation. The research needs for stellarators include:
• Quantitative understanding of the conditions for elm onset.
• develop elm-free operating regimes with low impurity accumulation.
• develop an integrated solution to high-performance core parameters and a high-density
cold edge.
aLPHa PaRtiCLES EjECtED by PLaSMa inStabiLitiES
The scientific issues associated with loss of alpha particles by plasma instabilities are similar in
iteR and demo. The large increase in fluence to the first-wall components prior to planned replacement
of first-wall components results in greater concern about loss of alpha particles by plasma
instabilities and/or ripple, since in d-t reactions both an alpha particle and a neutron are generated.
Thus, the effects such as blistering become even more serious.
an intrinsic advantage of stellarators is that they can operate at higher densities than comparable
tokamaks (not constrained by the Greenwald limit). operation at higher density and lower temperature
would reduce the drive for alpha-driven instabilities since the energetic particle density
scales as t 5/2 at fixed thermonuclear power. in contrast with tokamaks, in which noninductive
current drive efficiency is reduced at high density, stellarators, in principle, do not require noninductive
current drive. energetic fast ion driven instabilities have been observed in stellarators
and would need to be studied further. The list of research tasks identified in Theme 1 would have
to be extended to include stellarator magnetic configurations. one specific research need is:
• Understand and control particle loss induced by alpha-particle driven instabilities in
stellarators.
tRanSiEnt EVEnt tOLERant WaLLS
an alternative approach to solid divertor targets is to consider liquid divertor targets in a fusion
facility. The potential advantage of liquid divertor targets and more generally liquid first-wall
components is that an off-normal transient event, such as a disruption, large elm, or alpha particle
loss, might result in a plasma termination or erosion of the surface but not in damage to firstwall
components. experiments using lithium capillary-pore systems have demonstrated the ability
to withstand much larger heat fluxes than either cFc or tungsten targets. Work in this area is
just beginning and entails both research and technology development, and would have to be done
in concert with Theme 3. Research requirements include:
• characterize the impact of liquid metals on pedestal structure and stability.
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