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

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central to edge temperature can be maintained. The property of itG turbulence does not appear<br />

to be universal. in numerical simulations using the Gene code, the W7-X and ncsX stellarator<br />

designs have different properties from tokamaks. With appropriate design, it may be possible to<br />

maintain good confinement with a cool plasma edge, which would eliminate elms and permit detached<br />

divertor operation. The research needs <strong>for</strong> stellarators include:<br />

• Quantitative understanding of the conditions <strong>for</strong> elm onset.<br />

• develop elm-free operating regimes with low impurity accumulation.<br />

• develop an integrated solution to high-per<strong>for</strong>mance core parameters and a high-density<br />

cold edge.<br />

aLPHa PaRtiCLES EjECtED by PLaSMa inStabiLitiES<br />

The scientific issues associated with loss of alpha particles by plasma instabilities are similar in<br />

iteR and demo. The large increase in fluence to the first-wall components prior to planned replacement<br />

of first-wall components results in greater concern about loss of alpha particles by plasma<br />

instabilities and/or ripple, since in d-t reactions both an alpha particle and a neutron are generated.<br />

Thus, the effects such as blistering become even more serious.<br />

an intrinsic advantage of stellarators is that they can operate at higher densities than comparable<br />

tokamaks (not constrained by the Greenwald limit). operation at higher density and lower temperature<br />

would reduce the drive <strong>for</strong> alpha-driven instabilities since the energetic particle density<br />

scales as t 5/2 at fixed thermonuclear power. in contrast with tokamaks, in which noninductive<br />

current drive efficiency is reduced at high density, stellarators, in principle, do not require noninductive<br />

current drive. energetic fast ion driven instabilities have been observed in stellarators<br />

and would need to be studied further. The list of research tasks identified in Theme 1 would have<br />

to be extended to include stellarator magnetic configurations. one specific research need is:<br />

• Understand and control particle loss induced by alpha-particle driven instabilities in<br />

stellarators.<br />

tRanSiEnt EVEnt tOLERant WaLLS<br />

an alternative approach to solid divertor targets is to consider liquid divertor targets in a fusion<br />

facility. The potential advantage of liquid divertor targets and more generally liquid first-wall<br />

components is that an off-normal transient event, such as a disruption, large elm, or alpha particle<br />

loss, might result in a plasma termination or erosion of the surface but not in damage to firstwall<br />

components. experiments using lithium capillary-pore systems have demonstrated the ability<br />

to withstand much larger heat fluxes than either cFc or tungsten targets. Work in this area is<br />

just beginning and entails both research and technology development, and would have to be done<br />

in concert with Theme 3. <strong>Research</strong> requirements include:<br />

• characterize the impact of liquid metals on pedestal structure and stability.<br />

103

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