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

ELM-FREE HigH PERFORManCE
stellarators can operate in high-performance, high-confinement discharge modes both with and
without elms. edge localized modes are often observed in stellarator h-mode discharges, but the
available data and understanding are more limited than in tokamaks at present. as in tokamaks,
stellarator elms are associated with strong pressure gradients at the plasma edge, and act to
flush out impurities. The appearance of stellarator elms depends sensitively on the edge rotational
transform. an elm-free, high-density h-mode attained in stellarators exhibits good thermal
confinement with low impurity confinement times. stellarator optimization may allow for good
core confinement with edge conditions amenable to elm suppression. The application of non-axisymmetric
fields in the edge region has been shown in some cases to suppress elms in tokamaks.
research requirements
• Understand the mechanisms of enhanced transport in the edge thought to be responsible
for elm suppression with non-axisymmetric perturbations.
• Understand impurity transport in stellarator discharges without elms that show low
impurity accumulation. determine whether the elm-free hdh mode can extrapolate to
a burning plasma.
• high-performance Qs plasmas are needed to determine whether lower edge pressure
gradients are compatible with large pressure gradients in the core.
SuPERCOnDuCting StELLaRatOR COiLS
high t c superconductors could have a large potential impact on fusion devices as identified in the
Fesac “Priorities, Gaps, and opportunities” report, and is a focus of Thrust 7. second generation
high-temperature superconductors (hts) offer many advantages for fusion magnets over present
technology. This is especially true for stellarators. The conductors can have small curvature radii,
advantageous for stellarator coils. demountable joints provide new possibilities for access and
maintenance. Quench protection can be built into the magnet without large amounts of stabilizer.
higher current densities, critical field, and broader temperature range ease engineering design
and cryo-loads. novel manufacturing techniques are possible with the second-generation material,
which might permit a direct application to an otherwise structural element. a staged program
is needed to evaluate the suitability of hts for stellarator coils or flux shaping components.
research requirements
design and prototype work should be done so that, if advantageous, this technology could be applied
to all fusion systems in the iteR-era.
• Undertake an engineering analysis of an example stellarator coil to determine critical
elements in an R&d strategy for application of high t c to stellarators.
• carry out appropriate R&d through fabrication and testing of a model coil.
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