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

esearch requirements
The development and understanding of effective 3-d divertors for stellarators is a high-priority
research need. essential research requirements are:
• modeling to determine how the interaction region can be expanded and/or removed
from proximity to the fusion plasma and reduce power levels and impurity generation to
acceptable values.
• experimental validation of divertor and edge modeling codes.
• design of divertor structures integrated into stellarator design codes to preserve the
quasi-symmetry of the configuration (at least in the core).
• collaboration on divertor experiments on lhd and W7-X.
iMPuRity anD FuSiOn aSH aCCuMuLatiOn
impurity accumulation has been observed experimentally in stellarators in some regimes, especially
at high density with improved confinement. to avoid radiation collapse for high-density,
long-pulse operation, it is important to screen the impurity influx at the edge and degrade the
impurity confinement in the core. Without a large pedestal, elms may not be available to prevent
impurity accumulation in a stellarator. however, very high-density, low-impurity stable discharges
without elms have been achieved during super dense core (sdc) plasmas in lhd and
high-density h-mode (hdh-mode) plasmas in W7-as. neoclassical calculations indicate that for
a tokamak in the banana collisionality regime the ion temperature gradient promotes impurity
expulsion. in contrast, for a conventional stellarator at low collisionality, impurities are predicted
to accumulate in the core. it is presently unknown how much symmetry-breaking is allowable
before temperature screening of impurities is defeated. There is presently no experiment that can
test impurity transport physics for quasi-symmetric configurations at low ion collisionality. an
experimental test is needed to determine whether temperature-screening or anomalous transport
can sweep the impurities out of the plasma.
research requirements
• Understanding of impurity transport in quasi-symmetric configurations with low enough
charge exchange losses to support an ion temperature gradient. such experiments would
study the role of elms and magnetic islands in impurity accumulation and the influence
of a divertor on impurity transport. The possibility of a radiative divertor should also be
explored.
• Gyrokinetic and neoclassical calculations of impurity and helium ash transport for
stellarators, especially to search for configurations that expel impurities.
• Participation in collaborative experiments on impurity transport in lhd and W7-X can
aid understanding of techniques to avoid impurity accumulation.
• to study temperature screening in a stellarator or Pe class experiment, comparable to
diii-d, may be required. The required plasma conditions are: bulk and impurity ions in
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