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

The key areas in which new knowledge is required are:
• integration of high b (normalized plasma pressure) performance with good confinement
and without transient events; understanding the physics underlying the benign nature of
pressure limits in stellarators, and the impact of pressure-driven currents on maintaining
quasi-symmetry.
• demonstration of the fundamental benefit predicted of Qs: tokamak-like confinement of
energetic ions in high-temperature, collisionless plasmas.
• Understanding of reduced viscous damping of plasma flows in Qs plasmas and its relation
to higher plasma confinement through suppression of plasma turbulence, similar to
tokamaks.
• control of impurity accumulation and helium ash expulsion. despite predictions that
stellarator plasmas exhibit impurity accumulation, experiments have realized highconfinement
regimes in which impurity confinement is acceptably low. extension of
these results to higher temperature Qs plasmas is required.
• Three-dimensional divertors for Qs configurations. validation of 3-d physics models of
divertor performance and development of appropriate coil configurations.
different types of quasi-symmetric 3-d shaping offer different trade-offs. Quasi-axial (Qa) symmetry
is similar to the axisymmetry of the tokamak, and, with finite plasma pressure, gives rise
to moderate levels of self-driven current parallel to the magnetic field (bootstrap current). Quasi-axial
symmetry would provide a direct comparison of quasi-symmetry to true symmetry, and
may answer why the density and pressure-limiting behavior is substantially different in stellarators
and tokamaks. Quasi-helical (Qh) and quasi-poloidal (QP) symmetry exhibit lower levels of
bootstrap current than the tokamak-like Qa configuration, rendering them less susceptible to
current-driven instabilities and the need for external control.
coordinated pursuit of both the low (Qh, QP) and moderate current Qa approaches would establish
the efficacy of Qs compared to true symmetry, and determine the magnitude of stellarator
3-d shaping required to provide stable sustainment with finite plasma current. Results from
these experiments combined with theory would inform the choice of the optimal type of quasisymmetry
for a single performance extension device.
key research steps to provide the requisite knowledge include:
• construction and operation of two intermediate-scale experiments: a Qa device with
moderate bootstrap current and a Qh or QP experiment with low bootstrap current. both
experiments would have sufficient pulse length and heating power to evaluate b-limits at
low collisionality to compare with theory. The two devices differ significantly in the details
of their magnetic configuration, such as field line curvature, shear, and trapped particle
fractions, allowing comparative tests of turbulent transport, impurity transport, and density
limits. The two experiments could also differ in the magnet coil and 3-d divertor design.
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