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

the banana regime, low turbulent transport, small density gradient, large ion temperature
gradient, and long pulse time to adequately capture impurity transport.
• exploration of helium ash removal on an intermediate or Pe class experiment. however,
d-t operation may ultimately be needed to adequately resolve this issue.
OPERatiOnaL LiMitS
it appears that typically neither density nor beta is limited by either mhd instabilities or disruptions
in low-current stellarators. in both lhd and W7-as, the achievable b was limited by the
available heating power, with no hard b-limit seen. mhd activity seen in the experiments is consistent
with the theoretical predictions, but the modes saturate and they do not impede access to
higher 〈b〉 values. Three-dimensional equilibrium codes that do not assume the existence of nested
flux surfaces find a substantial region of stochastic (wandering) magnetic field lines in the outer
region of the plasma at the highest values of b in both W7-as and lhd.
densities of 4 × 10 20 to 10 21 m -3 have been achieved in W7-as and lhd. Without large plasma
currents there is minimal magnetic energy dissipated at termination. abnormal termination of
stellarator discharges does take place, however, when densities become too large. The maximum
achievable plasma density in a stellarator is determined by a soft radiative collapse on the confinement
time scale when there is balance between the heating power and radiation losses.
research requirements
• develop a theoretical understanding of the beta limit in stellarators.
• Understand at what level of plasma current are there tokamak-like limits with respect to
pressure and density.
• determine discharge termination behavior with significantly more poloidal field due to
large plasma currents in Qs configurations.
• model and validate the deterioration of magnetic surfaces with plasma pressure; this may
be a crosscutting issue for all of toroidal confinement.
anOMaLOuS tRanSPORt REDuCtiOn
Quasi-symmetric stellarators have reduced neoclassical transport and consequently turbulentdriven
transport becomes dominant. This has been demonstrated experimentally in hsX. Furthermore,
a number of theoretical and experimental results suggest that the reduction of neoclassical
transport will also result in the reduction of turbulent transport. Quasi-symmetric stellarators
have a minimum in damping due to parallel viscosity in the direction of symmetry. This
opens the possibility that strong flow shear or reduced zonal flow damping may reduce turbulent
transport in stellarators. hsX can examine flows and anomalous transport in a quasi-symmetric
stellarator with a hot electron plasma. turbulence and transport in hot ion plasmas can presently
only be studied in nonsymmetric and quasi-omnigenous stellarators such as lhd and W7-X.
close collaboration between theory and experiment can help to understand why stellarator profiles,
under some conditions, may be less stiff than for tokamaks.
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