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

esearch requirements
• new quasi-symmetric experiments are needed to study turbulence and transport barrier
formation in a hot ion, high-beta plasma. Quasi-symmetric stellarators optimized to
reduce both neoclassical and turbulent transport are needed.
• collaboration on turbulence and modeling with lhd and W7-X.
• nonlinear gyrokinetic calculations are needed to understand zonal flow formation and
critical gradients in stellarators. The role of magnetic shear and e x b stabilization needs
to be addressed specifically for stellarators as has already been done for tokamaks.
• neoclassical calculations are needed to understand whether transport barriers are possible
in quasi-symmetric stellarators due to electron/ion root proximity.
• The application of 3-d shaping to tokamaks with respect to intrinsic rotation to control
turbulence and resistive wall modes would be useful. This might be done most effectively
through the modification of an existing tokamak.
EnERgEtiC PaRtiCLE inStabiLitiES
alfvénic instabilities and energetic particle modes (ePms) are frequently observed in high-performance
tokamak regimes, and have also been observed in stellarators. since stellarators encompass
a wide range of magnetic geometries and symmetries, a larger set of alfvén instabilities
can be present than in tokamaks. Fast ion driven instabilities have been observed for all stellarator
experiments in which energetic tail heating populations are present. Regimes have been observed
both where the effects of such modes are relatively benign, as well as cases where they lead
to lowered core confinement. stellarators offer an effective means for suppressing these instabilities
through high-density operation. it is important to understand how lowered core confinement
may prevent the path to high-density operation.
research requirements
• continued refinement in spatial, velocity and time resolution of fast ion diagnostics will
be essential in understanding these phenomena and their implications and control in
future devices.
• Further improvements will be needed in the theory and modeling of fast ion instabilities
in stellarators in mode identification, linear thresholds, and fast ion loss prediction, along
with a greater focus on the nonlinear physics, to attain the desired predictive capabilities.
experimental validation is essential.
• since the form of fast ion instabilities can be influenced by the 3-d magnetic field structure,
a target for energetic particle instability suppression should eventually be included in
stellarator physics optimization efforts.
DiSRuPtiOnS anD PROFiLE COntROL
disruptions in low-current stellarators do not occur in typical operation, even at plasma pressures
above the calculated ideal stability limits. disruptions can be forced by fast current-ramps
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