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

theory predictions for alfvén instability-free regimes and nonlinear simulations for
tolerable alfvén instability activity.
• develop new diagnostics for plasma oscillation and eP profile measurements: This is
needed to establish an experimental database of eP losses and wall heat loads to help
verification and validation and foster reliable projections to next-step st devices.
• develop active control tools for linear and nonlinear eP transport control: notably in the
nonlinear regime, control over hole-clump generation is desirable.
Effect of energetic particle driven modes on thermal plasma: The electron transport induced
by energetic particle driven instabilities such as global alfvén eigenmodes is unknown, and
energetic particle energy channeling in which fast ion-induced multiple compressional alfvén eigenmodes
(caes) transfer their energy from the eP source to thermal ions by a transient resonance
is not well understood. The following studies are needed to fill the knowledge gaps:
• conduct nonlinear theoretical studies of multiple cae, Gae and energetic particle
interactions to understand their effect on fast ions.
• develop a program to study these effects experimentally by measuring internal mode
structures and polarization, and the effects on ePs on a fast time scale during Gae and
cae bursts (on the order of 1 msec and faster).
• engage in active antenna studies of wave-particle interaction with external excitation of
high-frequency modes.
neoclassical teaRinG modes
neoclassical tearing modes are prominent in high-performance tokamak and st operation, and are
among many instabilities that can reduce confinement and/or cause disruptions. For steady-state
operation, a large fraction of current self-generated by the plasma (bootstrap current) is clearly desirable.
since this current is produced by the plasma pressure gradient, it exposes the pressure gradient
free-energy as a mechanism to destabilize macroscopic tearing instabilities, with associated
magnetic island formation. For high-performance st operation, it is highly desirable to avoid neoclassical
tearing modes in rotating plasmas near the ideal plasma pressure-driven stability limits.
an extensive database is available from conventional tokamak experiments describing ntm instability
threshold, formation, and saturation processes through a combination of analytic modeling
and experimental investigation. largely, different elements of the theory can be parameterized
by a set of dimensionless numbers, normalized gyroradius, measures of plasma toroidal rotation
and toroidal rotation shear, collisionality, etc., from which extrapolations and predictions of
future devices are predicted. Present st operation feeds into this physics program and helps firmly
establish the relevant physics. since sts operate in the extremes of normalized rotation shear,
plasma shaping, etc., it may be possible to identify a desirable operational region that makes sts
somewhat less sensitive to the deleterious effects of ntms. a comprehensive theory describing
all of ntm physics remains elusive, and by leveraging a unique operational space, st experiments
make a novel contribution in the development of theory.
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