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

• control alpha heating feedback loops for steady-state operation. develop control
techniques for alpha-driven instabilities and attempt to expand stable operating regimes.
Facilitate helium ash removal. attempt the direct transfer of alpha particle energy to the
core fuel ions.
introduction
This Thrust has three interrelated components. First, simulation and theory need to be improved
and validated so that stable and unstable burning plasma regimes can be delineated and levels of
alpha transport in unstable regimes predicted. second, new advanced diagnostics will need to be
developed for energetic particle populations and the instabilities driven by them. Third, based on
the understanding gained in the previous two elements, alpha physics control techniques need to
be developed for optimized fusion performance in iteR and demo.
improvements in alpha Physics Modeling and Simulation
The modeling and simulation components of this Thrust are essential to maximize the benefit of our
iteR participation and to bridge the gap between iteR and demo. This effort is also highly synergistic
with the diagnostic development component of this Thrust, since improved simulation results
motivate better diagnostics and vice versa. Regular testing and validation of these simulation efforts
against experimental data from new diagnostics is essential for the success of this effort. The first
step is to develop realistic linear stability threshold predictions that can apply to alpha-driven alfvén
instabilities in the iteR regime. since these modes always experience finite levels of damping, the reliable
prediction of instability depends on comprehensive models of both damping and drive. existing
calculations require improvements in areas such as: sound-wave coupling, flow shear effects, better
resolution of edge profiles and damping, improved radiative and continuum damping, inclusion
of 3-d effects near the boundary, coupling to core micro-turbulence, and finite orbit-width effects.
accurate treatment of these effects becomes especially demanding for the small normalized fast ion
gyroradius regime of iteR; in this regime, the most unstable mode number shifts to higher toroidal
mode number (n ~ 20), and the number of modes available for destabilization at high n increases as
n 2 . This aspect significantly increases the resolution requirements and the complexity of linear stability
threshold evaluation in iteR as compared to current experiments. improved stability threshold
prediction will be beneficial for iteR operational scenario development activities, integrated simulation
efforts, and alpha physics control strategies.
in parallel with this effort, better nonlinear alfvén stability models must be developed so that
alpha transport can be assessed in regimes that are above the instability threshold. such models
will need to be upgraded to include multi-physics, multi-scale coupling to other plasma modes
(e.g., sawteeth, microturbulence, resistive wall mode), and nonlinear avalanche dynamics. For
large-scale plasma magnetohydrodynamic (mhd) instabilities (e.g., sawteeth and resistive wall
mode), there are several concerns. one issue is that these modes can stimulate energetic particle
avalanches and enhanced transient levels of alpha particle transport. a second issue is that energetic
particles can temporarily stabilize core plasma mhd modes, allowing pressure and current
gradients in the plasma to reach levels that can suddenly become unsustainable as energetic particle
instability-driven transport removes the stabilizing influences of the alpha particles. This
can lead to strong relaxation oscillations and prevent steady-state operation. coupling between
alpha-driven alfvén modes and core plasma microturbulence becomes especially important for
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