Abstract
The observed characteristics of toroidicity induced Alfven eigenmodes (TAE) in DIII-D tokamak discharges are compared in detail with predictions of various theories of TAE mode stability, and good qualitative agreement is found. The observed range of unstable toroidal mode numbers (n approximately 3-6) is consistent with theoretical predictions. For DIII-D parameters, low mode numbers are damped by coupling to the stable Alfven continuum, while high mode numbers are clamped by electron kinetic effects including coupling to kinetic Alfven waves. A threshold for destabilization is observed experimentally at a fast ion beta of approximately 1%. The predicted driving and damping rates, estimated from experimental data, balance within about a factor of two for discharges at the threshold. It is demonstrated experimentally that the damping of TAE modes can be increased by current profile control, in this case with a peaked current profile produced by a negative current ramp, in qualitative agreement with theoretical predictions. A possible stabilizing effect of discharge elongation is also observed
Highlights
Future fusion devices with significant amounts of alpha particle heating may be subject to instabilities driven by the free energy of the non-Maxwellian velocity distribution and non-uniform spatial distribution of the alpha particles
An understanding of the driving and damping mechanisms for this instability is essential in order to predict whether it poses a threat to fusion reactors. This understanding can be gained from present experiments, in which the toroidicity induced Alfvtn eigenmodes (TAE) mode can be studied by destabilizing it with fast ions produced by neutral beam injection
For DIII-D parameters, we find that the damping is dominated at low mode numbers by continuum coupling, and at high mode numbers by coupling to kinetic AlfvCn waves through electron kinetic effects
Summary
Future fusion devices with significant amounts of alpha particle heating may be subject to instabilities driven by the free energy of the non-Maxwellian velocity distribution and non-uniform spatial distribution of the alpha particles. The toroidicity induced AlfvBn eigenmode (TAE mode) is thought to be one of the most dangerous such instabilities, because the birth velocity of the alpha particles can be near the AlfvBn velocity, allowing the mode to be driven resonantly. The mode could be destabilized by fast ions from high energy neutral beams injected for heating or non-inductive current drive. An understanding of the driving and damping mechanisms for this instability is essential in order to predict whether it poses a threat to fusion reactors. This understanding can be gained from present experiments, in which the TAE mode can be studied by destabilizing it with fast ions produced by neutral beam injection
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