Title: Onset of nonlinear structures due to eigenmode destabilization in tokamak plasmas,
Abstract: A general methodology is proposed to differentiate the likelihood of
energetic-particle-driven instabilities to produce frequency chirping or
fixed-frequency oscillations. The method employs numerically calculated
eigenstructures and multiple resonance surfaces of a given mode in the presence
of energetic ion drag and stochasticity (due to collisions and
micro-turbulence). Toroidicity-induced, reversed-shear and beta-induced
Alfven-acoustic eigenmodes are used as examples. Waves measured in experiments
are characterized and compatibility is found between the proposed criterion
predictions and the experimental observation or lack of observation of chirping
behavior of Alfvenic modes in different tokamaks. It is found that the
stochastic diffusion due to micro-turbulence can be the dominant energetic
particle detuning mechanism near the resonances in many plasma experiments, and
its strength is the key as to whether chirping solutions are likely to arise.
The proposed criterion constitutes a useful predictive tool in assessing
whether the nature of the transport for fast ion losses in fusion devices will
be dominated by convective or diffusive processes.