Speaker
Description
The kinetic destabilization of gap modes, e.g., toroidal Alfvén eigenmodes, results in both diffusive and convective transport of energetic particles [1]. In scenarios where fast relaxation events result in very rapid redistribution of fast ions or rapid changes to the mode resonance conditions, a mode can be excited in a regime far from the instability threshold. For example, strong Alfvén eigenmode activity has been observed in both JET [2] and JT-60U [3] directly following sawtooth crashes, which can redistribute fast particles on a timescale faster than the mode growth rate. While the wave saturation level of these interactions has been established in the strongly driven regime within a perturbative approximation [4,5], no analytical description of the time evolution of the instability has been developed. We examine the dynamics of a strongly driven kinetic instability at a single resonance in the presence of collisional and dissipative processes. The wave evolution can be approximated as occurring in two phases: first, linear growth driven by the positive distribution gradient, then slower, weakly nonlinear evolution driven by the balance of the wave drive and dissipation. In the second phase of evolution, we find that the distribution evolves time locally with the evolution of the mode, and by exploiting this time locality, we construct a piecewise-continuous, closed-form analytical solution for the time evolution of the mode amplitude. This result agrees closely with nonlinear kinetic simulations performed using the BOT code [6].
[1] W. W. Heidbrink, Phys. Plasmas 15, 055501 (2008).
[2] J. Ruiz Ruiz et al., Phys. Rev. Lett. 134, 095103 (2025).
[3] G. J. Kramer et al., Nucl. Fusion 41, 1135 (2001).
[4] H. L. Berk and B. N. Breizman, Phys. Fluids B 2, 2226 (1990).
[5] N. Petviashvili, Ph.D. Thesis, University of Texas at Austin (1999).
[6] M. K. Lilley et al., Phys. Plasmas 17, 092305 (2010).