Experimental Evidence of Nonlinear Avalanche Dynamics of Energetic Particle Modes

TL;DR

This paper provides experimental evidence of nonlinear avalanche behavior in energetic particle modes within a tokamak, showing mode propagation, frequency changes, and implications for energetic particle transport relevant to future fusion devices.

Contribution

It presents the first experimental observation of EPM avalanche dynamics, confirming theoretical predictions and introducing a simplified model for interpretation.

Findings

Mode propagates radially outward during avalanche

Mode frequency self-adjusts to optimize wave-particle interaction

Results align with EPM convective amplification theory

Abstract

Recent observations in HL-2A tokamak give new experimental evidences of energetic particle mode (EPM) avalanche. In a strong EPM burst, the mode structure propagates radially outward within two hundred Alfv\'en time, while the frequency of the dominant mode changes self-consistently to maximize wave-particle power exchange and mode growth. This suggests that significant energetic particle transport occurs in this avalanche phase, in agreement with theoretical framework of EPM convective amplification. A simplified relay runner model yields satisfactory interpretations of the measurements. The results can help understanding the nonlinear dynamics of energetic particle driven modes in future burning plasmas, such as ITER.