Theoretical analysis of the saturation phase of the $1/1$ energetic-ion-driven resistive interchange mode

TL;DR

This study uses nonlinear simulations to analyze the saturation phase of the energetic-ion-driven resistive interchange mode in LHD plasma, reproducing experimental features and revealing mode interactions and stabilization mechanisms.

Contribution

The paper introduces a reduced MHD and gyrofluid model simulation that captures the nonlinear saturation and stabilization of the 1/1 EIC mode, aligning with experimental observations.

Findings

Simulation reproduces EIC saturation phases observed experimentally.

Identifies the destabilization of overtones during burst events.

Shows stabilization linked to pressure profile flattening.

Abstract

The aim of the present study is to analyze the saturation regime of the energetic-ion-driven resistive interchange mode (EIC) in the LHD plasma. A set of non linear simulations are performed by the FAR3d code that uses a reduced MHD model for the thermal plasma coupled with a gyrofluid model for the energetic particles (EP) species. The hellically trapped EP component is introduced through a modification of the averaged drift velocity operator to include their precessional drift. The non linear simulation results show similar 1/1 EIC saturation phases with respect to the experimental observations, reproducing the enhancement of the n/m = 1/1 resistive interchange modes (RIC) amplitude and width as the EP $\beta$ increases, the EP beta threshold for the 1/1 EIC excitation, the further destabilization of the 1/1 EIC as the population of the helically trapped EP increases and the triggering of burst events. The frequency of the 1/1 EIC calculated during the burst event is 9.4 kHz and the 2/2 and 3/3 overtones are destabilized, consistent with the frequency range and the complex mode structure measured in the experiment. In addition, the simulation shows the inward propagation of the 1/1 EIC due to the non linear destabilization of the 3/4 and 2/3 EPMs, leading to the partial overlapping between resonances during the burst event. Finally, the analysis of the 1/1 EIC stabilization phase shows the excitation of the 1/1 RIC as soon as the flattening induced by the 1/1 EIC in the pressure profile vanishes, leading to the retrieval of the pressure gradient at the plasma periphery and the overcoming of the RIC stability limit.