Theoretical analysis of energetic-ion-driven resistive interchange mode stabilization strategies using a Landau closure model

TL;DR

This study provides a theoretical analysis of strategies to stabilize energetic-ion-driven resistive interchange modes in LHD plasma using a combined reduced MHD and gyrofluid model, aligning with experimental observations.

Contribution

It introduces a Landau closure model to analyze EIC stabilization strategies, including plasma density, temperature, magnetic configuration, and NBI parameters, with results consistent with experiments.

Findings

Higher plasma density and temperature stabilize EIC.

Modifying magnetic shear and rotational transform can suppress EIC.

Increased NBI energy and tangential power enhance stability.

Abstract

The aim of the present study is to perform a theoretical analysis of different strategies to stabilize energetic-ion-driven resistive interchange mode (EIC) in LHD plasma. We use a reduced MHD 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 drift frequency to include their precessional drift. The stabilization trends of the 1/1 EIC observed experimentally with respect to the thermal plasma density and temperature are reproduced by the simulations, showing a reasonable agreement with the data. The LHD operation scenarios with stable 1/1 EIC are identified, leading to the stabilization of the 1/1 EIC if the thermal plasma density and temperature are above a given threshold. The 1/1 EIC are also stabilized if the rotational transform is modified in a way that the 1/1 rational surface is located further away than 0.9 times the normalized radius, or the magnetic shear in the plasma periphery is enhanced. Also, LHD discharges with large magnetic fields show a higher EIC destabilization threshold with respect to the thermal plasma density. If the perpendicular NBI deposition region is moved further inward than 0.875 times the normalized radius the 1/1 EIC are also stabilized. In addition, increasing the perpendicular NBI voltage such that the EP energy is higher than 30 keV stabilizes the 1/1 EIC. Moreover, Deuterium plasmas show a higher stability threshold for the 1/1 EIC than Hydrogen plasmas. The experimental data shows a larger time interval between EIC events as the power of the tangential NBI is increased providing that the perpendicular NBI power is at least 13 MW. This implies a stabilizing effect of the tangential NBI.