Linear gyrokinetic simulation of kinetic infernal mode

TL;DR

This paper uses gyrokinetic simulations to explore the kinetic infernal mode (KIM), revealing its connection to the kinetic ballooning mode (KBM) and how magnetic shear influences its behavior and growth.

Contribution

It demonstrates that KIM and KBM are physically related modes with a smooth transition, and details how magnetic shear and pressure gradients affect KIM's growth and localization.

Findings

KIM transitions smoothly to KBM as magnetic shear weakens.

KIM prefers to grow near the minimum magnetic shear surface.

Growth rate depends on magnetic shear and temperature gradient.

Abstract

Kinetic infernal mode (KIM) is an electromagnetic instability driven by thermal ions in weak magnetic shear region with a frequency similar to the kinetic ballooning mode (KBM). Gyrokinetic simulations of KIM using Gyrokinetic Toroidal Code (GTC) found that the electromagnetic instability shows a smooth transition from KBM to KIM in both frequency and growth rate when magnetic shear varies from strong to weak, which suggests that KIM and KBM may belong to the same mode physically. The mode structure analysis reveals that the mode transition is induced by the change in distance between adjacent mode rational surfaces. The magnetic shear and driving source effects are investigated in detail. The simulation results show that KIM prefers to grow on the mode rational surface nearest to the minimum magnetic shear, i.e., where the shear stabilizing effect is weakest, instead of at the maximum of density gradient or temperature gradient. However, the magnitude of the growth rate is determined by magnetic shear and temperature gradient simultaneously. These findings suggest that KIM can be effectively regulated by modifying the strength and position of magnetic shear, as well as pressure gradients.