Transition from fishbone mode to $\beta$-induced Alfv\'en eigenmode on HL-2A tokamak

TL;DR

This paper investigates the transition between fishbone instability and $eta$-induced Alfvén eigenmodes in HL-2A tokamak plasmas with energetic particles, using advanced modeling to identify different regimes based on safety factor and EP pressure.

Contribution

The study employs the hybrid kinetic-MHD model in NIMROD to identify and analyze the transition between fishbone and $eta$-induced Alfvén eigenmodes in HL-2A plasmas, revealing dependence on safety factor and EP pressure.

Findings

Fishbone mode transitions to BAE as $q_0$ approaches one.

BAE dominates at higher EP pressure when $q_0$ is slightly above one.

Different Alfvén eigenmodes are excited depending on safety factor and EP fraction.

Abstract

In the presence of energetic particles (EPs) from auxiliary heating and burning plasmas, fishbone instability and Alfv\'en modes can be excited and their transition can take place in certain overlapping regimes. Using the hybrid kinetic-magnetohydrodynamic model in the NIMROD code, we have identified such a transition between the fishbone instability and the $\beta$-induced Alfv\'en Eigenmode (BAE) for the NBI heated plasmas on HL-2A. When the safety factor at magnetic axis is well below one, typical kink-fishbone transition occurs as the EP fraction increases. When $q_0$ is raised to approaching one, the fishbone mode is replaced with BAE for sufficient amount of EPs. When $q_0$ is slightly above one, the toroidicity-induced Alfv\'en eigenmode (TAE) dominates at lower EP pressure, whereas BAE dominates at higher EP pressure.