Hybrid simulations of sub-cyclotron compressional and global Alfv\'en Eigenmode stability in spherical tokamaks

TL;DR

This study uses hybrid numerical simulations to analyze the stability of sub-cyclotron Alfvén Eigenmodes in spherical tokamaks, revealing mode behaviors and excitation conditions relevant to plasma stability and experimental observations.

Contribution

It provides the first comprehensive simulation-based analysis of CAEs and GAEs in low aspect ratio plasmas, linking mode excitation to beam parameters and experimental phenomena.

Findings

GAEs are more prevalent and have higher growth rates than CAEs.

Unstable modes are excited at lower beam energies, matching experimental data.

Local theory explains mode excitation dependence on beam injection and velocity.

Abstract

A comprehensive numerical study has been conducted in order to investigate the stability of beam-driven, sub-cyclotron frequency compressional (CAE) and global (GAE) Alfv\'en Eigenmodes in low aspect ratio plasmas for a wide range of beam parameters. The presence of CAEs and GAEs has previously been linked to anomalous electron temperature profile flattening at high beam power in NSTX experiments, prompting further examination of the conditions for their excitation. Linear simulations are performed with the hybrid MHD-kinetic initial value code HYM in order to capture the general Doppler-shifted cyclotron resonance that drives the modes. Three distinct types of modes are found in simulations -- co-CAEs, cntr-GAEs, and co-GAEs -- with differing spectral and stability properties. The simulations reveal that unstable GAEs are more ubiquitous than unstable CAEs, consistent with experimental observations, as they are excited at lower beam energies and generally have larger growth rates. Local analytic theory is used to explain key features of the simulation results, including the preferential excitation of different modes based on beam injection geometry and the growth rate dependence on the beam injection velocity, critical velocity, and degree of velocity space anisotropy. The background damping rate is inferred from simulations and estimated analytically for relevant sources not present in the simulation model, indicating that co-CAEs are closer to marginal stability than modes driven by the cyclotron resonances.