Ideal MHD theory of low-frequency Alfven waves in the H-1 Heliac

TL;DR

This paper combines analytical and numerical ideal MHD methods to analyze low-frequency Alfven waves in the H-1 stellarator, revealing significant wave interactions and mode structures despite low plasma beta.

Contribution

It provides the first detailed 3D ideal MHD spectrum for H-1, identifying beta-induced Alfven eigenmodes and addressing computational challenges due to complex magnetic geometry.

Findings

Identification of quasi-discrete Alfven-acoustic modes

Reproduction of observed frequency dependencies with hollow temperature profiles

Demonstration of computational convergence issues in complex geometries

Abstract

A part analytical, part numerical ideal MHD analysis of low-frequency Alfven wave physics in the H-1 stellarator is given. The three-dimensional, compressible ideal spectrum for H-1 is presented and it is found that despite the low beta (approx. 10^-4) of H-1 plasmas, significant Alfven-acoustic interactions occur at low frequencies. Several quasi-discrete modes are found with the three-dimensional linearised ideal MHD eigenmode solver CAS3D, including beta-induced Alfven eigenmode (BAE)- type modes in beta-induced gaps. The strongly shaped, low-aspect ratio magnetic geometry of H-1 causes CAS3D convergence difficulties requiring the inclusion of many Fourier harmonics for the parallel component of the fluid displacement eigenvector even for shear wave motions. The highest beta-induced gap reproduces large parts of the observed configurational frequency dependencies in the presence of hollow temperature profiles.