High-order geodesic coupling of shear-Alfv\'{e}n and acoustic continua in tokamaks

TL;DR

This paper demonstrates that high-order plasma shaping can create gaps in the coupled shear-Alfvén and acoustic continua at higher frequencies, revealing new unstable global eigenmodes relevant for tokamak operation and fusion stability.

Contribution

It introduces the concept that high-order plasma shaping opens gaps in the coupled continua, leading to the discovery of new unstable eigenmodes at higher frequencies.

Findings

High-order shaping opens gaps in shear-Alfvén and acoustic continua.

Global eigenmodes in these gaps are unstable to hot-ion populations.

Eigenmodes resonate near the geometric mean of Alfvén and sound speeds.

Abstract

High-order plasma shaping (mainly elongation and shift, as opposed to low-order toroidicity) is shown, under certain conditions, to open gaps in the coupled shear-Alfv\'{e}n and acoustic continua at frequencies significantly above the values predicted by previous theories. Global eigenmodes in these gaps, which lie between those of geodesic acoustic modes (GAMs) and toroidicity-induced Alfv\'{e}n eigenmodes (TAEs), are found unstable to hot-ion populations typical of tokamak operation, whilst their fundamental resonances with circulating particles are shown to take place at velocities near the geometric mean of the Alfv\'{e}n and sound speeds. Therefore, such eigenmodes are expected to be observed near the predicted frequencies at operating tokamaks, playing a still unexplored role in magnetohydrodynamic spectroscopy as well as in the stability of next-step fusion experiments.