High-order coupling of shear and sonic continua in JET plasmas

TL;DR

This paper models the coupling of shear-Alfvén and acoustic continua in JET plasmas, explaining observed high-order geodesic acoustic eigenmodes (HOGAEs) near TAE frequencies, and compares predictions with experimental data.

Contribution

It introduces a high-order coupling model explaining HOGAEs in JET plasmas and validates predictions with experimental measurements.

Findings

HOGAEs are predicted to arise from high-order harmonics of geodesic curvature.

Model predictions of HOGAE frequency and location agree with JET measurements.

Wave-particle interactions influence HOGAE stability in JET plasmas.

Abstract

A recent model coupling the shear-Alfv\'{e}n and acoustic continua, which depends strongly on the equilibrium shaping and on elongation in particular, is employed to explain the properties of Alfv\'{e}nic activity observed on JET plasmas below but close to the typical frequency of toroidicity-induced Alfv\'{e}n eigenmodes (TAEs). The frequency gaps predicted by the model result from high-order harmonics of the geodesic field-line curvature caused by plasma shaping (as opposed to lower-order toroidicity) and give rise to high-order geodesic acoustic eigenmodes (HOGAEs), their frequency value being close to one-half of the TAEs one. The theoretical predictions of HOGAE frequency and radial location are found to be in fair agreement with measurements in JET experiments, including magnetic, reflectometry and soft x-ray data. The stability of the observed HOGAEs is evaluated with the linear hybrid MHD/drift-kinetic code CASTOR-K, taking into account the energetic-ion populations produced by the NBI and ICRH heating systems. Wave-particle resonances, along with drive/damping mechanisms, are also discussed in order to understand the conditions leading to HOGAEs destabilization in JET plasmas.