Double-resonant fast particle-wave interaction

TL;DR

This paper investigates how fast particles interact with multiple Alfvén eigenmodes in fusion plasmas, revealing that double-resonance can both enhance and stabilize modes depending on their radial separation.

Contribution

It introduces the concept of double-resonance in fast particle-wave interactions and analyzes its effects on mode growth and stabilization in fusion devices.

Findings

Double-resonance can significantly increase mode growth rates.

Small radial mode distances can cause nonlinear stabilization.

No radial overlap is necessary for double-resonance effects.

Abstract

In future fusion devices fast particles must be well confined in order to transfer their energy to the background plasma. Magnetohydrodynamic instabilities like Toroidal Alfv\'en Eigenmodes or core-localized modes such as Beta Induced Alfv\'en Eigenmodes and Reversed Shear Alfv\'en Eigenmodes, both driven by fast particles, can lead to significant losses. This is observed in many ASDEX Upgrade discharges. The present study applies the drift-kinetic HAGIS code with the aim of understanding the underlying resonance mechanisms, especially in the presence of multiple modes with different frequencies. Of particular interest is the resonant interaction of particles simultaneously with two different modes, referred to as 'double-resonance'. Various mode overlapping scenarios with different q profiles are considered. It is found that, depending on the radial mode distance, double-resonance is able to enhance growth rates as well as mode amplitudes significantly. Surprisingly, no radial mode overlap is necessary for this effect. Quite the contrary is found: small radial mode distances can lead to strong nonlinear mode stabilization of a linearly dominant mode.