Energetic-particle-modified global Alfv\'{e}n eigenmodes

TL;DR

This study uses hybrid simulations to show how energetic particles significantly modify the frequency and characteristics of global Alfvén eigenmodes in NSTX-like conditions, revealing potential new modes driven by energetic particles.

Contribution

It demonstrates that energetic particles cause substantial frequency shifts in GAEs, suggesting the existence of a new energetic-particle-modified GAE mode, independent of equilibrium changes.

Findings

Mode frequency varies with beam parameters, especially v_0/v_A.

Large frequency shifts occur without structural changes, indicating a new mode.

Frequency shifts are driven by Doppler-shifted cyclotron resonances.

Abstract

Fully self-consistent hybrid MHD/particle simulations reveal strong energetic particle modifications to sub-cyclotron global Alfv\'{e}n eigenmodes (GAE) in low-aspect ratio, NSTX-like conditions. Key parameters defining the fast ion distribution function -- the normalized injection velocity $v_0/v_A$ and central pitch -- are varied in order to study their influence on the characteristics of the excited modes. It is found that the frequency of the most unstable mode changes significantly and continuously with beam parameters, in accordance with the Doppler-shifted cyclotron resonances which drive the modes, and depending most substantially on $v_0/v_A$. This unexpected result is present for both counter-propagating GAEs, which are routinely excited in NSTX, and high frequency co-GAEs, which have not been previously studied. Large changes in frequency without clear corresponding changes in mode structure could indicate the existence of a new energetic particle mode, referred to here as an energetic-particle-modified GAE (EP-GAE). Additional simulations conducted for a fixed MHD equilibrium demonstrate that the GAE frequency shift cannot be explained by the equilibrium changes due to energetic particle effects.