Fast excitation of geodesic acoustic mode by energetic particle beams

TL;DR

This paper introduces a novel mechanism for exciting geodesic acoustic modes (GAM) using energetic particle beams, explaining experimental observations by showing how specific particle distribution features can strongly drive GAM instability.

Contribution

It reveals that energetic particle beams with certain velocity space gradients can excite GAM without a threshold, with higher frequencies and larger growth rates than previously known mechanisms.

Findings

EGAM can be excited without a pitch angle threshold.

EGAM frequency can exceed local GAM frequency.

Growth rate of EGAM is significantly larger than with fully slowed EP beams.

Abstract

A new mechanism for Geodesic acoustic mode (GAM) excitation by a not fully slowed down energetic particle (EP) beam is analysed to explain experimental observations in Large Helical Device. It is shown that the positive velocity space gradient near the lower-energy end of the EP distribution function can strongly drive GAM unstable. The new features of this EP-induced GAM (EGAM) are: 1. no instability threshold in the pitch angle; 2. the EGAM frequency can be higher than the local GAM frequency; and 3. the instability growth rate is much larger than that driven by a fully slowed down EP beam.