Electromagnetic waves destabilized by runaway electrons in near-critical electric fields

TL;DR

This paper investigates how anisotropic runaway electron distributions near the critical electric field destabilize high-frequency electromagnetic waves, especially oblique whistler waves, with growth rates depending on magnetic field and damping effects.

Contribution

It provides a detailed analysis of the linear instability growth rates of electromagnetic waves destabilized by near-critical runaway electrons, highlighting the conditions for wave destabilization.

Findings

Oblique whistler waves are most unstable.

Wave frequencies and propagation angles depend on magnetic field.

Runaway electron density threshold for wave destabilization is established.

Abstract

Runaway electron distributions are strongly anisotropic in velocity space. This anisotropy is a source of free energy that may destabilize electromagnetic waves through a resonant interaction between the waves and the energetic electrons. In this work we investigate the high-frequency electromagnetic waves that are destabilized by runaway electron beams when the electric field is close to the critical field for runaway acceleration. Using a runaway electron distribution appropriate for the near-critical case we calculate the linear instability growth rate of these waves and conclude that the obliquely propagating whistler waves are most unstable. We show that the frequencies, wave numbers and propagation angles of the most unstable waves depend strongly on the magnetic field. Taking into account collisional and convective damping of the waves, we determine the number density of runaways that is required to destabilize the waves and show its parametric dependences.