Excitation of a global plasma mode by an intense electron beam in a dc discharge

TL;DR

This study numerically investigates how an intense electron beam interacts with a finite, inhomogeneous plasma, revealing two regimes of two-stream instability that excite plasma waves at different locations and generate suprathermal electrons.

Contribution

It identifies and characterizes two distinct regimes of two-stream instability in inhomogeneous plasma caused by an intense electron beam, including their conditions and effects.

Findings

Two regimes of instability: central and edge plasma excitation.

Edge regime occurs sporadically at high beam currents.

Instability leads to generation of suprathermal electrons.

Abstract

Interaction of an intense electron beam with a finite-length, inhomogeneous plasma is investigated numerically. The plasma density profile is maximal in the middle and decays towards the plasma edges. Two regimes of the two-stream instability are observed. In one regime, the frequency of the instability is the plasma frequency at the density maximum and plasma waves are excited in the middle of the plasma. In the other regime, the frequency of the instability matches the local plasma frequency near the edges of the plasma and the intense plasma oscillations occur near plasma boundaries. The latter regime appears sporadically and only for strong electron beam currents. This instability generates copious amount of suprathermal electrons. The energy transfer to suprathermal electrons is the saturation mechanism of the instability.