Absolute Instability of the Bernstein Wave in a Beam-Plasma System

Abstract

The backward wave along the magnetic field is observed in a Maxwellian plasma. The propagation characteristics of the wave is explained in terms of the dispersion relation of the Bernstein wave. When an electron beam with an intensity above a certain critical value is injected into the plasma, a strongly excited wave is observed, whose frequency spectrum is very sharp. It can be shown from the measurement of the amplification that the waves excited are not explained in terms of a growth rate of thermal noise amplified by the convective instability but in terms of a self-oscillation due to the absolute instability. Dependence of the spontaneously excited wave on the plasma electron temperature and the electron beam parameters is investigated in detail.