Absolute Instabilities of Perpendicularly Propagating Cyclotron Harmonic Plasma Waves

Abstract

Computed dispersion characteristics are presented for perpendicularly propagating electron cyclotron harmonic waves in an infinite plasma having a delta-function transverse electron velocity distribution and an arbitrary longitudinal velocity distribution. It is shown that in general the propagation occurs in passbands centered on the cyclotron harmonic frequencies and that a given mode can carry positive or negative rf energy depending on the frequency and wavenumber. This suggests the possibility of growing waves in finite plasmas due to interaction with slow wave circuits. Even in an infinite plasma, as the ratio of plasma frequency to cyclotron frequency increases, modes in successive passbands couple to each other, leading to wave growth in time. This represents absolute instability of a type which may be closely related to effects observed in some thermonuclear fusion studies and other laboratory plasmas. There may also be important applications of these modes to the construction of practical microwave oscillators and amplifiers.