Excitation of lower hybrid waves by a density-modulated electron beam in a plasma cylinder

Abstract

A density-modulated electron beam propagating through a plasma cylinder drives electrostatic lower hybrid waves to instability via Cerenkov interaction. The lower hybrid wave instability has the largest growth rate γ when the frequency and wave number of the modulation are comparable to that of the unstable wave. The growth rate of the instability increases with the modulation index Δ and is maximized for Δ=1. For Δ=0, γ turns out to be ∼0.43×107 sec−1. The growth rate scales as the one-third power of the beam density. The real frequency of the unstable wave increases as almost the square root of the beam voltage. The results of the theory are applied to explain some of the experimental observations of Chang [Phys. Rev. Lett. 35, 285 (1975)].