Cyclotron Excitation of Electromagnetic Waves by a Gyrating Electron Beam

Abstract

When a tenuous electron stream gyrates around the field lines of a static magnetic field in the presence of a stationary plasma, electromagnetic waves are excited which propagate perpendicular to the magnetic field. Previous theoretical studies involved the assumption that these excited waves are almost longitudinal (quasistatic approximation). Such an assumption is not made in this analysis. It is shown that the excited waves have the electron cyclotron frequency or some multiple thereof. The electric intensity vector rotates in a plane perpendicular to the magnetic field; the phase velocities are of the order of, or exceed, the velocity of light, and for certain plasma-beam systems lower harmonics of the electron cyclotron frequency cannot be excited. Expressions are given for the rate of growth of the waves as a function of the plasma-beam parameters and the harmonic number.