Spiral electron beam interaction with whistler waves at cyclotron resonances

Abstract

The interaction of a radially bounded electron beam spiraling in a magnetoplasma with a quasi-monochromatic whistler at the Cherenkov and Doppler-shifted resonance conditions is investigated. For the linear stage of the interaction, the instability growth rate as well as the radial flux of wave energy emitted by the thin spiral beam are calculated. Compared to previous works considering a cylindrical beam or electrostatic lower hybrid waves, the present study gives the possibility to extend the investigation of the whistler-beam interaction to wider ranges of physical parameters, and, in particular, to situations where the beam is injected obliquely with respect to the ambient magnetic field. The main physical process occurring during the dissipative beam-wave interaction—that is, which takes into account the energy leakage out of the bounded beam volume—is the nonlinear self-organization of part of the beam electrons which leads to the formation of dynamically stable bunches that are continuously decelerated or accelerated while keeping resonance with the emitted wave. In a long time evolution, bunches are the main cause which supports the wave emission whereas the nonresonant beam electrons practically do not exchange energy with the wave. It is shown that the efficiency of the electromagnetic wave emission by a thin spiral beam is less at the cyclotron resonances than at the Cherenkov resonance.