Spiral electron beam interaction with whistler waves at cyclotron resonances

The interaction of a radially bounded electron beam spiraling in a magnetop lasma with a quasi-monochromatic whistler at the Cherenkov and Doppler-shif ted resonance conditions is investigated. For the linear stage of the inter action, the instability growth rate as well as the radial flux of wave ener gy emitted by the thin spiral beam are calculated. Compared to previous wor ks considering a cylindrical beam or electrostatic lower hybrid waves, the present study gives the possibility to extend the investigation of the whis tler-beam interaction to wider ranges of physical parameters, and, in parti cular, to situations where the beam is injected obliquely with respect to t he ambient magnetic field. The main physical process occurring during the d issipative beam-wave interaction-that is, which takes into account the ener gy leakage out of the bounded beam volume-is the nonlinear self-organizatio n of part of the beam electrons which leads to the formation of dynamically stable bunches that are continuously decelerated or accelerated while keep ing resonance with the emitted wave. In a long time evolution, bunches are the main cause which supports the wave emission whereas the nonresonant bea m electrons practically do not exchange energy with the wave. It is shown t hat the efficiency of the electromagnetic wave emission by a thin spiral be am is less at the cyclotron resonances than at the Cherenkov resonance. (C) 2001 American Institute of Physics.