Raman scattering in a partially filled plasma waveguide near cyclotron resonance

Abstract

The parametric instability of a predominantly transverse magnetic mode in a waveguide, partially filled with a magnetized plasma, is considered. Raman backscattering of the pump wave off the space-charge wave leads to the scattered radiation. Waveguide eigenmodes relevant to the pump wave and the excited waves are discussed. Formulas are derived for the temporal and spatial growth rates of the backscattered wave and scatterer wave with the pump wave frequency near the electron cyclotron frequency. A numerical study indicates that the phase-matching conditions permit the approach to cyclotron resonance with substantial enhancement of the growth rates. Growth rates show considerable dependence on the ratio of the plasma radius to the waveguide radius a/R and their maximum values are found to be for a/R≈0.3. The growth rate found from the electromagnetic treatment was about 20% less than the one calculated with electrostatic assumption on the space-charge wave. It is also shown that the electromagnetic effect of the space-charge wave has considerable influence on the maximum growth rate near cyclotron resonance by preventing the pump wave from approaching the resonance. Possible use of the EH waveguide mode as a pump for a free-electron laser is discussed. The results of this theory are found to be comparable with the results of previous theoretical and experimental investigations.