Raman scattering in a partially filled plasma waveguide near cyclotron resonance

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 sca ttered radiation. Waveguide eigenmodes relevant to the pump wave and the ex cited waves are discussed. Formulas are derived for the temporal and spatia l growth rates of the backscattered wave and scatterer wave with the pump w ave frequency near the electron cyclotron frequency. A numerical study indi cates that the phase-matching conditions permit the approach to cyclotron r esonance with substantial enhancement of the growth rates. Growth rates sho w considerable dependence on the ratio of the plasma radius to the waveguid e radius a/R and their maximum values are found to be for a/R approximate t o 0.3. The growth rate found from the electromagnetic treatment was about 2 0% 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 cycl otron 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 th e results of previous theoretical and experimental investigations. (C) 1999 American Institute of Physics. [S1070-664X(99)03508-9].