PONDEROMOTIVE EFFECTS IN PLASMA-FILLED BACKWARD-WAVE OSCILLATORS

A numerical model is presented for analyzing plasma-filled backward-wa ve oscillators (BWO's) operating near cutoff. The model allows for the investigation of the effects of the ponderomotive potential of the hi gh-frequency electromagnetic waves on the motion of plasma electrons. As a result of their motion the electron plasma density is modified, a nd this affects the high-frequency radiation by modifying the dispersi on characteristics of the slow wave structure, Two approaches for mode ling the plasma are considered, a fluid model and a particle-in-cell m odel. Nonlinear simulations are performed to investigate the possible excitation of plasma waves over a range of background plasma densities . Results from nonlinear simulations show that for low plasma densitie s, electrons clump in regions of low high-frequency electric field. At somewhat higher densities nonlinear instabilities of the Raman type a re excited. The model does not indicate the cause of the observed effi ciency enhancement in plasma filled backward wave oscillators.