Ponderomotive effects in plasma-filled backward-wave oscillators

A numerical model is presented for analyzing plasma-filled backward-wave oscillators (BWOs) operating near cutoff. The model allows for the investigation of the effects of the ponderomotive potential of the high-frequency electromagnetic waves on the motion of plasma electrons. As a result of their motion the electron plasma density is modified, and this affects the high-frequency radiation by modifying the dispersion characteristics of the slow wave structure. Two approaches for modeling the plasma are considered, a fluid model and a particle-in-cell model. 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 densities, electrons clump in regions of low high-frequency electric field. At somewhat higher densities nonlinear instabilities of the Raman type are excited. The model does not indicate the cause of the observed efficiency enhancement in plasma filled backward wave oscillators.