Recent progress in the development of plasma-filled traveling-wave tubes and backward-wave oscillators

The presence of a controlled amount of background plasma inside microwave tubes can possibly lead to improvement in their characteristics beyond what is available in evacuated devices. In particular, recent results clearly demonstrated that the presence of plasma can significantly increase the bandwidth, efficiency and power handling capabilities of nonrelativistic microwave oscillators and amplifiers and allow operation without a guiding magnetic field. In the present paper, recent scientific advances in this field-both theoretical and experimental-are reviewed with emphasis on basic processes. We review some fundamental physical issues like the formation of hybrid waves in plasma-filled slow wave structures (SWSs) and the role of these modes in improving the beam/wave coupling; the beneficial role of ac space charge in plasma-filled devices and the effect of plasma on electron beam transport (efficiency and bandwidth enhancement due to the presence of the plasma). Also reviewed are recent experimental results on plasma-loaded traveling-wave tube (TWT) amplifiers and backward-wave oscillators (BWO). Plasma-loaded microwave devices have the potential to advance the technological and scientific base of microwave tubes and have an impact on commercial and industrial applications through the development of commercially viable technologies.