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

Summary form only given. 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 non-relativistic 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 such as the formation of hybrid waves in plasma-filled slow-wave structures 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; and efficiency and bandwidth enhancement due to the presence of the plasma. Also reviewed are recent experimental results on plasma-loaded TWT amplifiers and BWO oscillators. Plasma-loaded microwave devices have the potential to advance the technological and scientific base of microwave tubes, and also to have an impact on commercial and industrial applications through the development of commercially viable technologies.