Progress in the pasotron theory and experiments

Summary form only given, as follows. Plasma-assisted slow wave oscillators (pasotron) are unique high-power microwave sources, which can operate without (or at small) guiding magnetic fields. Beam transport is provided by plasma, which compensates for the defocusing space charge forces of the electron beam. Plasma assisted microwave devices have the potential to advance the technological and scientific base of microwave tubes. A joint University of Maryland/Hughes Research Lab. program is focused on the basic physics processes in pasotrons. An analysis of coherent Cherenkov radiation in the absence of guiding magnetic field, and mode interaction in backward-wave oscillators with strong end reflections were completed. Studies of operation with weak magnetic field demonstrated that the weak magnetic field required for protecting the slow-wave structure from electron bombardment depends on the microwave power. In situ diagnostics were developed to characterize a 1.2 GHz, 30% efficient helix pasotron operating at 40 kV and 30-100 A. The axial beam current distribution, beam focusing, as well as gas and plasma density measurements were performed. The most important results will be presented, as well as current efforts aimed at improving the characteristics, reducing size, and increasing the overall efficiency of pasotrons.