Recent advances in the theory and experiments on plasma-assisted microwave source

Summary form only given, as follows. A plasma-assisted slow wave oscillator (Pasotron) may be operated without external magnetic field allowing for radial motion of the electrons. This phenomenon has been exploited to achieve an increase in the interaction efficiency from /spl sim/20% to /spl sim/50% by injecting electrons into the weak RF field region near the axis and having them bunched as they move axially and radially toward the surface of the slow wave circuit where RF fields are strong and efficient energy extraction occurs. The absence of a guide field, which gives the electrons the freedom to move not only axially but also transversely under the action of the RF fields, explains the high efficiency. This high efficiency is unattainable in conventional backward wave oscillators driven by linear electron beams in the presence of an externally applied guiding magnetic field. In our paper, recent theoretical and experimental studies of this device with two-dimensional electron motion will be described. Both in-situ diagnostics and theoretical analysis confirmed that RF forces dominate the beam dynamics. The innovation of simultaneously controlling the beam cross-section and optimizing the quality factor enables stable, high efficiency operation. These results greatly improve the prospects for developing advanced, plasma assisted, megawatt-class microwave sources.