Microwave radiation from rotating, annular electron beams

Generation of microwave radiation at high harmonics of the electron cyclotron frequency has been studied in both large-orbit gyrotron and circulating FEL configurations. In the large-orbit gyrotron experiments, a low-energy annular rotating electron beam (27 kV, 1 A, 3 μsec) interacts with the modes of a surrounding magnetron-type waveguide to produce microwave radiation (5-10 GHz). The axis-encircling E layer is produced by passing a linearly streaming hollow beam through a magnetic cusp. Narrow-band microwave radiation was obtained from the device at various power levels in various modes of operation; the dominant mode being an eight-cyclotron-harmonic synchronous resonance having a power level of 9.1 kW at 6.78 GHz, corresponding to an efficiency of tens of percent. A synchronous resonance at the tenth-cyclotron harmonic was also excited as well as a cyclotron-mode resonance at the eighth harmonic. Measurements of the radiated power spectra and confining magnetic field allowed identification of operating waveguide modes and instability mechanisms. In the circulating FEL studies, a linear growth-rate analysis reveals that fast and slow space-charge waves slightly alter the ponderomotive beam-wave resonance condition