High-power microwave generation from large-orbit devices

Experimental and theoretical studies conducted at the University of Maryland on the production of high-power microwave radiation in cusp-injected, large-orbit devices are reviewed. Three classes of devices belonging to this category are discussed. In the first case, an axis-encircling rotating electron beam interacts via the negative mass instability with the modes of a smooth cylindrical waveguide, producing broadband radiation at multiple harmonics of the electron cyclotron frequency. In the second case, the beam interacts with a multiresonator magnetron circuit designed to provide mode control, resulting in high-power radiation at a desired cyclotron harmonic. In the third case, the beam interacts with a transverse wiggler magnetic field produced by samarium-cobalt magnets placed interior and/or exterior to the beam. In this last case the interaction is analogous to a circular-geometry free-electron laser. Experimental results form all three configurations are reviewed and compared with theoretical expectations. The possibility of enhanced operational efficiency in all of these devices by electron energy recovery is discussed and a design for a first experiment to test this concept is presented