Beam loading in magnicon deflection cavities

Analysis of the beam-deflection cavity interaction in a magnicon is presented and compared with experiment. For a driven cavity a dispersion relation is obtained wherein the interaction modifies the cold-cavity quality factor and the resonance frequency. In terms of a lumped-parameter equivalent circuit the interaction corresponds to a complex-valued beam admittance Y_b in parallel with the cavity admittance. The response of the gain cavities is modified by the same admittance. In a magnicon, Y_b is a sensitive function of the solenoidal focusing magnetic field B₀, thus providing a convenient means of adjusting the cavity properties in experiments. When the relativistic gyrofrequency is twice the drive frequency, Im Y_b =0 and the beam does not load the cavity. Analytical expressions of the variation of the detuning, instantaneous bandwidth (i.e., loaded quality factor) and gain with B₀ are derived. Simulation results are presented to verify the linear analysis with ideal beams and to illustrate the modifications due to finite beam emittance. Results of the magnicon experiment at the Naval Research Laboratory are examined in the light of the analysis