A study of the saturated output of a TE01gyrotron using an electromagnetic finite size particle code

An adaptation of a finite size particle code has been used to predict the saturated output of gyrotron amplifiers operating in the TE01 mode. The initial laboratory frame code solves for the self-consistent axial variations of the electromagnetic field along with the trajectories of several thousand arbitrarily distributed finite size beam particles. Weakly varying guide wall profiles can be prescribed along with absorbing regions which have the same output match characteristics as the real device. Since the fields are solved using Fourier transforms, the addition of a k-space dependent absorption term to Maxwel's equations can simulate the effect of frequency selective wall loss. The saturated bandwidths of TE0l amplifiers similar to ones being developed at Hughes Electron Dynamics Inc. and the Naval Research Laboratory arc determined for several current and magnetic field settings. Simulations reveal that for operation near grazing incidence and largo beam powers, unwanted oscillations near cutoff are oxcited which strongly modulates the output power. The oscillations can be suppressed and stable operation restored by applying sufficient, wall loss in a narrow range of frequencies near cutoff. Finally the effects of velocity spread on efficiency are presented.