The effect of the time-dependent self-consistent electrostatic field on gyrotron operation

The time-dependent self-consistent electrostatic field is shown to have a deleterious effect on gyrotron operation. As the electron beam density increases, the nonlinear efficiency is degraded seriously by the self-electrostatic field. Time-dependent multimode simulations demonstrate that at sufficiently large beam densities the electron cyclotron instability is quenched and the oscillation will not start. Contrary to previous investigations, electrostatic effects do not necessarily increase the linear growth rate of the electromagnetic cavity mode and, depending on the beam density, electrostatic effects can actually stabilize the mode. Typically, however, considerations of linear theory are not important in an overmoded, open resonator gyrotron because the system evolves nonlinearly into a state consisting of a single mode which is linearly stable, but nonlinearly the most efficient mode.