Nonlinear beam loading and dynamical limiting currents in a high power microwave gap

Summary form only given, as follows. The successful operation of the relativistic klystron amplifier (RKA) at a power level of 3 GW raised the question of whether higher power can be generated or whether a saturation mechanism is limiting the output power. The answer to this question depends on the degree of beam loading and the maximum amount of current modulation which can be transmitted across an RF gap. A simple model has been used to show that the intense space charge of a high-current beam may increase the gap capacitance by a factor of two to three over the vacuum value. This increase in capacitance due to beam loading would (a) provide additional shunts of the current to the load, leading to saturation of efficiency: (b) cause detuning of cavities: and (c) result in a longer buildup time for the current modulation. Some of these features were observed in recent RKA experiments and simulations. The limiting current under an AC gap voltage was computed and compared with the steady-state value. The implications of these findings on operation efficiency and frequency stability in other high-power microwave devices have been investigated