Investigation of rising-sun magnetrons operated at relativistic voltages using three-dimensional particle-in-cell simulation

This work is an attempt to elucidate effects that may limit efficiency in m agnetrons operated at relativistic voltages (V similar to 500 kV). Three-di mensional particle-in-cell simulation is used to investigate the behavior o f 14 and 22 cavity, cylindrical, rising-sun magnetrons. Power is extracted radially through a single iris located at the end of every other cavity. Nu merical results show that in general output power and efficiency increase a pproximately linearly with increasing iris width (decreasing vacuum Q) unti l the total Q becomes too low for stable oscillation in the pi-mode to be m aintained. Beyond this point mode competition and/or switching occur and ef ficiency decreases. Results reveal that the minimum value of Q (maximum eff iciency) that can be achieved prior to the onset of mode competition is sig nificantly affected by the magnitude of the 0-space-harmonic of the pi-mode , a unique characteristic of rising suns, and by the magnitude of the elect ron current density (space-charge effects). By minimizing these effects, up to 3.7 GW output power has been produced at an efficiency of 40%. (C) 2000 American Institute of Physics. [S1070-664X(00)03502-3].