Effects that limit efficiency in relativistic magnetrons

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 (3-D) particle-in-cell (PIC) simulation is used to investigate th e behavior of 14- and 22-cavity, cylindrical, rising-sun magnetrons, Power is extracted radially through a single iris located at the end of every oth er cavity, Numerical results show that in general output power and efficien cy increase approximately linearly with increasing iris width (decreasing v acuum Q) until the total Q becomes too low for stable oscillation in the pi -made to be maintained. Beyond this point, mode competition or switching o ccur and efficiency decreases. Results reveal that the minimum value of Q ( maximum efficiency) that can be achieved prior to the onset of mode competi tion is significantly affected by the magnitude of the 0-space-harmonic of the ir-mode, a unique characteristic of rising-suns, and by the magnitude o f the electron current density (space-charge effects). When these effects a re minimized, numerical simulation predicts that gigawatt-level power produ ction at 55% efficiency may be possible at relativistic voltage.