Cathode priming of a relativistic magnetron using multi-emission zones on projection ablation lithography cathodes.

Abstract

A novel priming technique of magnetrons, cathode priming, has been demonstrated experimentally and computationally to reduce microwave start-oscillation in The University of Michigan relativistic magnetron. In cathode priming, N/2 emission zones are constructed in discrete patches around the azimuth of the cylindrical cathode in an N-cavity magnetron. This emission geometry favors excitation of the pi-mode, the most efficient mode in the relativistic magnetron. Microwave oscillation builds-up from noise, therefore priming is advantageous. Advantages of priming include faster start-oscillation, potentially longer pulse-lengths, suppression of mode competition, and frequency locking. Relativistic magnetron experiments demonstrated that cathode priming reduced the start-oscillation times from 114 ns to 99 ns when compared to non-cathode priming shots. Generated magnetron frequencies operated in pi-mode range more frequently with priming (33%) than without priming (20%). Peak electronic efficiency yielded the highest average with cathode priming at 17%. Three-dimensional, electromagnetic, particle-in-cell simulations verified start-oscillation times were reduced with cathode priming, up to a factor of 3. In 3-D simulations, mode competition was suppressed in the startup phase with cathode priming. Without cathode priming the simulated relativistic magnetron initially operated in the 2pi/3-mode before transitioning into the pi-mode. The emission zones on the cathode have been fabricated by a new technique for explosive emission cathodes. This technique, denoted as Projection Ablation Lithography (PAL), involved micro-texturing of solid metal (Al 6061) cathodes by a KrF excimer laser to provide the electric field enhancement. Advantages to the PAL cathode compared to previously-used cotton cathode are: less out-gassing, resulting in lower base vacuum pressures; emission regions are heat-sinked to the cathode; and emission regions can be varied to provide the desired amount of electric field enhancement and cathode priming. PAL cathodes are capable of producing current densities of kA/cm<super>2</super> with total crossed-field currents (for 6 cm<super>2</super> area) between 2--10 kA. Experimentally, this all metal PAL cathode out-performed or was equal to the previously-used cotton fiber in every category (energy, pulselength, start-time) when used in the relativistic magnetron. Inferred plasma closure velocities were a factor of two lower with the PAL cathodes and in some shots exhibited zero plasma closure. Microwave pulselengths with PAL cathodes increased 57% to 212 ns, while microwave power averaged the same at 165 MW.