Magnetic Priming of a Relativistic Magnetron.

Abstract

Experiments and simulations have been performed testing magnetic priming at the cathode and at the anode of a relativistic magnetron to study the effects on high power microwave generation. Magnetic priming consists of N/2 azimuthal magnetic perturbations applied to an N-cavity magnetron for rapid generation of N/2 of electron spokes for the π-mode. Magnetic perturbations were imposed utilizing three high-permeability nickel-iron wires embedded beneath the emission region of the cathode, spaced 120 degrees apart. Magnetic priming was demonstrated to increase the percentage of π-mode shots by up to 30% over the baseline case. In general, the shortest wire length cases (4 cm and 8 cm-long cathode wires and 10 cm-long anode wires) yielded the best π mode performance. The start-oscillation time for the π mode in the magnetically primed magnetron was reduced to as little as 57% of the unprimed case. In an unbalanced loading condition, mean peak power for π-mode shots was found to be higher in the magnetically primed case by almost a factor of 2. Additionally, increases in mean microwave pulse width were also observed in the magnetically primed case when compared to the unprimed case (66 ns primed versus 50 ns unprimed) under unbalanced loading conditions. Under balanced loading conditions, measured power and pulse width in the magnetically primed cases were observed to be statistically the same as the unprimed baseline case, with the exception of the 4 cm cathode wire and 15 cm anode wire cases which were slightly lower. Additional experimental and simulation work was performed to study and correct vacuum side window breakdown occurring on the dielectric microwave windows used on the UM/L-3 relativistic magnetron. Simulations indicated that electrons emitted from the microwave apertures of the magnetron were striking the microwave windows in their original configuration, located 3 cm from the apertures. The microwave windows were redesigned and moved 33 cm from the apertures, resulting in the elimination of flashover and a threefold increase in measure microwave output power and pulse width.