Microwave Breakdown Studies of He — N2 Mixtures in a Pillbox Cavity for Repetitively Pulsed High Power Systems

Abstract

Microwave breakdown of various He-N2 mixture combinations is investigated for the prevention of electrical and thermal breakdown in repetitively pulsed high power systems. The objective is to determine the best combination of the two gases to maximize the mixture’s ability to cool a dielectric window surface while maintaining high electrical breakdown thresholds. Helium is known to have a high thermal conductivity for heat transport off the dielectric window surface, but it also has a low microwave breakdown threshold. Nitrogen does not transport heat as well, but has a much higher electrical breakdown threshold. Initial studies focused on generating a series of microwave power versus gas mixture pressure curves for electrical breakdown to help identify optimum He - N2 mixture ratios. The gas mixtures flow through ports in a 333 cm3 S-band double window pillbox cavity. The double window cavity is placed in a traveling wave resonant ring (TWRR) coupled to a 2.85 GHz, 4 MW, magnetron. This combination of double window pillbox and TWRR allows for testing power levels up to 40 MW. High speed diagnostics are used to measure the incident/reflected power and discharge luminosity. An ICCD camera provides time integrated images of the breakdown event. Coupled mass flow controllers maintain the gas mixture ratio and continuous gas flow through the cavity. Investigations can be conducted in either single or multiple pulsed configurations. Results included several gas mixtures from pure helium to pure nitrogen at 760, 1520, and 3040 torr, and showed a significant increase in breakdown levels by adding a small percentage of nitrogen to helium.