The Microwave Spark

Electrical breakdown of air was studied at 3 microwave frequencies: wave-lengths of 1.25 cm, 3 cm, and 10 cm. Breakdown field strength was determined as a function of pressure, gap height, intensity of initial ionization, width of a single microwave pulse which sufficed for breakdown, and the pulse repetition frequency.It is found that the field strength for breakdown is strongly dependent upon the width of the microwave pulse, the intensity of initial ionization, and, of course, upon gap height and pressure of enclosed gas. A lesser dependence is found on the repetition rate of the pulses. The validity of Paschen's law at these high frequencies is investigated, and a semitheoretical formulation of breakdown is obtained based on electron ionization competing with the process of electron attachment to neutral molecules (relatively immobile in the gas). Prediction is made from this theory as to certain expected results. For experimental conditions these were not realizable at the time the experiments were performed but they may now be realized in certain laboratories.