Observations on the electrical breakdown of gases at 2800 Mc/s

Electrical discharges at 2800 Mc/s are produced in a nosed-in cavity type of resonator by energizing it from a pulsed magnetron. Individual pulses of various durations in the range 0.25 microsec to 2.5 microsec are employed. Electrons are provided in mid gap by the short ultra-violet radiation from an auxiliary d.c. spark actuated from the circuit used to trigger the power pulse. Descriptions and illustrations are given of the form of the resulting discharge, which in typical cases begins in mid gap and advances towards the electrodes, crossing a 1-cm gap in times of the order of 1 microsec. The discharges in different gases at atmospheric pressure show wide differences in appearance. Oscillograms of the envelope of the field in the resonator show a very rapid collapse when the discharge occurs; in certain gases, notably hydrogen, a striking consistency of behaviour is exhibited. Studies of the absorption coefficient of the active radiation from the auxiliary gap agree with the supposition that it lies in the short ultra-violet region, and it is found possible to calculate the variation of sparking probability with distance of the irradiating spark from the resonator. Confirmatory experiments with pulsed d.c. sparks support the argument.