Nanosecond Triggering of Air Gaps with Intense Ultraviolet Light

Abstract

Measurements are presented of the breakdown time of a conventional two-electrode air gap. The applied voltage is maintained below the sparking threshold and breakdown is caused by the emission of a 6-nanosec burst of photoelectrons from the cathode, which produces space-charge distortion of the electric field. An auxiliary trigger spark provides the necessary light and results in cathode emission up to ∼10 ma/cm2. The dominant wavelength region is found to be ∼1100 A because of the relatively low air absorption and high photoelectric yield in this region. For a fixed gap spacing and using the highest light intensity available, the time delay is typically found to decrease from ∼5t- to a minimum delay t- as the main gap voltage is increased from ∼8% below threshold up to threshold. The minimum delay ranges from 10–60 nanosec for the gap spacings studied and agrees with calculated values of gap spacing/electron drift velocity. The techniques developed have direct application to the triggering of conventional spark-gap switches and to pulsed light sources and may provide an additional tool for investigating some of the basic parameters of gaseous electronics.