Development of the spark discharge in air with uniform applied field and long gaps

The optical phenomena occurring during the initiation of breakdown in a uniform-field gap in air have been investigated for values of pd extending from 7600 to 20 000 mmHg-cm. The investigations were made using an image convertor and a photomultiplier. Measurements have also been made of the breakdown voltage, probability of breakdown with impulse voltage applied, time lags to breakdown, and current flowing during the breakdown process. The manner of spark development was generally similar throughout the range of pd investigated. The process began with a faint, filamentary discharge, which progressed across the gap; the point of origin and direction of propagation depended on the experimental conditions, and the maximum velocity of propagation was approximately 2.5×108cm/s. This was followed by a much brighter discharge which originated at the same point in the gap as the first channel, and travelled across the gap with an approximate velocity of 4×107cm/s. Finally, a very bright third discharge occurred, with a rate of propagation several times higher than that of the second. A theory is put forward to explain the breakdown process.