The Expansion of Gaseous Spark Channels

Abstract

A rotating mirror camera with a temporal resolution on the film of 1.5 mm/μsec. has been used to determine the rate of expansion of long gaseous spark channels during their first 10 μsec. of growth. The spark channels conducted aperiodic impulse currents in the range of 60 to 500 amp. peak, which attained their peak values in about 1/4 μsec. and decayed to half-value in 10 or 28 μsec. Sparks have been investigated in air at atmospheric and reduced pressures, and in nitrogen, oxygen and hydrogen at atmospheric pressure. The measured areas of cross section of these spark channels bear a linear relation to the peak currents, except in hydrogen, and are little influenced by the rate of current decay in the range studied. Measurements have also been made with the rotating mirror scanning an image of the spark across a photomultiplier connected to a high speed oscillograph; by this method a direct record is obtained of the radial light distribution across the spark channels. The mechanism of spark channel expansion is discussed, and estimates are made of the average ion densities in spark channels.