Radiance Saturation and Afterglow Emission of High Pressure 20-nsec Spark Channels in Argon and Helium.

Time functions of the radiance (ITT FW-114, spectral response S-4) were observed, as well as the radiant intensity of high pressure spark channels in argon and helium with pressures between 1 atm and 40 atm, and a constant gap of 0.65 mm. The radiance, as well as radiant intensity, first increases exponentially with pressure and breakdown voltage; later, the radiance curves bend into a saturation plateau in both gases. On the other hand, the radiant intensity does not reach saturation, but shifts its maximum up to 50 nsec after the 8-nsec current maximum. The shift of the radiant intensity and saturation are related and result from the increasing opacity of the channel. The observed 1.65 ratio of the saturation radiances of helium and argon agrees with values reported in microsecond spark channels which have a diameter approximately six times wider.