Streamer Mechanism in Filamentary Spark Breakdown in Argon by Fast Photomultiplier Techniques

Using techniques developed by Hudson on air and reported in the preceding paper, the phenomena were studied in Linde's spectroscopically pure grade Ar gas admitted to a system using Alpert vacuum techniques, and on that gas further purified by gas cataphoresis. Study was made in a point-to-plane gap with a 2.36-mm hemispherically capped cylinder opposite a 3-cm distant thin out-gassed Ni plane in the pressure range from 300 mm to 50 mm. For the spectroscopically pure grade Ar, transition from a positive point corona through a fine filamentary spark to an incipient arc breakdown on a time scale of ${10}^{-7\mathrm{}}$ sec down to 100 mm pressure proceeds by movement of primary and secondary streamers progressing from anode to cathode at slower speeds and lower luminous intensities than air at 760 mm. Unlike the case for air, the main stroke in Ar appears to move from anode towards the cathode. At 50 mm a somewhat diffuse spark channel did not reveal any streamer-like progression but the time scale was still in the ${10}^{-7\mathrm{}}$-sec range. One percent air in Ar at 60 mm restored streamers. Purified Ar at 240 mm revealed a 2-mm wide diffuse channel breakdown occurring across the whole gap by a process unknown with a rise time of several microseconds and sustained luminosity for tens of μsec with no indication of streamers. This demonstrates the necessity of adequate photoionizable impurities in Ar for the development of the filamentary streamer spark transition.