Positive Coaxial Cylindrical Corona Discharges in Pure N2, O2, and Mixtures Thereof

Abstract

Dc corona studies were made on coaxial cylinders with inner cylinder at positive potentials in pure N₂, pure O₂, and mixtures of 1 percent, 10 percent O₂ in N₂ and on clean air from 27‐mm to 760‐mm pressure. Observations of current‐potential relations from field intensified ionization currents to as near breakdown as possible were made noting thresholds for current transitions, pulsed discharges, and other oscillographic as well as visual appearances. Positive corona in pure N₂ does not exist as such. Predischarge Townsend currents by photon action on the cathode condition the outer Ni cathode until it spontaneously develops a glow discharge focusing in a spot on the anode wire. The discharge sets in at high potential and persists with lowering by hundreds of volts. Increase in current gives multiple spots and irregular current fluctuations. At lower pressure potential overshoot gradually disappears and the discharge begins as a regular glow discharge. Positive coronas with 1 percent O₂ show burst pulses at threshold, these being longer in duration (1000 μsec) than in air because of spread down the wire. At lower pressure burst pulses lasted still longer. Breakdown streamers were observed at lower pressures where the applied potential was adequate. The Townsend equation for current in coaxial geometry applies above threshold both for N₂ and N₂+1 percent O₂. In air the positive corona sequence showing only burst pulses at threshold and breakdown streamers were observed. Burst pulses lasted two to four times those with 1‐mm diameter point to plane corona in air owing to spread along the wire, which was less extensive than in 1 percent O₂. In pure O₂ the high absorption of photons ionizing the gas led to pre‐onset streamers and no burst pulses at thresholds above 200‐mm pressure. Below 200 mm streamer pulses and burst pulses were observed and below 50 mm only burst pulses were observed. The streamers above 200‐mm spread down the whole length of the tube by transparency of O₂ to photons liberating electrons from the Ni cathode cylinder.