Breakdown Processes in Nitrogen, Oxygen, and Mixtures

Abstract

The studies of corona mechanisms in coaxial cylindrical geometry with central anode using $\alpha$-particle triggering, initiated by Colli and Facchini and by Lauer in inert gases, have been extended to pure nitrogen, pure oxygen, and mixtures of these gases. Pure gases were studied in a well outgassed system with nickel cathode. In pure nitrogen, from 25 to 600 mm pressure, the predominant mechanism is secondary liberation of electrons from the cathode by positive ion impact, with ${\gamma }_i$ varying between 1.4×${10}^{-3\mathrm{}}$ and 0.6×${10}^{-3\mathrm{}}$. Correction for back diffusion following Theobald gives a ture ${\gamma }_i$ of 2-3×${10}^{-2\mathrm{}}$. The ion is the ${\mathrm{N}}_4^{+}$ observed by Varney, with reduced mobility 2.53±0.08 ${\mathrm{cm}}^2$/volt sec at 760 mm and 20°C. Addition of the order of 1 percent oxygen reduces ${\gamma }_i$ by a factor of $\frac{1}{5}$ and gives a weak photoelectric ${\gamma }_p$ at the cathode, both effects resulting in part from action of oxygen on the cathode as noted by Parker and Theobald. Photoionization in the gas also appears, and with 5 percent oxygen the self-sustaining corona consists to a large extent of Geiger counter-like pulses propagating along the wire. In air, there is no longer any observable cathode action at threshold. With the use of transverse and longitudinal α particle trajectories relative to the axis, the velocity of propagation of the discharge down the wire has been measured for various pressures in air, and for constant pressure and potential as a function of oxygen concentration of between 5 and 20 percent. Velocities ranged between ${10}^8$ and ${10}^7$ cm/sec, decreasing as oxygen concentration increased. In pure oxygen above 200-mm pressure, the absorption of photoionizing radiation is so intense that, as indicated by Miller and Loeb, instead of spreading along the wire, the discharge propagates outward in the form of radial streamers, which have a duration of less than ${10}^{-7\mathrm{}}$ sec, contain some ${10}^8$ ions, and are spaced at regular intervals closely related to the positive-ion transit time. The latter yields a reduced mobility of 2.2±0.1 ${\mathrm{cm}}^2$/volt-sec at 760 mm Hg and 20°C. in conformity with Varney's assumed ${\mathrm{O}}_2^{+}$ ion.