Ionizing Space-Charge Waves in Gases

Abstract

Luminous waves have been produced along the positive column of a glow discharge in N2 by applying a fast-risetime step voltage to an electrode at one end of the column. Such waves have been identified as a primary mechanism in the buildup of ionization in many kinds of electrical breakdown in gases. Voltage-sensing probes and a photomultiplier were used to observe how the waves were affected by the electron density in the positive column of the glow discharge. The most noticeable effect was that the velocity increased from 1×109 cm/sec at zero density to around 7×109 cm/sec at a density of 3×109 /cm3. The velocity also depended upon the magnitude of the voltage pulse, pressure, and the diameter of the coaxial shielding surrounding the glow-discharge tube. The shape of the wavefront also depended on the electron density, but it was more strongly affected by the pressure. There were three striking differences between positive waves (those produced by a positive step voltage) and negative waves: (1) The velocities of positive waves depended more strongly on the initial electron density. (2) The wavefront of positive waves had a more complicated shape than that of negative waves. (3) Negative waves had to be initiated from a special electrode. The electron density resulting from the ionizing waves has been calculated.