Spark cross size in a sliding submicrosecond discharge of opposite polarities in rare gases: experiment and application of ionization wave front propagation theory to analysis

摘要

The results from an experimental measurement of the spark channel radius in a sliding multichannel discharge of opposite polarities in Ne, Ar and Xe are presented and discussed. The experiments were performed at submicrosecond discharge pulse duration (90-190 ns current pulse FWHM) and gas pressures of 30 and 100 kPa, with alumina ceramics as the dielectric substrate. The data showed the optical radius of the negative polarity spark channels to exceed 1.27-1.6 times those of the positive channels, depending on the gas type and its pressure. The earlier theoretical study by U Ebert, W van Saarloos and C Caroly on the propagation of opposite polarity ionization wave fronts was applied to the analysis of experimental results. From the two approaches of theory, only the 'non-localized initial conditions' one, which in addition to the gas impact ionization, drift and diffusion of electrons also takes into account the free initial electrons in the gas ahead of the ionization wave front, is capable of explaining concurrently the experimental observations on positive and negative channel radii, their ratios and the dependence on gas type and pressure. Numerical solutions of the channel expansion equation specified that the expansion speed is governed by both the drift electrons and free electrons generated by a short-range source in a narrow layer around the channel lateral surface. The depth of the latter layer was estimated to be comparable to the expansion wave front depth and, thus, much less than the channel radius.