Spark propagation mechanisms in ambient air at the surface of a charged dielectric. I. Experimental: The main stages of the discharge

Abstract

The fast propagation (v≊106 ms−1) of an ionized and thermalized channel (ne≊1018 cm−3, T≊2.8×104 K) is studied in ambient air at atmospheric pressure, using gliding discharges produced over a charged dielectric slab. For surface voltages of about 100 kV, 1-m-long gliding sparks follow a straight line without any preionization of the gas. In this way, the discharges can be investigated with full diagnostics, including measurements of the current and of the propagation velocity, recording of the light emission (electronic image converter, spectroscopy, Lichtenberg figures), holographic interferometry of the spark channels, and detection of transient electric fields by capacitive probes. The various measurements are synchronized from optical fiber devices located close to the sparks. The study shows that the thermalized spark channel is produced by the three following stages: (a) a predischarge stage where the electron temperature Te≊2 eV is much greater than the gas temperature T0<1500 K; (b) a transient arc stage, lasting about 10 ns where energy is transferred from the electric field to the gas ionization; and (c) a heating stage, lasting from 25 to 60 ns, where the electric field has been largely reduced.