Corona Discharges in Liquid and Gaseous Hydrocarbons: The Influence of Pressure

Previous studies1,2 in a point-plane electrode assembly of prebreakdown phenomena in very pure liquid cyclohexane have shown that the results depended greatly on the radius, rp of the point. For sharp points of rр of the order of 0.1 µm, the observed phenomena were found in good agreement with a field emission (point cathode) or field ionization (point anode) regime. In this case, an electrical field strength, Es, at the surface of the emitter of about 20 MV/cm is required for current onset. For larger point radii (rp>0.5 µm) the phenomena were completely different. For negative points, above a threshold electrical field of about 7 MV/cm (a field much lower than required for field emission) a highly regular pulse regime, very similar to the Trichel one found in air3 can be observed. However, for positive points in the same range of point radius and voltage, no current instability and pulse regime can be detected. Extended studies in other pure liquid hydrocarbons (propane and n-pentane) and in compressed gases (air, propane and methane),2,4 later showed that the same phenomena were observed in all these media. However, in liquids, contrary to results in gases, the characteristics of the Trichel-like pulse regime displayed no pressure dependence. All these results led us to consider a mechanism of charge multiplication in purified liquid hydrocarbons, although the existence in these liquids of hot electrons and avalanches is questionable.