The Onset of Long Electrical Discharges in Clean Air

Abstract

Understanding of the propagation of electrical discharges leading to a stepped leader process requires knowledge of discharge mechanisms that can operate over long distances in the absence of metal electrodes. However, as normally studied in the laboratory, the different stages in a long air discharge are known to be strongly influenced by the metallic properties of the electrodes used. This study reports on the formation and propagation of discharges that avoid the use of metal surfaces. It is shown that local thermalization of a section of the discharge region in the air, is a prerequisite for the propagation of ionization over distances of the order of 0.1 m. Lack of thermalization leads to charge redistributions that lower the electric stress and stop the discharge. Conversely, the thermalization process inherently produces impulse high voltage waves with rise times in the order of 10 nsec. These waves result in further ionization that extends the range of the discharge. Using water and polyester surfaces, it is shown that the procurement of a critical electron density required for thermalization is controlled not directly by the local electric field but by the availability of charge from regions located outside the channel that becomes thermalized. The critical thermalization condition of 1023m−3 electrons is verified using oscilloscope recordings of the current in the discharge in conjunction with the ignition of propane-air mixtures. The technique is approximate but much easier than time-resolved spectroscopic observation of the discharge.