A non-thermal mechanism of spark breakdown in Ar

Experiments are described in which streamer breakdown, producing short circuiting, is observed in long non-uniform Ar gaps under standard conditions. The breakdown is developed for a few microseconds and the average breakdown field is around 650 V cm-1. These results differ substantially from the previous observations in long air gaps. A 1.5-dimensional simulation is carried out which supports the general observed properties of the streamer propagation in long Ar gaps and the main features of the streamer breakdown. There are two distinct phases of the breakdown. The first phase consists of an ionizing wave traversing the gap a few times. The second phase consists of a uniform increase in electron density along the whole channel length. The breakdown is of a non-thermal nature and is due to two-step electron-impact ionization through excited states of Ar atoms and due to the effect of electron-electron collisions which drive the non-equilibrium electron energy distribution towards a Maxwellian distribution. (author)