A non-thermal mechanism of spark breakdown in Ar

Experiments are described in which streamer breakdown, producing short circ uiting, is observed in long non-uniform Ar gaps under standard conditions. The breakdown is developed For a Few microseconds and the average breakdown held is around 650 V cm(-1). These results differ substantially from the p revious observations in long air gaps. A 1.5-dimensional simulation is carr ied out which supports the general observed properties of the streamer prop agation in long Ar gaps and the main features of the streamer breakdown. Th ere are two distinct phases of the breakdown. The first phase consists of a n ionizing wave traversing the gap a few times. The second phase consists o f a uniform increase in electron density along the whole channel length. Th e breakdown is of a non-thermal nature and is due to two-step electron-impa ct ionization through excited states of Ar atoms and due to the effect of e lectron-electron collisions which drive the non-equilibrium electron energy distribution towards a Maxwellian distribution.