Self-initiated volume discharge in nonchain HF lasers based on SF₆—hydrocarbon mixtures

A self-sustained volume discharge without preionisation, i.e. a self-initiated volume discharge (SIVD), in nonchain HF lasers with SF₆—C₂H₆ mixtures was investigated. It was established that, after the primary local electrical breakdown of the discharge gap, the SIVD spreads along the gap in directions perpendicular to that of the electric field by means of the successive formation of overlapping diffuse channels under a discharge voltage close to its quasi-steady state value. It is shown that, as new channels appear, the current flowing through the channels formed earlier decreases. The volume occupied by the SIVD increases with increase in the energy deposited in the plasma and, when the discharge volume is confined by a dielectric surface, the discharge voltage increases simultaneously with increase in the current. The possible mechanisms which explain the observed phenomena, namely the dissociation of SF₆ molecules and electron attachment SF₆ molecules, are examined. A simple analytical model, which makes it possible to describe these mechanisms at a qualitative level, was developed.