The transition between two glow discharge phases in the SF₆spark

Abstract

The glow phases of sparks in SF6 have been studied at pressures around 1 Torr, and for discharge gaps of between a few cm and 1 m. Applied voltage pulses were maintained low enough so that the sparks did not proceed to their final arc phase. Measurements were made of discharge current, voltage and axial potential distribution as functions of time, and information on neutral particle density as a function of time was obtained using a polarization interferometer. The SF6 spark was found to develop through two glow phases. The first is a low current (tens of mA) phase, in which little molecular dissociation occurs; it is dominated by electron attachment. Gas heating during this phase leads to dissociation in times of the order of 100 mu s, and a transition occurs to a higher current (around 1 A) glow phase. In this dissociated phase, electron attachment is not so important. The interferometer study showed the increase in discharge refractivity due to dissociation, which accompanies the glow-glow transition. Further evidence for the role of dissociation in the transition was obtained from comparison of the SF6 spark with sparks in hydrogen, argon and SF6/argon mixtures.

The electrostatic forces acting on constricted discharges

“…The instability, which can take various forms, has been shown to be capable of explanation in a number of ways (cf. Nighan 1977, Emeleus and Coulter 1983, Woolsey et al 1984. The object of the present note is to call attention to a further cause of instability which does not appear to have been noticed in this connection, namely that the discharges may be acted upon by forces produced by the surface electric fields, and that charges cannot be in equilibrium under the action of electrostatic forces alone (Earnshaw's theorem, Smythe 1939, p.13).…”