Spark discharges in iodine vapour

Abstract

Sparks in iodine vapour have been studied at pressures between 0.08 and 0.80 Torr, and for gaps of the order of 10 cm. Sparks were initiated in non-uniform fields, using voltage pulses between 1 kV and 3 kV, corresponding to overvoltages of up to 300%. Development of the spark proceeds through a glow phase to a filamentary arc. The glow-arc transition is a two-stage process, involving the formation of a cathode spot, with a corresponding drop in cathode fall voltage, followed by constriction of the positive column, during which molecular dissociation occurs. Following construction, the arc expands radially and a temperature of 5000K and electron density of 6*1012 cm-3 are reached before decay sets in. During spark decay, a transition occurs from an atomic to a molecular arc phase.

Influence of the water surface on the glow-to-spark transition in a metal-pin-to-water electrode system

“…They observed that a glow discharge is formed before the spark and that an electro-dynamical constriction of the anode layer in two anode spots triggers the spark formation. Other researchers ( [6][7][8] and references therein) report a constriction of the glow discharge in the near cathode region to a narrow filament. This constriction is observed to progress towards the anode in [7].…”

“…Other researchers ( [6][7][8] and references therein) report a constriction of the glow discharge in the near cathode region to a narrow filament. This constriction is observed to progress towards the anode in [7]. Chalmers and Duffy [9] observed that in oxygen arc-initiating regions appear to be always located in the gap and never at the electrodes.…”

The transition between two glow discharge phases in the SF6spark

“…Calculations by Rogoff (1972), using the hydrogen data presented by Cavenor and Meyer, confirm that at 60 ns after the diffuse glow in hydrogen 0022-3727/84/112255 + 11 $02.25 @ 1984 The Institute of Physics is established, the gas temperature reaches a value at which the effects of dissociation should begin to be significant. Power input calculationsfor the hydrogen spark (Chalmers et a1 1972) and for the iodine spark (Lewis and Woolsey 1981) provide further support for a relationship between a glow column transition and molecular dissociation. In the present work, this relationship is examined in low pressure sulphur hexafluoride, where a transition occurs between two forms of glow discharge, involving a change in positive column behaviour but not in cathode mechanism.…”

“…Similar observations were made by Emeleus and Woolsey (1970) in low current DC discharges in iodine vapour, which is electronegative. Furthermore, Lewis and Woolsey (1981), studying the low current phase of a 0.36 Torr iodine spark, reported values of cathode fall in excess of 2400 V and concluded that this phase represented an abnormal glow disharge.…”

“…In low pressure hydrogen (Kekez et a1 1972) and in nitrogen, on the other hand (Doran 1968, Chalmers eta1 1972, Kekez and Savic 1974, collapse of the cathode fall occurs before constriction of the positive column. In low pressure iodine vapour, where the glow discharge phases are dominated by attachment, Lewis and Woolsey (1981) found that collapse of the cathode fall occurs before column constriction. Important differences between the low pressure electronegative iodine spark and sparks in non-attaching gases include substantially longer time scales, with quasi-stable phases existing for times of the order of p, and glow currents of the order of mA rather than A.…”