An image converter and a 4-stage image intensifier have been used to investigate the development of spark breakdown in a 3-cm, uniform-field, gas-discharge gap. The growth of ionization, initiated by a burst of electrons a t the cathode, has been observed for overvoltages up to 25 % in nitrogen, oxygen and sulphur hexafluoride a t pressures in the range 13 to 40kPa (100 to 300 Torr). In nitrogen, time resolved shutter and streak records have been obtained which demonstrate, that, at low overvoltages, breakdown is preceded by the formation of a diffuse glow discharge whereas, at voltages well in excess of the static breakdown voltage, the arc discharge is formed directly in the track of the initial electron avalanche, as a result of space-charge distortion of the applied electric field. This change with overvoltage has previously been ascribed to a transition from a Townsend to a streamer breakdown mechanism; the present results, however, do not entirely support this view. In oxygen and sulphur hexafluoride, no such change in the form of ionization development has been observed and it is concluded from the evidence obtained that, even a t very small overvoltages, the development of ionization is strongly affected by space-charge distortion of the applied field. Photographs obtained in sulphur hexafluoride have been correlated with measured formative-time-lag data showing the time-lag-free region reported previously by Kuffel. Some conclusions have been drawn regarding the breakdown mechanism under these conditions.
Breakdown of liquid dielectrics