Breakdown in sulphur hexafluoride and nitrogen under direct and impulse voltages

Breakdown studies have been carried out in uniform fields, in nitrogen and SF6, for pressures varying between 200 torr and 2830 torr, with both 0.1/320¿s impulse voltages and direct voltages. Formative timelags in nitrogen and SF6 have been measured, and the effect of conditioning the electrode surfaces and the gas, by sparking, has been studied. Irradiation has been provided by a spark-discharge source situated inside the anode of the test gap, and breakdown is triggered by the pulse of ultraviolet light on the cathode. The mean time lags in SF6 and nitrogen have been measured for overvoltages between 0.5 and 20%. They are found to be of similar magnitude in the two gases, and are in the range 10¿6 to 10¿8. These values contrast sharply with much longer time lags for SF6 reported elsewhere. The differences are attributed to differing methods of irradiating the cathode. At pressures of 200 torr and 760 torr in SF6, repeated sparking is found first to decrease the mean time lag, and later to increase it. This behaviour is repeated if the test is continued after the gas has been changed. Time lags in SF6 are found to be much less consistent than in nitrogen, with a much greater standard deviation in a set of results. Particularly erratic behaviour was observed in SF6 at 2830 torr, for both impulse and direct voltages. This is a consequence of breakdown stresses reaching 350kV/cm, since the characteristics then became very dependent on the precise state of the electrode surface.