Electrical breakdown of a point-plane gap in high vacuum and with variation of pressure in the range 10−7−10−2 Torr of air, nitrogen, helium, sulphur hexafluoride, and argon

The dc breakdown potential of a point-plane electrode configuration is measured in high vacuum (∼10−7 Torr) using the positive and the negative voltage polarity of the point electrode as a function of gap separation. Air, nitrogen, helium, sulphur hexafluoride, and argon are used in turn to alter the electrode coverage by adsorption of various gases in high vacuum. It has been found that helium gives the highest breakdown voltage at a given gap length, in high vacuum. It has also been found that very high sparking potential values are obtained using positive-point-negative-plane electrodes (90 kV at 0.3 mm gap length, helium at 3×10−7 Torr). The effect of ac (50 Hz) glow discharge conditioning on the dielectric strength of the gap is investigated and found to give considerable improvement in the voltage that the gap can withstand before a vacuum breakdown occurs. The effect of introducing various gases in the pressure range 10−7−10−2 Torr on the breakdown potential of point-plane gaps is investigated. Maxima are observed in the breakdown voltage and pressure curves in the range 10−4−10−3 Torr. Helium and nitrogen give the highest breakdown voltage of about 90 kV for a gap length of 0.2 mm at about 10−4 Torr. The observed improvements in the breakdown potential that the gap can withstand with certain gases are attributed to the increase in the work function of the combined metal-gas system.