Breakdown characteristics of nitrogen at different pressures in alternating non-uniform electric fields

AC breakdown characteristics of nitrogen at different pressures in electric fields with different degrees of non-uniformity provided by a sphere-plane electrode system have been investigated. It is found that the measured breakdown voltages obey Paschen's law at the values of the field factor (the ration of the maximum field strength between electrodes to the average field strength) below similar to 2. Deviations start to appear and increase gradually if the field factor increases above this threshold. Respectively, the dependence of the breakdown voltage U b on the product of pressure p and gap distance d changes from been linear U b similar to p xd to a more complex relationship of type U b similar to p xln(d). It is found that the maximum field strength corresponding to the breakdown increases with increasing gas pressure and decreasing electrode radius but it is independent on the gap distance. The breakdown voltages predicted by the streamer criterion agree well with the experimental data. The results obtained for nitrogen were compared with the breakdown characteristics of air and it was found that at the values of the field factor below 9, the breakdown strength of nitrogen was higher than that of ambient air.