Field Shaping to Improve Vacuum Diode Voltage Holdoff

In our work we need a small vacuum diode which will reliably hold off a pulse voltage in excess of 150 kV. These diodes typically have an interelectrode spacing of 6 to 10 mm with a glass insulator length 3 to 4 times the interelectrode spacing. We found that the overall holdoff voltage for a diode with a fixed insulator and interelectrode gap length was strongly affected by the position of the interelectrode gap. The highest voltage holdoff was obtained when the interelectrode space was at the negative end of the insulator. Experiments were made with two sizes of diodes, varying the location of the interelectrode space from the negative end to the positive end. The large diode high voltage holdoff ranged from 240 kV with the interelectrode space at the negative end to a minimum of 50 kV in the middle and returning to a 180 kV with the interelectrode space at the positive end. The smaller diode high voltage holdoff ranged from a high of 200 kV with the interelectrode space at the negative end to 130 kV with the interelectrode space at the positive end. The minimum at the middle was not as pronoucned in this diode. Since most of the HV breakdowns were across the glass insulator, electron avalanching was suspected. Computer-generated field plots confirmed this possibility.