Current Evolution in a Pulsed Overstressed Radial Vacuum Gap

The evolution of current in a radial, two element, vacuum gap has been studied as a function of applied voltage. Gap spacings of 1 and 2 mm, between coaxial brass electrodes, at a pressure of 1×10-2 Pa, have been used. Voltage pulses between 20 kV and 176 kV, with 15 ns risetimes and 50 ns durations, have been applied to the vacuum gap using a water-filled Blumlein generator having a source impedance of 10 ¿. The data indicate that voltages on the order of 32 and 60 kV can be applied to 1 and 2 mm gaps, respectively, without drawing currents larger than 100 A during the excitation time. For applied voltages greater than 60 kV, current approximately equal to the short circuit value was observed in a 1 mm gap. For higher voltage excitations, breakdown delays on the order of 5 ns, with an initial current rate of rise of up to 8×1011 A/s, have been obtained.