PHOTOGRAPHIC OBSERVATIONS OF IMPULSE BREAKDOWN IN SHORT VACUUM GAPS

An image intensifier and image converter with a maximum sweep speed of 1 ns mm-1 have been used to observe the development of impulse breakdown in short vacuum gaps of up to 1 mm. Electrodes of stainless steel, copper and aluminium have been used and it has been found that in each case metal vapour is always produced first at the cathode surface and later at the anode surface; the final arc discharge develops by propagation of the anode vapour. Time delays between voltage application and the appearance of the anode vapour have been measured in the range 10-50 ns and their dependence upon electrode separation and electrode material has been explained using a simple model involving anode heating by the emission current from a vaporised emitting site at the cathode.