Failure of Vacuum Insulation at Low Field Strengths

Abstract

PRE-BREAKDOWN currents in vacuum-insulated systems are usually regarded as due to field emission. If the electrodes are not subjected to heat treatment, however, these currents occur at unexpectedly low field strengths. We have recently examined a form of conduction at 10–150 kV. D.C. across gaps of a few centimetres in continuously evacuated systems. The current is in this case distinct from field emission because (i) it is critically dependent on anode conditions, and (ii) the voltage required is only slightly changed by large variations in gap-length. Other characteristics of interest are that the current is self-extinguishing, after a charge of the order of micro-coulombs has crossed the gap, with only slight drop in voltage; and that the voltage required may be increased by factors up to 2 by increasing the pressure of residual gas in the system.