Pulsed electric current and the value of critical current at cryogenic temperatures

Decreasing the temperature of a cathode changes its thermophysical properties. The transition to the superconducting state causes the cathode resistance to vanish and, hence, makes impossible any Joule heating of the cathode micropoints (CMs) by the self-emission current. The results of an experimental and theoretical investigation of how cooling a Nb cathode to cryogenic temperatures and its transition to the superconducting state affect the prebreakdown current and the pulse electric strength of the vacuum gap are reported. It is shown experimentally that the superconducting state of a cathode has no effect on the delay time of pulsed vacuum breakdown. Cooling the cathode from 300 to 4.2 K results in a monotonic increase of the explosion delay time as well as of the prebreakdown current flowing through the CM. This is due to the intensification of heat removal at low temperatures. The field emission current rises spontaneously at voltages across the vacuum gap close to the breakdown voltages on a time scale of ⩽1000 ns. The period and the rate of current rise are determined by the initial cathode temperature. The phenomenon observed is due to the heating of the emitter by self-emission current and to the rapid propagation of the temperature maximum deep into the point. An essential role of the space charge in the heat balance at the point and in the rate of field-emission current rise is revealed