Electrical breakdown in nonuniformly heated gases and the upper temperature limit of the applicability of Paschen's law

The electrical breakdown of nonuniformly heated gas-discharge gaps with a u niform electric field at temperatures of 1200-3600 K is studied experimenta lly. It is found that the thermionic emission from the cathode has an effec t on the breakdown parameters. This effect starts manifesting itself at the critical density of the emission current J(cr) similar to 10(-8) A/cm(2) a ttained in experiments with a tungsten cathode at a temperature T-cr simila r to 1700 K. The increase of the cathode temperature to similar to 2200 K r esults in a decrease of the breakdown voltage U-br by more than an order of magnitude due to the thermionic emission. The heating of the anode to a te mperature of up to 3600 K does not result in a decrease of the breakdown vo ltage below the values defined by the gas density. In this case, the decrea se of the values of Ubr is due only to the change of the gas density distri bution along the gap length and obeys the generalized law of similarity. Th e existence of a critical temperature TT is predicted, above which one shou ld expect a drastic decrease of the breakdown voltage due to the thermal io nization of gas. The value of TT depends on the configuration of the therma l field between the electrodes and the type of gas. In the thermal fields t ypical of our experimental conditions involving the heating of one of the e lectrodes, the critical temperature for xenon is about 4700 K. The studies were carried out in argon, krypton, acid xenon.