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 uniform electric field at temperatures of $1200$–$3600$ K is studied experimentally. It is found that the thermionic emission from the cathode has an effect on the breakdown parameters. This effect starts manifesting itself at the critical density of the emission current $J_{cr}\sim10^{-8}$ A/cm$^2$ attained in experiments with a tungsten cathode at a temperature $T_{CT}\sim1700$ K. The increase of the cathode temperature to $\sim2200$ K results 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 temperature of up to $3600$ K does not result in a decrease of the breakdown voltage below the values defined by the gas density. In this case, the decrease of the values of $U_{br}$ is due only to the change of the gas density distribution along the gap length and obeys the generalized law of similarity. The existence of a critical temperature $T_T$ is predicted, above which one should expect a drastic decrease of the breakdown voltage due to the thermal ionization of gas. The value of $T_T$ depends on the configuration of the thermal field between the electrodes and the type of gas. In the thermal fields typical of our experimental conditions involving the heating of one of the electrodes, the critical temperature for xenon is about $4700$ K. The studies were carried out in argon, krypton, and xenon.