THE THERMAL-BEHAVIOR OF TANTALUM CARBIDE CATHODES IN PSEUDOSPARK SWITCHES

Pseudospark cathodes made of either tungsten, tungsten with an additio n of rhenium (WRe26), or tungsten with inclusions of tantalum carbide grains (WRe3TaC30) were investigated by laser-induced fluorescence on metal vapors and by measurements of the cathode power loss, Excitation temperatures of tungsten vapor are considerably higher (T-ex less tha n or equal to 8300 K) for the purely metallic cathodes than for WRe3Ta C30 (T-ex less than or equal to 4000 K), Furthermore;the increase of t he excitation temperature with the current amplitude, dT(ex)/di, is ab out ten times less for WRe3TaC30 than for the other materials, Similar ly, the cathode surface temperature T-s estimated from a comparison of the ratio of vapor densities of tungsten and rhenium with the ratio o f their equilibrium vapor pressures is much higher (T-s approximate to 7000 K) for WRe26 than for WRe3TaC30 (T-s approximate to 4500 K, Fina lly, the power loss to the cathode is about equal for the tungsten and WRe26 cathodes (e,g,, 200 W at a repetition rate of 230 Hz and 9-kA p eak current), but it is 30% less for the tantalum carbide cathode, irr espective of the pulse repetition rate, Consequently, due to their low er power dissipation, WRe3TaC30 cathodes can be used for much higher c urrents than the purely metallic cathodes. The physical reason for the favorable thermal behavior of the WRe3TaC30 cathode is attributed to the fact that the electrical conductivity of tantalum carbide increase s with increasing temperature.