The thermal behavior of tantalum carbide cathodes in pseudospark switches

Pseudospark cathodes made of either tungsten, tungsten with an addition 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/sub ex//spl les/8300 K) for the purely metallic cathodes than for WRe3TaC30 (T/sub ex//spl les/4000 K). Furthermore, the increase of the excitation temperature with the current amplitude, dT/sub ex//di, is about ten times less for WRe3TaC30 than for the other materials. Similarly, the cathode surface temperature T/sub s/ estimated from a comparison of the ratio of vapor densities of tungsten and rhenium with the ratio of their equilibrium vapor pressures is much higher (T/sub s//spl ap/7000 K) for WRe26 than for WRe3TaC30 (T/sub s//spl ap/4500 K). Finally, 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 peak current), but it is 30% less for the tantalum carbide cathode, irrespective of the pulse repetition rate. Consequently, due to their lower power dissipation, WRe3TaC30 cathodes can be used for much higher currents 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 increases with increasing temperature.<
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