Metal vapour densities in a pseudospark switch

Abstract

A pseudospark switch with a cathode made of a composite material containing 95% of tungsten and small amounts of iron, copper, and nickel was subjected to current pulses of 12 kA in amplitude and 1.6 µs duration. Laser-induced fluorescence was used to measure the contribution of each metal to the number density of neutral vapour released by the pseudospark discharge. The vapour composition differed considerably from the composition of the cathode material. In particular, the maximum copper vapour density amounted to 1.5 *1018 m-3 while the maximum tungsten density was as low as 2 *1017 m-3. It is concluded that the loss of copper from the electrode material may result in disintegration of the electrode material after sufficient number of discharges. Hence low-melting additions to the cathodes of pseudospark switches should be avoided. A comparison of the ratios of the vapour densities of the constituents of the cathode material with the corresponding ratios of the equilibrium vapour pressures revealed that there are regions of extremely high (T>5000 K) and comparatively low (T approximately=1500 K) temperature on the cathode. This adds evidence to the view that the electron emission at the cathode of pseudosparks occurs from individual microscopic emission sites rather than from a surface heated homogeneously to a uniform temperature.