Spot Conduction of Currents between Moving Plasma and Cold Electrodes

The electrical impedances of several pairs of cold metal electrodes immersed in moving shock-heated argon have been measured by applying constant, increasing and decreasing voltages to the electrodes. Two distinct conduction régimes are found to exist - one where the electrode impedance is very high and is approximately inversely proportional to electrode surface area, and a second régime where the electrode impedance is virtually independent of electrode area for electrode current densities less than 10 6 A cm -2 . It is deduced that the first régime arises when few electrons are emitted from the cathode, whereas the second régime exists when many electrons are emitted from a small fraction of the cathode area. A simple theory of a cathode spot is proposed which shows at least order-of-magnitude agreement with the experimental observations and, moreover, explains various unusual characteristics of the impedance.