Correlation of slow rates of rise of current with times to pinch in transient mercury-vapour plasma

Measurements of time of maximum pinch in a mercury-vapour-plasma discharge over a range of voltages up to 5kV are not, at first sight, in agreement with values calculated by using the Leontovich-Osovets theory, which has been shown to apply in similar experiments with noble gases. The influence of the external circuit involving the ignitron switch, measuring shunt and associated stripline connecting links, is deduccd from the oscillographic records of instantaneous v and i for the plasma discharge. The ratios v/i for both the external circuit and the plasma discharge are plotted against time to show the times at which the minima and the pinch maximum are reached. A graph showing measured times to pinch is compared with values calculated using the Leontovich-Osovets formula, using the initial rate of rise of current, and shown to be in poor agreement. A further plot of calculated values, obtained by using the mean rate of rise of current up to the pinch point as a fixed value, is shown to be in good agreement with the experimental results.