Growth of Surface Instabilities in a Linear Pinched Discharge

Abstract

RECENT experiments at the Imperial College of Science and Technology, London, have been concerned with the growth of instabilities in a linear pinched discharge in argon. These discharges were produced in a tube 15 cm. in diameter and 50 cm. long at pressures of 0.1–1 mm. mercury and with currents of 40–150 k.amp. The experiments have revealed the presence of surface instabilities of the type shown in Fig. 1, which is typical of many single-shot photographs of the discharge taken through the side of the tube with a Kerr cell camera. The irregularities on the plasma surface first appear when the column is expanding from the first axial bounce, and grow while the plasma is being accelerated towards the axis of the discharge tube between the first and second ‘bounces’. In Fig. 2 the diameter of the column, together with the amplitude of the instability, and its mean wave-length, are shown as functions of time for a particular set of initial conditions. In this diagram the diameter of the column has been found by taking the mean of fifty measurements of the local diameter at different points along the tube axis. A numerical estimate of the amount of instability was obtained from the standard deviation of these measurements about their mean.