Optical and x-ray observations of carbon and aluminium fibre Z-pinch plasmas

Abstract

This paper reports investigations of a Z-pinch plasma formed from carbon and aluminium fibres by a pulsed power generator delivering 100 kA with a rise time (10 - 90%) of 55 ns. For carbon fibres the breakdown was observed to proceed from the cathode and was coincident with a pulse of hard x-rays consistent with an electron beam of about 4 kA and > 50 keV. For aluminium fibres plasma emission was observed to be uniform over the whole length of the fibre and there was no hard x-ray signal. A radial expansion with a velocity of of the plasma column was seen for both fibres. During the expansion, at about 4 ns for carbon fibres and later for thicker fibres, m = 0 perturbations distributed along the length of the column became evident with a scale length which increased with time as the column expanded. Also for carbon fibres, two groups of bright spots separated by about 10 ns emitting optical and x-ray radiation were observed; the second group was more intense, lasted longer and showed bifurcation. Coincident with the second group was an electron beam of 16 keV for and 7 keV for carbon fibres. Bright spots were also evident for aluminium fibres but these were in a single group, lasted for a much longer time and were accompanied by emission from the anode consistent with 7 keV electron beam. Analysis of time-integrated x-ray pinhole images gave temperature of the bright spots between 75 - 135 eV for carbon fibres and 80 - 100 eV for carbon fibres. The very intense bright spots observed were found to coincide with density islands which were observed later in time but only for carbon fibres. The density in these islands was measured by interferometry to be greater than . After 110 ns plasma was not observable for carbon fibres; for carbon and aluminium fibres, the plasma was observed for longer times.