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

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 approximate to 10(4) m s(-1) of the plasma column was seen for both fibres. During the expansion, at about 4 ns for 7 mu m 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 7 mu m and 7 keV for 33 mu m 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 7 mu m carbon fibres and 80-100 eV for 33 mu m carbon fibres: The very intense bright spots observed were found to coincide with density islands which were observed later in time but only for 7 mu m carbon fibres. The density in these islands was measured by interferometry to be greater than 10(25) m(-3) After 110 ns plasma was not observable for 7 mu m carbon fibres; for 33 mu m carbon and 25 mu m aluminium fibres, the plasma was observed for longer times.