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Studies of the Z-pinch discharge in high pressure helium Preston, Jonathan Melvin

Abstract

Improvements to the laser excited interferometer previously constructed in this laboratory are described. The use of a rotating retro-reflector in the interferometer cavity, and electronic circuitry, permit direct recording of electron densities. The sensitivity of the interferometer is 5 x 1015 cm-2, and the temporal resolution is 0.1 usee. Pyrex tubes, terminated by windows, enclosing the laser beam, have been used to improve spatial resolution. The effects of the tubes, and of all other sources of error associated with interferometric measurements of electron densities, have been carefully assessed. A Z-pinch discharge in 4 torr helium has been studied with this instrument, and with spectroscopic determinations of electron temperature. Under the conditions chosen, the pinching plasma does not reach the axis of the discharge tube, but stops at a radius of 2.7 cm. The shock front caused by the pinching action has been shown to be weak. Longitudinal structure, due to heat flux, characteristic cathode behaviour, and variations in timing of the pinching action, has been found. The discharge has been assessed as a spectroscopic source and as a medium for laser scattering experiments. The characteristics of two suitable regions are presented. These are the axial region and the hollow cylinder of plasma formed by the arrest of the pinching plasma. Both are free of instabilities. At the appropriate time the latter is also current free, and shows longitudinal electron density variations of less than 2%. The length of time that the current spends near the wall of the vessel is determined by the balance between kinetic and magnetic pressures.

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