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. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Experimental profiles of the He II lines at 3203 Å and 4686 Å have been obtained from a well diagnosed helium plasma. The plasma was produced in a low power z-pinch discharge contained in a vessel of 15 cm diameter and 60 cm length. Uniform plasma columns were located and studied by means of two moveable quartz limiter tubes. The effect of the tubes on the measured electron temperature and density due to cooling and evaporated impurities was measured and the tubes were positioned so as to select a uniform plasma column for study. Electron temperatures were determined from the intensity ratio of the lines He II 4686 and He I 5876 using a theory that considered finite escape probabilities for the Lyman a resonance photons. Electron densities were determined from the width of the He I 5876 line. Corrections were applied for ion broadening due to doubly charged ion perturbers. An electron temperature of 4.0±0.4 eV and an electron density of (6 .1 ±0.6) xl 0 2 3m"^;.wlere determined for the axial plasma occurring soon after the pinch phase. The experimental profile of He II 4686 as measured by a spectrometer-optical multichannel analyzer arrangement was compared to theoretical profiles resulting from a treatment of the electron broadening collisions as single event impacts (Kepple (1972)) and from a treatment of the electron broadening which considers the time development in the collision process (unified classical-path theory) (Greene (1976)). The experimental profile was found to lie midway between the theoretical profiles. Agreement between the experimental and unified classical-path profi1e in the near line wings was good. Agreement between experimental and electron impact results for He II 3203 was not good with a 60% disagreement in line widths. The He II line had a double peak with the peak at shorter wavelengths being 9% higher than the other peak. This is similar to the behavior observed in the Balmer line, Hβ, in the atomic hydrogen spectrum. / Science, Faculty of / Physics and Astronomy, Department of / Unknown

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