Optical studies of plasma jets

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Abstract
Various adaptations of Schlieren and shadow methods were used to study the highly turbulent region in the wake of a magnetically rotated DC arc plasma jet to elucidate the velocity and mixing pattern of the plasma jet with the ambient atmosphere; in this case argon with air. The main tools were spark- and fast cine-shadowgraphy, and a novel form of laser Schlieren velocimetry based on focusing the dark-field Schlieren image of the wake region on to a series of fine slits backed by a photomultiplier and oscilloscope. The passage of the bright edge of each eddy is thus characterised by a modulated signal which yields its velocity component in the direction perpendicular to the orientation of the slits. Probe methods were used to monitor the optical study. Complete velocity distributions were obtained and used to infer relative local densities by conservation of mass. The results include volume entrainment from the surroundings and local relative turbulence energies. In the innermost zone the density distribution suggests gradual degradation of nontranslational arc energy while the flow in the outer zone appears to be independent of events within the nozzle.

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