Long exposure time schlieren photography of plasma focus discharges

Abstract

An effect of the oscillating acceleration of the current sheet during the compression phase in a plasma focus discharge has been observed using long (60 ns) exposure time schlieren photography. This effect had not been previously observed in experiments using exposure times of a few nanoseconds or less. A 1 J Q-switched ruby laser has been used as a source of light. The photograms are taken in dark-background schlieren with a circular stop (disk) instead of a usually used knife edge. The various acceptance ranges of refraction angle have been selected by the size of the stop and aperture in order to limit the integral of the electron density gradient along the beam within the apparatus range, which is between 3X10'* and 3X 102' cmm3. Raising the sensitivity threshold to 2X1019 cm-a (refraction angle of 4 mrad) has helped to clear the pictures by limiting their image to high gradients of density only. Using this technique and other diagnostic methods, the dynamics of 6-W, 16-kV plasma focus discharges have been investigated. These discharges were made in deuterium at 5 Torr, with a 10% decrease of the magnetic insulation at the breech. The average radial velocity of the current sheet, 5 X lo4 m/s, obtained with this effect agrees with those measured by the smear effect. The peak value of 1 X lo" m/s is consistent with results received with magnetic probes. The electron density gradient has been determined for several instants; its maximum value at the pinch is (1.2?0.2)X1021 cmm4. 8