Plasma Motion during the Formation Phase of the PS-3 and PS-3.5 Spheromaks

Spectroscopic Doppler shift measurements of a CIII impurity ion in the ultraviolet are reported from the University of Maryland spheromak experiments PS-3 and PS-3.5. A new time and space resolved Doppler shift diagnostic with absolute wavelength calibration to +/- 0.02 A is described. The spheromak is a member of the compact toroid class of magnetic confinement devices. Spheromak formation is thought to proceed through a magnetic reconnection or relaxation process characterized by evolution to a minimum energy equilibrium subject to the constraint that the magnetic helicity is invariant. Large velocity fields (on the order of the Alfven speed) observed in both the PS-3 and PS-3.5 experiments during the formation phase, but not during the equilibrium, suggest that flow fields may play an important role in the reconnection and relaxation process. The standard theory of Taylor relaxation may need to be modified in order to include the effects of bulk plasma motion. Doppler shift measurements of the CIII 2296.87 A impurity emission line in the PS-3.5 spheromak were made side-on above and below the machine axis. During the formation phase, blue shifts are observed below and red shifts observed above the axis suggesting a toroidal rotation in the direction of the confined plasma diamagnetic drift. The plasma emissivity near the CIII wavelength was obtained from Abel inversion of the line-integrated intensities. Computer modelling of velocity fields, emissivity profiles and line shapes is used to interpret the experimental data. Variation of the flow fields as a function of the static fill pressure indicated the possible significance of the density profile to bulk motion of the plasma. A quadarature interferometer was used to obtain the time history of the line-integrated density through two chords of the plasma. Double-floating probe measurements gave the radial component of the electric field. The University of Maryland PS experiments utilize a combined theta and z discharge to produce the spheromak magnetic field configuration. The theory of rotation in theta pinch devices is briefly reviewed and a comparison is made with the results of the present experiment. The theoretical implications of including velocity terms in the energy minimization problem are also discussed. The decay rate of an ideal MHD invariant, the cross helicity, is considered in view of viscous and resistive effects in the experiments.