Alfv\'enic plasma flow during spheromak preformation stage

Spheromak formation consists of a series of dynamic steps whereby highly localized plasma near the electrodes evolves towards a Taylor state equilibrium. The dynamical evolution stage is often modeled as a series of equilibrium states. However, the experiments at the Caltech Spheromak facility have revealed that non-equilibrium Alfv\'enic flows are driven during these preliminary stages by unbalanced $\vec{J}\times\vec{B}$ forces. The flow velocity was measured using time of flight measurements using a novel He-Ne density interferometer with low phase ambiguity $\sim 1^\circ$(D. Kumar and P. M. Bellan, Rev. Sci. Instrum. {\bf 77}, 083503 (2006)). The flow velocities depend on the gas species inertia and lead to a collimated plasma jet with $\beta \sim 1$(P. M. Bellan, Phys. Plasmas {\bf 10} Pt2, 1999 (2003)). Experiments are underway to characterize how the flow velocity depends on the initial neutral gas density profile. Under some conditions, a layer of neutral gas with density $\sim 10^{23}/\textrm{m}^3$ and thickness $\sim 1$ cm is observed to move in front of the plasma jet. The neutral gas density in the layer was estimated using the Gladstone-Dale relation(F. J. Weinberg, Optics of flames (1963)).