Investigation of the ion dynamics in a multispecies plasma under pulsed magnetic fields

The interaction between a moving magnetic-field front and a low-collisionality plasma consisting of different ion species is investigated using spatially and temporally resolved spectroscopic techniques. The experiment is carried out in a plasma-opening-switch configuration, in which a current rising to $150 kA$ in $400 ns$ is conducted through a plasma that prefills the region between two planar electrodes. Ion-species separation is found to occur, similarly to the results reported for a $80 ns$ duration plasma-opening-switch experiment of cylindrical geometry, which was not necessarily expected since in the present experiment plasma pushing is more substantial. The separation, in which the light-ion plasma (protons) is reflected while the heavy-ion plasma (carbon) is penetrated by the propagating magnetic-field, is investigated by determining the electron density from the temporal evolution of spectral lines, the nonprotonic ion velocities from line-emission Doppler shifts, and the proton velocity distribution from Doppler shifts of line emissions from hydrogen atoms produced by proton charge exchange. The ion dynamics is shown to be consistent with the acceleration expected from the one-dimensional Hall electric field, based on the previously published magnetic-field and electron density evolutions. Significant acceleration of the nonprotonic ions behind the magnetic-field front is observed. It is found that a significant fraction of the protons acquire a velocity that is more than twice the velocity of the magnetic piston. This phenomenon is shown to result from the time dependence of the accelerating electric field and the broad acceleration region. The lateral motion of the nonprotonic ions is also discussed.