Electron density evolution during a fast, non-diffusive propagation of a magnetic field in a multi-ion-species plasma

We present spectroscopic measurements of the electron density evolution during the propagation of a magnetic-field front (peak magnitude ∼8 kG) through low-resistivity, multi-ion species plasma. In the configuration studied, a pulsed current, generating the magnetic field, is driven through a plasma that pre-fills the volume between two electrodes. 3D spatial resolution is achieved by local injection of dopants via an optimized laser blow-off technique. The electron density evolution is inferred from the intensity evolution of Mg II and B II-III dopant line-emission. The Doppler-shifted line-emission of the light boron, accelerated by the magnetic field is also used to determine the electric-potential-hill associated with the propagating magnetic field. Utilizing the same spectral line for the determination of both the density and the electric potential allowed for exploring the precise correlation between these two key parameters. For these measurements, achieving a high spatial resolution (a small fraction of the magnetic-field front) was necessary. The density evolution is found to be consistent with a scenario in which ions with relatively high charge-to-mass ratios are reflected by different potential heights, namely, reflected off the magnetic-field front at different field magnitudes, whereas the plasma of ions with low charge-to-mass ratios is penetrated by the magnetic field.