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

Abstract

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.

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

“…The significant additional density-rise observed was attributed to the proton reflection, a process later verified using charge exchange measurements. 31 The subsequent density drop could not have been explained by the proton reflection only. The present, high-resolution measurements, that show a much larger density-rise and a faster subsequent densitydrop, thus call for a revisit of the underlying processes, which can now be performed using the correlation between the electron density and the ion velocity.…”

“…Plasmas 23, 122126 (2016) We stress that unlike the reflection of the protons that was directly observed by means of charge-exchange techniques, 31 the inferred reflection of C IV-V depends on the validity of the electric-potential hill model. We note that other possible phenomena, such as instabilities or other 3D effects, as well as energetic electrons ejected from the electrodes, may also play a role in the interaction and their investigation may require better resolutions.…”

“…29 In these experiments, the heavier-ion plasma component (carbon) is penetrated by the magnetic field at a rate much faster than that expected from diffusion, 30 whereas the light-ion plasma (protons) is reflected off the field front, acquiring velocities that are about twice the magnetic-field front propagation velocity. 31 The ions in the plasma penetrated by the magnetic field are also found to acquire non-negligible velocities relative to the magnetic-field propagation velocity. This complex plasma dynamics, involving both magnetic field penetration and plasma pushing, should be manifested in the electron density evolution.…”

On the Role of the Plasma Composition in the Magnetic Field Evolution in Plasma Opening Switches

“…A complete report on the ion dynamics measurements, including a quantitative analysis of the results, is given in a subsequent paper [25]. Here, in Fig.…”

“…Two doping methods are used: a gas doping technique and a surface-flashover method (previously described in [25]). The dopant sources are mounted outside the A-K gap about 0.5 cm from the cathode, and the injection is in the direction.…”