Observation of faster-than-diffusion magnetic field penetration into a plasma

Abstract

Spatially and temporally resolved spectroscopic measurements of the magnetic field, electron density, and turbulent electric fields are used to study the interaction between a pulsed magnetic field and a plasma. In the configuration studied (known as a plasma opening switch) a 150 kA current of 400 ns-duration is conducted through a plasma that fills the region between two planar electrodes. The time-dependent magnetic field, determined from Zeeman splitting, is mapped in three dimensions, showing that the magnetic field propagation is faster than expected from diffusion based on the Spitzer resistivity. Moreover, the measured magnetic field profile and the amplitude of turbulent electric fields indicate that the fast penetration of the magnetic field cannot be explained by an anomalously high resistivity. On the other hand, the magnetic field is found to penetrate into the plasma at a velocity that is independent of the current-generator polarity, contradictory to the predictions of the Hall-field theory. A possible mechanism, independent of the current-generator polarity, based on the formation of small-scale density fluctuations that lead to field penetration via the Hall mechanism, is presented. It is suggested that these density fluctuations may result from the effect of the unmagnetized Rayleigh–Taylor instability on the proton plasma that undergoes a large acceleration under the influence of the magnetic field pressure.

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

“…Previous spectroscopic measurements of the electron density have shown a density rise of up to $50%, which generally accompanies the arrival of the magnetic field to the region of observation, followed by a sharp density drop. 30 This observation was consistent with earlier line-integrated density measurement in similar systems. 32,33 The spatially resolved spectroscopic measurements showed that part of the plasma is indeed penetrated by the magnetic field and remains behind the propagating field front.…”

“…2016) profile. 30 It was shown, under similar conditions, that ionization processes (due to the electron heating resulting from the magnetic energy dissipation) and the continuous plasma flow from the flashboard-plasma source cannot account for more than $30%-rise in n e during the magnetic field penetration. The significant additional density-rise observed was attributed to the proton reflection, a process later verified using charge exchange measurements.…”

“…28 Laboratory observations in such systems of low-resistivity, multi-ion-species plasmas (mostly protons and carbon ions) revealed a phenomenon of simultaneous field penetration and plasma pushing. 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.…”

“…However, a quantitative analysis showed that the proton reflection cannot account for the entire observed density drop. 30 Here, using the same multi-ion species system used in Ref. 30, we employ spectroscopic techniques with significantly improved spatial resolution to study in detail the electron density evolution during the plasma-magnetic field interaction.…”

“…30 Here, using the same multi-ion species system used in Ref. 30, we employ spectroscopic techniques with significantly improved spatial resolution to study in detail the electron density evolution during the plasma-magnetic field interaction. Furthermore, Doppler shifts of the same line emission that is utilized for the density measurements are also utilized for the determination of the electric potential (referred to as potential hill) that develops due to the magnetic-field propagation.…”