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.