The structure of a magnetic-field front propagating non-diffusively in low-resistivity multi-species plasma

Abstract

We report on the first experimental verification of the traveling-wave-like picture of a magnetic-field and an associated electric potential hill propagating non-diffusively in low resistivity plasma. High spatial resolution spectroscopic method, developed here, allowed for obtaining the detailed shape of the propagating magnetic-field front. The measurements demonstrated that the ion separation, previously claimed, results from the reflection of the higher charge-to-mass ratio ions from the propagating potential hill and from climbing the hill by the lower charge-to-mass ratio ions. This ion dynamics is found to be consistent with the observed electron density evolution.

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

“…This assumption is supported by recent measurements, 39 showing that the electric-and magnetic-front profiles remain nearly constant during their propagation at the relevant time-scale of the measurement. The force exerted on an ion by such an electric field is proportional to the ion's charge; hence the ion's velocity change (in time) is proportional to its charge-to-mass ratio, Z/m.…”

Fast magnetic field penetration into low resistivity plasma

“…2001; Rubinstein et al. 2016) showed that there is also a significant ion motion. Moreover, they demonstrated a most interesting process; ion separation in a multi-ion species plasma.…”

“…2004, 2008; Rubinstein et al. 2016), showed magnetic field penetration that was much faster than expected by classical resistivity. Theorists at the Kurchatov Institute (Kingsep, Mokhov & Chukbar 1984; Kalda & Kingsep 1989; Kingsep, Chukbar & Yan‘kov 1990; Gordeev, Kingsep & Rudakov 1994) and in other research groups (Fruchtman 1991, 1992 a , b ; Fruchtman & Gomberoff 1992, 1993; Fruchtman & Rudakov 1992, 1994; Oliver et al.…”

“…2004, 2008; Rubinstein et al. 2016), we assume that the magnetic field propagates as a travelling wave to the right with a constant velocity and that and are uniform in space and constant in time. In experiments, the magnetic field on the upstream side of the current layer is not exactly uniform due to the finite rise time of the current.…”

“…In a common configuration, the magnetic field has one component and the associated current flows in only one direction (perpendicular to the magnetic field). In high-power pulsed plasmas of such configuration, experimental evidence by probes (Weber et al 1984), followed by thorough studies employing spectroscopy at the Weizmann Institute (Sarfaty et al 1995;Shpitalnik et al 1998;Weingarten et al 2001;Arad et al 2003;Doron et al 2004Doron et al , 2008Rubinstein et al 2016), showed magnetic field penetration that was much faster than expected by classical resistivity. Theorists at the Kurchatov Institute (Kingsep, Mokhov & Chukbar 1984;Kalda & Kingsep 1989;Kingsep, Chukbar & Yan'kov 1990;Gordeev, Kingsep & Rudakov 1994) and in other research groups (Fruchtman 1991(Fruchtman , 1992aFruchtman & Gomberoff 1992Fruchtman & Rudakov 1992Oliver et al 1992;Huba, Grossmann & Ottinger 1994;Swanekamp et al 1996;Fruchtman, Ivanov & Kingsep 1998;Chuvatin, Ivanov & Rudakov 2004;Richardson et al 2013Richardson et al , 2016 explained the measurements by showing that the Hall electric field can induce a fast magnetic field penetration if the plasma is non-uniform, even if the resistivity is small (but not zero).…”