Magnetic field penetration and ion separation in POS plasma

Abstract

Summary form only given. A theory is presented that explains the puzzling process discovered experimentally recently of simultaneous magnetic field penetration and ion separation in the plasma of a plasma opening switch (POS). The ion separation was shown by spectroscopic measurements of high resolution to consist of a light-ion plasma being pushed ahead of, while a heavy-ion plasma lags behind the magnetic piston. The fast penetration of the magnetic field into low resistivity plasma is explained by the Hall field mechanism and is predicted by the theory to occur only if the current-carrying electron flow in the direction of the gradient of the electron density, and not to occur for an opposite current polarity. In recent experiments, however, fast magnetic field penetration in the POS configuration was observed even when the initial electron density nonuniformity seemed to be too small to explain the penetration and also to be independent of the current polarity. In addition, until now there was no model that described the ion separation that occurred simultaneously with the magnetic field penetration. In the talk we will present a model that explains the fast magnetic field penetration into plasma even of initially uniform density and the ion separation. We will show that if the plasma ion composition is nonuniform, then the different pushing of the different ion species by the magnetic field generates an electron density nonuniformity which, in turn, induces the field penetration, that increases further the ion separation. That will be shown to happen for a gradient of ion composition of either direction. Thus, the two processes, magnetic field penetration and ion separation, are closely linked, and occur simultaneously. Since very often plasmas in opening switches and in other pulsed devices are multi-ions with ion-composition nonuniformity, our analysis shows that magnetic field penetration is a much more general process that has been previously believed and that ion separation is likely to occur in many cases in which a pulsed current is driven through a multi-ion plasma. This mechanism of combined field penetration and ion separation is likely to have an important effect also in space and astrophysical plasmas.