Current channel evolution in ideal Z pinch for general velocity profiles

Abstract

Recent diagnostic advances in gas-puff Z pinches at the Weizmann Institute for the first time allow the reconstruction of the current flow as a function of time and radius. These experiments show an unexpected radially outward motion of the current channel, as the plasma moves radially inward [C. Stollberg, “Investigation of a small-scale self-compressing plasma column,” Ph.D. thesis (Weizmann Institute, 2019)]. In this paper, a mechanism that could explain this current evolution is described. We examine the impact of advection on the distribution of current in a cylindrically symmetric plasma. In the case of metric compression, |vr|∝r, the current enclosed between each plasma fluid element and the axis is conserved, and so the current profile maintains its shape. We show that for more general velocity profiles, this simple behavior quickly breaks down, allowing for nonconservation of current in a compressing conductor, rapid redistribution of the current density, and even for the formation of reverse currents. In particular, a specific inward radial velocity profile is shown to result in radially outward motion of the current channel, recovering the surprising current evolution discovered at the Weizmann Institute.