# Current channel evolution in ideal Z pinch for general velocity profiles

## TL;DR

This paper investigates how different plasma velocity profiles affect current distribution in Z pinches, explaining recent experimental observations of outward current channel motion through advection effects.

## Contribution

It introduces a theoretical framework analyzing current evolution under general velocity profiles, explaining the experimentally observed outward current channel motion in Z pinches.

## Key findings

- Current profile shape can be preserved under metric compression with proportional velocity.
- General velocity profiles can cause non-conservation and redistribution of current.
- Specific inward velocity profiles can lead to outward current channel motion.

## 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, 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, with |v_r| proportional to 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 non-conservation 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.