Pinches and micropinches in the SPEED 2 plasma focus

Abstract

Dynamical pinches (z-pinch, plasma focus) produce two different modes of pinch plasmas, unstable columns with successive short-lived micropinches or stable narrow columns with lifetimes around 50 ns. These two modes depend on the energy density of the plasma sheath and the atomic number of the working gas. Low energy density and/or heavy gases result in the micropinch mode; high energy density and/or light gases result in the stable column mode (SCM). Micropinches are actuated in pinch plasmas by local radiative collapses, the necessary condition of which is that the pinch column shows neckings (m = 0 type) at these places. In the SPEED 2 plasma focus with fixed energy input (70 KJ, 180 kV, 1.5 MA) neckings and micropinches appear only if the atomic number Z ≥ 18 whereas with Z < 18 macroscopically stable pinch columns are formed. Attempts to trigger micropinches on predetermined axial positions by inserting solid particles failed because of the short interaction time of the dynamic pinch (≤50 ns) with the target. Only surface plasma around the solid core is created and deep compression only takes place in front of the tip of solid fibres.