Ion beam formation in an m = 0 unstable Z pinch

Abstract

It is well known both in early Z-pinch experiments and more recently in plasma focus experiments that highly accelerated ion beams are formed, this being inferred from the axial centre-of-mass motion of the neutrons produced by nuclear reactions. Two acceleration mechanisms are discussed here. The first, proposed in ref. 1, concerns the acceleration of ions across an electrostatic sheath adjacent to the anode, this sheath having a thickness less than an ion Larmor radius but greater than an electron Larmor radius. The electrons in the sheath, being magnetised, have a c E × B/B 2 drift through which the J r B θ force is provided to satisfy momentum conservation. The second is a new mechanism arising from the occurrence of a violent m = 0 instability, and relies on large ion Larmor radius effects. It is shown that the inclusion of the Hall terms and finite ion Larmor radius (FLR) terms in the MHD equations removes the mirror symmetry each side of an m = 0 neck. A detailed consideration of particle orbits during the formation of the highly compressed neck shows that on the cathode side of the neck an energetic ion beam is formed close to the axis, whilst on the anode side an equal and opposite momentum is distributed among a larger number of ions throughout the cross-section.