Acceleration and compression of Compact Toroid plasmas

For several years, Phillips Laboratory has been investigating, experimentally and computationally, magnetized plasma structures known as Compact Toroids (CTs). These plasmas, first proposed for laboratory acceleration by Hartman and Hammer at Lawrence Livermore National Laboratory, are axially symmetric, donut-shaped configurations with embedded toroidal and poloidal magnetic fields, which relax naturally to a minimum-free-energy state between concentric electrodes. As a result, they are stable to resistive and MHD perturbations. Once created, and under suitable conditions, they may be accelerated and compressed by magnetic forces in a coaxial gun configuration. In our laboratory, these forces are provided by the discharge of our Shiva Star 9.4 MJ fast capacitor bank, with currents in excess of 2 MA. In this paper we present recent results obtained in converging electrode accelerators at the 1 MJ stored energy level. Other parameters associated with the experiments are 10µs acceleration time scale, 50 cm initial major radius, 10 cm initial minor radius, 1 m gun length, 1 mg CT mass, and various gaseous loads. In previous PL experiments, CTs were accelerated intact over meter lengths in straight (nonconverging) coaxial electrodes at average accelerations in excess of 10 billion gravities to speeds of roughly 50 cm/µs. Using O-D and 2-D models, converging electrode geometries were designed and tested over the last year. Experimental results include compression in radius and thickness of factor 9 each, and speeds greater than 40 cm/µs.