Structural Evolution and Formation of High Energy Density Plasmas in X-Pinches

Summary form only given. Results are presented from 3D MHD simulations of X-pinch experiments at Cornell University and the Pontificia Universidad Catolica in Santiago. The simulations reveal how the formation of the key features of X-pinch structure - the central microscopic Z-pinch and the supersonic axial jets - are the direct results of the non-linear evolution of the ablating wire plasmas and the distribution of the Lorentz force. The generation of synthetic diagnostic images from the simulation data allows direct comparison to absorption radiographs from the Cornell experiments and laser interferometry and gated soft X-ray images from Santiago. This comparison showed good agreement with the time dependent structure in both cases. The results of the 3D code were used to initialize 2D cylindrical simulations of the micro Z-pinch region including a recently developed radiation transport package, which could be run at much finer spatial resolution <1 mum. These simulations showed that the experimental results from Cornell and Santiago for the X-ray hot-spot region in X-pinches could be explained in terms of the non-linear development of a m=0 instability in the presence of strong radiative cooling. Both the 3D and the 2D results revealed that the main process determining the radial collapse of the hot-spot region is the reduction in the mass per unit length of the plasma caused by axial outflow. Results are also presented from a simple analytic model of the collapsed state, in which the plasma is assumed to be in radial pressure balance and the energy gained from ohmic heating balances a black-body like radiation loss. Under these conditions, equilibrium values for the radius, density, temperature, X-ray power, etc. can be derived in terms of the current and the mass per unit length. This model was found to be consistent with both the 2D and 3D simulation results and with data for density and temperature versus time during the intense X-ray burst phase, deter- ined spectroscopically in experiments at Cornell and at the Lebedev Institute. This model then allows a scaling to be obtained for peak densities and temperatures as functions of the driver current and the X-pinch material.