Investigation of K-shell emission from moderate-Z, low-η (-velocity), Z-pinch implosions

Abstract

Because of the large amount of kinetic energy and mass needed for efficient production of K-shell emission from moderate atomic number z-pinch plasmas (Z≳22), moderate Z experiments performed in the near future will likely take place at relatively low implosion velocities or low-η values, where η is defined as the ratio of the maximum kinetic energy per ion generated prior to stagnation to the minimum energy per ion, Emin, needed to instantly heat and to ionize a plasma into the K shell upon stagnation of the pinch. Since there has been no systematic theoretical or experimental investigations of K-shell yield scaling with mass and atomic number in the low-η regime η∼1–3, in anticipation of such future experiments, we report on the results of our theoretical investigation into this regime. In particular aluminum, argon, titanium, and krypton plasmas were studied using a 1D radiation hydrodynamics model that uses enhanced transport coefficients to phenomenologically attain the stagnation conditions of experimental z-pinches. The results of this study have identified the following important effects: low-η plasmas remain in an inefficient mass2 (or current4) scaling regime at much higher masses than predicted by our earlier work, which was based on η≳3.7 calculations. Thus, more energy than predicted by the earlier scaling laws will apparently be needed to obtain significant amounts of, for example, krypton K-shell emission. In addition, lags in ionization times become important increasingly with the Z of the plasma and adversely affect K-shell emission. The physics responsible for these time dependent effects and how they will influence the design of future experiments are discussed.