On the transparency of foam in low-density foam Z-pinch experiments

Abstract

Z-pinch experiments have been performed on the SATURN and Z machines at Sandia National Laboratories to study physics issues related to x-ray radiation generation and inertial confinement fusion. Some of these experiments utilize a CH foam located on-axis to convert energy to radiation and act as a radiative transfer volume. A significant issue for foam Z-pinch experiments is the transparency of the heated foam as a function of time and wavelength. Foam transparency will be important in future foam Z-pinch experiments both because it influences the time-dependent radiation field seen by an inertial confinement fusion capsule embedded in the foam, and because it is an important factor in making high-resolution spectral measurements of a capsule or tracers embedded in the foam. In this article, we describe results from simulations and experiments which address the issue of foam transparency. We discuss imaging data from one Z experiment in which x-ray emission from a half-Au/half-CH disk located at the bottom of a 1-cm-tall, 14 mg/cc TPX foam is observed. Simulation results predicting CH foam optical depths as a function of plasma conditions are presented. In addition, we present results from spectral calculations which utilize 2D magnetohydrodynamic (MHD) simulation predictions for the time-dependent foam conditions. Our results indicate that the observed x-ray framing camera images are consistent with early-time (several ns prior to stagnation) foam electron temperatures of ≳30 eV, which is somewhat hotter than the foam electron temperatures predicted from the 2D MHD simulations at early times.