Simulations of photoionization fronts on the Z-machine using a well-characterized radiation flux input*

In the early universe at the end of the dark ages, the first galaxies and stars started forming. This introduced a sustained ionizing photon flux into the intergalactic medium (IGN) in photoionization (PI) fronts, re-ionizing the universe. PI fronts are heat fronts where PI dominates the energy deposition at the interface. The Z-machine at Sandia is a very bright source of x-rays, emitting over 1 MJ of soft x-ray energy. This is an attractive platform to make measurements of photoionization fronts. We discuss a study performed with the Helios-CR code for a N gas cell for a potential Z experiment. The radiation-hydrodynamic simulations included inline, self-consistent non-equilibrium atomic physics and photon-energy resolved radiation transport. They were driven with the time-history of a spectrally resolved x-ray flux obtained from VISRAD view factor modeling of the Z radiation environment constrained with power and monochromatic image measurements of the z-pinch. A parameter study over gas pressure and atomic model complexity explores the front propagation with Z as a driving source. A resolution study shows the importance of capturing the photon mean free path in PI front calculations.