Atomic Modeling of Photoionization Fronts in Nitrogen Gas
  Photoionization fronts play a dominant role in many astrophysical
environments, but remain difficult to achieve in a laboratory experiment.
Recent papers have suggested that experiments using a nitrogen medium held at
ten atmospheres of pressure that is irradiated by a source with a radiation
temperature of T$_{\rm R}\sim$ 100 eV can produce viable photoionization
fronts. We present a suite of one-dimensional numerical simulations using the
\helios\ multi-material radiation hydrodynamics code that models these
conditions and the formation of a photoionization front. We study the effects
of varying the atomic kinetics and radiative transfer model on the
hydrodynamics and ionization state of the nitrogen gas, finding that more
sophisticated physics, in particular a multi-angle long characteristic
radiative transfer model and a collisional-radiative atomics model,
dramatically changes the atomic kinetic evolution of the gas. A photoionization
front is identified by computing the ratios between the photoionization rate,
the electron impact ionization rate, and the total recombination rate. We find
that due to the increased electron temperatures found using more advanced
physics that photoionization fronts are likely to form in our nominal model. We
report results of several parameter studies. In one of these, the nitrogen
pressure is fixed at ten atmospheres and varies the source radiation
temperature while another fixes the temperature at 100 eV and varied the
nitrogen pressure. Lower nitrogen pressures increase the likelihood of
generating a photoionization front while varying the peak source temperature
has little effect.
