Laboratory-generated photoionization fronts relevant to astrophysics

In both astrophysics and the laboratory, sufficiently energetic ionizing radiation can create photoionized plasma that has complex opacity, emissivity, temperature, and density. Previous efforts have studied the steady state properties of photoionized plasmas, but little work has been done to look at the transient behavior at the leading edge of the photoionized regions. Here, we present a first-of-its-kind experiment to create a photoionization front in the laboratory and study its time-dependent evolution. In the experiments on the Z-Machine at Sandia National Laboratories, a Z-pinch dynamic hohlraum created a bright, 2 MJ x-ray source which drove a photoionization front into a gas cell with 1 atm of nitrogen. Using a combination of photon Doppler velocimetry and streaked visible spectroscopy, measurements of the front curvature and emission show the front had a supersonic velocity of 1580 ± 620 km/s, high ionization states of NIII and NIV, and curvature flattening consistent with non-diffusive radiation transport behavior. This new platform can be used to study transient photoionization physics similar to many radiation-driven systems, such as O-type stars emitting into molecular clouds, allowing detailed laboratory studies of this physics.