Design of Laboratory Experiments to Study Photoionization Fronts Driven by Thermal Sources

Abstract

This paper analyzes the requirements of a photoionization-front experiment that could be driven in the laboratory, using thermal sources to produce the necessary flux of ionizing photons. It reports several associated conclusions. Such experiments will need to employ the largest available facilities, capable of delivering many kJ to MJ of energy to an X-ray source. They will use this source to irradiate a volume of neutral gas, likely of N, on a scale of a few mm to a few cm, increasing with source energy. For a gas pressure of several to ten atmospheres at room temperature, and a source temperature near 100 eV, one will be able to drive a photoionization front through a system of tens to hundreds of photon mean free paths. The front should make the familiar transition from the so-called R-Type to D-Type as the radiation flux diminishes with distance. The N is likely to reach the He-like state. Preheating from the energetic photons appears unlikely to become large enough to alter the essential dynamics of the front beyond some layer near the surface. For well-chosen experimental conditions, competing energy transport mechanisms are small.

Atomic modeling of photoionization fronts in nitrogen gas

“…The top left panel of Figure 3 shows the ratio of the mass density to the ambient density. As expected for a photoionization front 33,34 there is little to no density evolution over the length of the simulation. These density profiles, including the density peaks, are nearly identical for these models, amounting to a difference of only a few percent.…”

“…We have presented a suite of one-dimensional Helios simulations in order to study the formation of a photoionization front in a nitrogen gas cell. This work builds upon the theoretical analysis done by Drake et al 33 and the two dimensional numerical simulations of Gray et al 34 By using Helios we are able to study the effect of changing the complexity of the physics in the formation of a photoionization front, which we were unable to do in the previous two-dimensional models. Specifically, we are able to study the effect of changing the radiation transport method between a flux limited diffusion model and a more complex multi-angle long characteristics (S n ) model.…”

“…In the case of nitrogen, there are seven distinct α and β values. These definitions proved useful in simplifying the analytic expressions found in Drake et al 33 and Gray et al 34 , but do not fully capture the physics involved. The time rate of change for a given ionization state is given by…”

“…Finally, the color-coded bars above each ionization state in Figure 6 represent the mean free path ( i = 1/n i σ i ) at the peak of each ionization state. As discussed in Drake et al 33 , a successful photoionization experiment will consist of a system with tens to hundreds of mean free paths. Except for N VII, all ionization state show mean free paths that are small compared to the system size.…”

“…As mentioned in Drake et al 33 the primary photoabsorption mechanism is photoelectric, that is, the freed electron has an energy equal to the photon energy minus the ionization energy. In astrophysical systems, stars providing the ionizing photons have temperatures of a few eV and only the photons in the high-energy tail drive the photoionization front.…”