Investigations of Laboratory Photoionized Plasmas with the 1MA Zebra Accelerator

Laboratory produced photoionized plasmas provide a key steppingstone between astrophysical observations and the models we use to interpret them. Understanding the behavior of photoionized plasmas is essential for our interpretation of astrophysical phenomena such as: x-ray binary systems, active galactic nuclei, and planetary nebulae. These plasmas are driven by a high-intensity broadband x-ray flux. Astronomers parameterize photoionized plasmas via the ratio of x-ray flux to particle number density, known as the ionization parameter,, which characterizes the balance of photoionization and recombination. In order for laboratory photoionized plasmas to be relevant, intense broadband x-ray sources are required. The 1MA Zebra pulsed-power accelerator at the University of Nevada, Reno is capable of producing an intense ~20kJ broadband 25ns x-ray pulse, via the implosion of a gold 8-wire cylindrical wire-array. The x-ray flux is used to photoionize a supersonic gas jet. Astrophysically relevant photoionized plasmas have an ionization parameter , which can be tuned by two main parameters: the distance the gas jet is from the radiation source and the atomic density of the gas jet. The bulk of the spectral distribution serves to heat and ionize the gas, while a portion of the high-energy tail end of the spectral distribution, 13-16 Å, backlights the plasma for x-ray absorption spectroscopy. Also, laser diagnostics are employed. Neon, argon, and nitrogen photoionized plasmas have been studied as well as hydrogen/neon mixtures. Neutral gas jet atomic densities are in the range of 10¹⁷-10¹⁸cm^-3. The electron temperature is extracted from the K-shell line absorption spectrum using a novel method developed for laboratory photoionized plasmas that is independent of atomic kinetics modeling¹. Analysis of spectroscopy and laser interferometry measurements are consistent and measure electron areal densities of the order 10¹⁸cm^-2. For the case of neon photoionized plasmas, spectroscopy analysis shows the fractional ion population to be dominated by B-, Be-, and Li-like neon ions, with an average charge state of 5.8, and electron temperatures in the range from 10eV to 30eV. With continued systematic surveys of the parameter space detailed above, we are developing a quality data set. That will enable us to test modeling codes and thus improve our ability to interpret celestial phenomena.

¹R. C. Mancini et al, Physical Review E 101, 051201(R) (2020)