Modeling the Radiation Drive, Heating, and Ionization of a Photoionized Neon Plasma Experiment at the Z Facility 

Thomas E. Lockard, University of Nevada, Reno

January 13, 2:00 - 3:00pm, EBUII 584

Abstract:  Motivated by gas cell photoionized plasma experiments performed by our group at the Z facility of Sandia National Laboratories, we discuss a modeling study of the x-ray drive, and plasma heating and ionization. Photoionized plasmas are non-equilibrium systems driven by an intense and broadband x-ray radiation flux. They are commonly found in astrophysics but rarely seen in the laboratory. Several modeling tools have been employed: (1) a view-factor computer code constrained with side x-ray power and gated monochromatic image measurements of the z-pinch radiation, to model the time-history of the photon-energy resolved x-ray flux driving the photoionized plasma, (2) a Boltzmann self-consistent electron and atomic kinetics model to simulate the electron distribution function and configuration-averaged atomic kinetics, (3) a radiation-hydrodynamics code with inline non-equilibrium plasma atomic kinetics to perform a comprehensive numerical simulation of the experiment and plasma heating, and (4) steady-state and time-dependent collisional-radiative atomic kinetics calculations with fine-structure energy level description to assess transient effects in the ionization and charge state distribution of the plasma. The results indicate that the photon-energy resolved x-ray flux impinging on the front window of the gas cell is very well approximated by a linear combination of three geometrically-diluted Planckian distributions representing contributions from z-pinch radiation as well as re-radiation from the hardware set up. Knowledge of the spectral details of the x-ray drive turned out to be important for computing both the heating and ionization of the plasma. The free electrons in the plasma thermalize quickly relative to the timescales associated with the time-history of the x-ray drive and the plasma atomic kinetics. Hence, electrons are well described by a Maxwellian energy distribution characteristic of a single temperature. This finding is important to support the application of a radiation-hydrodynamic model to simulate the experiment. It is found that the computed plasma heating compares well with experimental observation when the effects of the Mylar window transmission, hydrodynamics, and non-equilibrium neon emissivity and opacity are taken into account. The atomic kinetics shows significant time-dependent effects because the timescale of the x-ray drive is too short compared to that of the photoionization process. These modeling and simulation results are important to test theory and modeling assumptions and approximations, and also to provide guidance on data interpretation and analysis of experiments performed over an order of magnitude range in ionization parameter.

This work was sponsored in part by the National Nuclear Security Administration under the High Energy Density Laboratory Plasmas grant program through DOE Grant DE-FG52-09NA29551, the Z Facility Fundamental Science Program, and by SNL.