Residency Abstracts

The Plasma Theory and Applications group at Sandia National Laboratories, with whom I worked, maintains the EMPIRE plasma simulation software suite. EMPIRE is unique in that it is capable of simultaneously using fluid methods, which are computationally efficient but not applicable to all physical situations, alongside kinetic methods, which tend to be slower but have a much wider regime of validity. The goal of the residency was to develop algorithms to dynamically switch between the two models depending on the physics and to apply these algorithms to large-scale simulations of real devices. In particular, I planned to develop an algorithm that we referred to as “hybrid split,” whereby a fluid is able to detect when it is about to enter a regime where a fluid description is invalid and partially reverts to a kinetic description to stabilize. In addition, the “hybrid merge” method developed during the previous residency, which was designed to identify fluid-like portions of the kinetic model and turn them into a fluid, was to be exploited during simulations of a pulsed-power device. Ultimately we chose to simulate the relativistic klystron amplifier (RKA), a device maintained by France’s CEA.

There is a need to reliably transition electrical power from a conductor immersed in dielectric medium to vacuum. At the boundary between these two media there exists an insulating wall which serves to provide structural support and electrical isolation. It is highly desirable to make this wall as small as possible to enable faster pulses and relax the design requirements of other portions of the system. The electrical limit is set by the formation of arcs across the vacuum facing surface of the insulator. This limit can be address by exploring geometries this reduce electrical stress at what are believed to be critical junctions and by careful material selection. Aspects of both techniques were investigated with emphasis placed on quantifying the statistical likelihood of flashover for given materials.

My residency supervisor Steve Slutz first showed via simulation that compression of FRCs on the Z-machine may be interesting from a magneto-inertial fusion prospective and could also produce high fusion yield. My work at Michigan focuses on experimentally characterizing FRCs produced at the hydrodynamic scale needed for this concept. The goal of my Sandia residencies was to gain access to and learn to use some computational resources to begin exploring the physics of these FRCs in simulation.

the Z-machine may be interesting from a magneto-inertial fusion prospective and could also produce high fusion yield. My work at Michigan focuses on experimentally characterizing FRCs produced at the hydrodynamic scale needed for this concept. My virtual residency allowed me to get set up with the computational resources of Sandia needed to model aspects of my Michigan experiments. I was also able to design hardware for an experimental part of my residency which will involve using the Mykonos pulsed power facility to study helical liner breakdown and/or instability mitigation on imploding liners relevant to FRC compression on Z.

This project involves producing spectroscopic data from an expanding foil photoionized plasma using the laboratory platform on the Z machine at Sandia National Laboratories. The broad scientific goal is to test astrophysical photoionized plasma codes to evaluate the fidelity of the underlying physics. Currently, the main specific motivation for the work is the persistence of the supersolar iron abundance problem. This problem refers to the fact that there are many black hole accretion disk systems which models suggest contain far more iron in the accretion disk than seems reasonable. Neglect of high density physics effects in the models is one proposed hypothesis that could explain the models' predictions of excess iron. The intent of the majority of the experimental data collection efforts is to test whether these high density effects are properly implemented and whether they can inform the supersolar Fe abundance problem.

Computer simulations of plasmas typically either use fluid models, which are fast but cannot capture far-from-equilibrium effects, or kinetic models, which are more complex but have a wider regime of validity. Hybrid models combine the two, simulating a portion of a plasma as a fluid and the remainder using a kinetic model. However, there has been little research investigating how these models should interact. Collisional and other processes can cause the kinetic portion of the plasma to rapidly overwhelm computing resources if nothing is done to move plasma from the kinetic model to the fluid. The goal of this residency was to develop, implement, and test a method for reducing the computational cost of hybrid simulations by identifying portions of the kinetic plasma that would be well-modeled as a fluid and transferring these portions into the fluid. Such a method is especially important for the hybrid simulation of the plasma in pulsed-power devices, which is initially far enough from equilibrium that a kinetic model is required, but rapidly gains sufficient mass to be computationally intractable if it were fully modeled kinetically.

This residency project was a detailed study of Fe X-ray fluorescence driven by photoionization in high-energy-density plasmas. We started by laying the theoretical atomic foundation, which consisted of analytical and numerical calculations of photoionization cross sections, radiative decay rates, and autoionization rates for Fe inner-shell K and L- shell transitions over a range of ionization stages from Fe I to Fe XXV. This was then used to calculate the Fe fluorescence yield for all Fe charge states. This atomic theory was then codified to build a Screened-Hydrogenic Atomic Process Package. This is a numerical package that calculates a range of atomic processes and information using a screened-hydrogenic, super configuration treatment. Finally, this atomic theory and associated numerical atomic package was built into a brand new Monte Carlo Radiation Transport code to simulate photon transfer in a Magnetized Liner Inertial Fusion (MagLIF) plasmas. Overall, the project resulted in the production of two new computational packages to study HED plasmas.

We studied how the wall (also called the "liner") moved in MagLIF (Magnetized Liner Inertial Fusion) style targets at the OMEGA laser facility. The main diagnostics included a 4 omega probe to take a pre and durning experiment shot showing the location of the wall at a given time and spherical crystal imager to obtain radiographs at different experimental times to analyze the density profiles of the wall.

This is a long term project which consisted of two parts. The first part was a smaller project to find line identifications in two high resolution Silicon emission data set obtained on Z. The goal was to measure transition wavelengths from a variety of Silicon charge states (He-like to Be-like) observed in the data, many of which had never been observed before. These transition wavelengths were then compared against laboratory and astrophysical atomic databases and the values updated and revised. Some discrepancies between our measured values and those encoded in NIST were also identified. In the broader context, establishing more accurate transition wavelengths is beneficial for next generation X-ray observations from upcoming satellite telescopes, namely XRISM and Athena. The second and major component of the project was to establish a clear motivation and design for the next steps of the Accretion Powered Photoionized Plasma Experiment. This experiment is part of a collaboration that encompasses 5 different experiments collectively known as Z Astrophysical Plasma Properties (ZAPP) under the fundamental science program at Sandia. The project involved designing, preparing for, and fielding experiments on the Z-machine, conducting a literature review to establish the state of the field and identify astrophysical questions which could be informed by the experimental results, and proposing for the next allocation of fundamental science program shots on Z. Specifically, we intend to perform experimental tests of new high density effects which have recently been incorporated in XSTAR (an astrophysical accretion disk modeling code) and time resolved emission measurements to inform the time-dependence of atomic kinetics in highly dynamical plasmas such as those found in warm absorbers around Active Galactic Nuclei. The work ranged from activities as diverse as crystal calibration using the Manson X-ray source at Z, performing high fidelity plasma simulation calculations using both laboratory and astrophysical codes like PrimSPECT and XSTAR, and interacting with a wide cross-section of senior scientists, technicians, diagnostics, and engineers in both astrophysics and laboratory based atomic physics to design and field the experiments.

The project focuses on the study of conditions and mechanisms associated with the voltage hold-off limits. Better understanding of these limits is desired to build smaller devices/improve device operation limits under existing geometric limitations.

Theoretical Modeling was performed on experimental spectral data from x-ray irradiated Fe foil on the Sandia Z-machine. Two different collisional-radiative spectral modeling codes, PrismSPECT and SCRAM 7.5, were utilized to analyze K-shell spectra of Fe foil photoionized by x-ray backlighting originating from nested cylindrical wire arrays of Tungsten Z-pinches on the Z-machine. I worked closely with the ZAPP research group to further their experimental goals on photoionized plasma in accretion-powered zones around active galactic nuclei.

I worked on a concept called "Laser Gate." This project is in support of the MagLIF preheat experiments on the Z Machine. I implemented work I have been doing at Michigan on Sandia scale targets and in an already built test facility at Sandia. I was studying our ability to open laser entrance hole windows by melting the perimeter instead of ablating them with a laser. The current laser ablating method has a lot of associated energy coupling losses.