Session CP15: Poster Session: HED: Analytic, Diagnostic, and Computational Techniques (2:00pm - 5:00pm)

The Ohio State University Scarlet Laser Facility -- Open To External Users Via LaserNetUS

The Scarlet laser facility [1] at The Ohio State University features a 300 TW, 30 fs, 815 nm pulsed laser system capable of firing up to one shot per minute at a focused intensity exceeding 5 x 10$^{\mathrm{21}}$ W/cm$^{\mathrm{2}}$. The facility includes a large 76 inch diameter, 85 inch tall experimental chamber with 38 large aperture diagnostic ports and 7 large doors for each access. A range of diagnostics are available including electron and ion spectrometers. Scarlet is available for external users via an independent proposal system as part of the DOE supported LaserNetUS network of 10 high-power laser facilities. See https://www.lasernetus.org. In addition to supporting user targets, our liquid crystal technology [2] is available to provide on-demand, free-standing thin film targets and plasma mirrors with thicknesses ranging from \textasciitilde 10 nm to \textasciitilde 1 $\mu $m. Recent upgrades include an improved pre-pulse contrast with better than 10$^{\mathrm{-10}}$ contrast without a plasma mirror up to 50 ps before the main pulse, a long focal length (F/17) configuration in addition to our standard short focal length (F/2) setup, and a \textgreater 100 mJ probe pulse.   

PlasmaPy for HEDP Regime

PlasmaPy is a Python package being developed to foster an open source software ecosystem focused around plasma physics research and education. The high-energy-density (HED) physics regime refers to systems with an energy density \textgreater 1 Mbar or 10\textasciicircum 6 atm. In this regime, plasmas behave differently from ideal plasmas and require additional functionality to describe them. For example, ionization, magnetic fields, and relativity can be important in this regime. I am tasked with adding functionality to PlasmaPy that is relevant to HED plasma physics. One example is the Saha equation: which estimates the ratio of ions of a plasma in one ionization state to those in another. This becomes more accurate in the HED regime.   

Measuring the distribution of non-thermal electrons in hot dense plasmas

Understanding dense plasma dynamics at the level of electron collisional interactions is a challenging problem in plasma physics, dictating a host of plasma properties including stopping powers, opacities, and energy transport and equilibration timescales. Here we present an approach to investigate this ultrafast interaction dynamics via measurements of the evolution of the non-thermal electron distribution in a hot dense plasma. Using the LCLS x-ray free-electron laser we demonstrate how tailored non-thermal electron distributions can be created on-demand in highly ionized Fe plasmas and measured using single-photon-counting spectroscopy. Our results suggest a promising way to track experimentally the relaxation dynamics of a non-thermal electron distribution function on femtosecond timescales for the first time.   

Calibration of the NIF Electron Positron Proton Spectrometers (NEPPS) for Intense Laser Solid Interactions

Electron-positron pairs, produced in intense laser-solid interactions, are diagnosed using magnetic spectrometers with imaging plates. NEPPS are such spectrometers which capture the electron, positron, and proton energy spectra. We have calibrated the NEPPS with six electron beams ranging in energy from 3-15 MeV from a Siemens Oncor accelerator. The Geant4 TOPAS Monte-Carlo simulation was set up with an accurately characterized beam source to match depth dose curves and lateral profiles measured in water. Analysis of the scanned imaging plates together with the determined electron fluence and energy distribution arriving in the spectrometer provide improved dispersion curves for the NEPPS and electron dosage responses of the photo-stimulated luminescence effect. Notably, the background signal on the imaging plates during this calibration resembles the background profiles measured in laser-solid experiments. We use the Monte-Carlo simulation to characterize sources of this background by tracking photons originating from the interaction of the calibration beams and the NEPPS structure.   

Monochromatic Talbot-Lau X-ray deflectometry diagnostic for high-energy-density experiments

Talbot-Lau X-ray deflectometry (TXD) is a refraction-based technique that has been developed into a diagnostic for high-energy-density physics (HEDP) experiments. A sample located along a probing X-ray beam path causes small-angle X-ray deflections induced by changes in line-integrated density along the path. This information provides a temporally and spatially resolved map of electron density gradients within the sample. TXD can improve visibility of small features in a plasma due to the higher contrast refraction-based imaging can provide for low-Z materials when probed with X-rays in the 1-100~\mathrm{keV} range. Recent experiments to benchmark the diagnostic on the OMEGA EP and MTW laser facilities have shown that X-ray backlighter emission outside the interferometer contrast curve can reduce electron density mapping accuracy. In order to suppress this unwanted emission, a monochromatic TXD (M-TXD) system has been developed through implementation of a graded multilayer X-ray mirror. This mirror reflects only the desired X-ray energy band, preventing higher energies from reaching the system's detector and reducing performance. Results from laboratory experiments to characterize the M-TXD system are presented in preparation for upcoming MTW and OMEGA EP campaigns   

Millimeter-wave to middle ultraviolet radiometry and phase contrast imaging for studies of power balance in COBRA gas puff pinch plasmas*

Radiometer diagnostics from millimeter-wave to near-infrared and middle ultraviolet bands along with a phase contrast imaging diagnostic are being developed in order to characterize radiated power and turbulent density fluctuations in gas puff plasmas in the COBRA high energy density science facility at Cornell University. An initial millimeter-wave radiometer channel will operate in the 94 GHz range, with three near-infrared channels operating at 1100, 1310, and 1550 nm, and a UV channel at 214 nm. The multiple channels in the IR and UV range along with an envisioned expansion of the millimeter-wave radiometer to a number of channels covering the 10-300 GHz range will allow for a detailed characterization of emission as a function of frequency across the UV, IR, and microwave band. The phase contrast imaging system will operate at 1052 nm in the Bragg scattering limit, where small scattering angles corresponding to density fluctuation wavelengths ranging from 1 to 0.1 mm can be resolved spatially and temporally in order to characterize the evolution of the density fluctuation energy spectrum. Diagnostic system design, acquired data, and an explanation of the results will be presented.   

An Analysis of Laser Deflection in a Turbulent Plasma using Synthetic Diagnostics

We have established a synthetic pipeline to investigate laser deflection in turbulent High Energy Density (HED) Plasmas using a combination of ray-tracing and ray-transfer-matrix techniques. This has been done to support the development of a new diagnostic system which measures directly deflection angles in one direction. The spectrum of deflection angles is directly linked to the spectrum of density perturbations within a turbulent plasma. Conventional shadowgraphy and schlieren techniques retrieve the spectrum of deflection angles using a digital Fourier transform of an image, which leads to a limited dynamic range and is ineffective when caustics are present. A 3-Dimensional Gaussian field with a given power law is used to generate synthetic plasma electron density perturbations which allows us to compare conventional shadowgraphy and schlieren imaging techniques with the new diagnostic system. We analyse the angular deflections of the probing laser beam in order to assess the performance of these imaging systems for HED Plasma turbulence applications..   

Advances in X-Ray Imaging Crystal Spectrometer Design Through Raytracing

X-ray imaging crystal spectrometers are powerful diagnostics for both magnetic confinement fusion (MCF) and high energy density physics (HEDP) experiments. Recent advances in spectrometer design have led to improvements in spectrometer capabilities, physical characteristics and calibration quality. These improvements have been driven, in part, by recently developed x-ray raytracing capabilities. In addition, ray tracing has also enabled improvement in analysis capabilities, both in improved accuracy of spectral analysis and in verification of tomographic inversion techniques. In this contribution we explore advancements, enabled by x-ray ray tracing, for several existing and proposed diagnostic systems including the W7-X XICS spectrometer, the ITER XRCS-Core spectrometer, and several variable crystal radii spectrometers for HEDP applications (multi-cone, modified toroid and sinusoidal spiral). We also introduce a new ray tracing software package, XICSRT, and discuss current capabilities and opportunities for future applications.   

FAC Atomic Data in SPECT3D

SPECT3D is a collisional-radiative spectral analysis package used to compute detailed emission, absorption and XRTS spectra, as well as filtered images, for multiple 1D, 2D, and 3D geometries. SPECT3D computes LTE and non-LTE populations using detailed atomic physics models. Here we introduce the initial implementation of SPECT3D support for atomic data produced by the Flexible Atomic Code (FAC) in addition to the standard ATBASE atomic data. Users can now use the Prism application Atomic Model Builder to read binary FAC output and produce atomic model files which can be read by SPECT3D and other Prism plasma codes. We demonstrate good agreement with the well-validated ATBASE data for a Local Thermodynamic Equilibrium (LTE) spectral calculation from an aluminum plasma using the single-cell code PrismSPECT.   

SPECT3D, Imaging and Spectral Analysis Package.

SPECT3D is a collisional-radiative spectral analysis package designed to compute detailed emission, absorption, or x-ray scattering spectra, filtered images, XRD signals, and other synthetic diagnostics. The spectra and images are computed for virtual detectors by post-processing the results of hydrodynamics simulations in 1D, 2D, and 3D geometries. SPECT3D can account for a variety of instrumental response effects so that direct comparisons between simulations and experimental measurements can be made. We will present new features of SPECT3D and highlight their application to the analysis of HEDP experiments, especially to K-$\alpha $ and K-$\beta $ emission spectroscopy.   

Open L-Shell Spectroscopy of Non-local Thermodynamic Equilibrium Plasmas

Spectral modeling codes are commonly used to infer plasma conditions from measured spectra. However, at non-local thermodynamic equilibrium conditions common in high-energy-density environments, such models produce conflicting results for open-shell systems. Improving the underlying atomic models would improve inference capabilities in such systems, particularly for employing L-shell spectra as a sensitive density diagnostic where traditional K-shell techniques are limited. To begin to provide high-quality data that can discriminate between spectral codes, we present results of time-resolved, open L-shell Ge spectroscopy from Ge and Sc buried layers in 10-$\mu $m-thick Be irradiated by the OMEGA laser. Time-resolved temperature and density are constrained by Sc K-shell spectra and images of the emitting volume. Comparisons to the spectral model SCRAM are explored. This platform will enable systematic measurement of high-resolution, temporally-resolved spectra of open L-shell mid-$Z$ elements.   

Simulation development for multi-ion species shock interaction using discontinuous Galerkin method

The interaction of multi-ion species in plasma shocks is not well understood due to kinetic effects that are necessary to understand the physics of species separation and diffusion. The discontinuous Galerkin method (DGM) can capture sharp gradients, discontinuities, and shocks to effectively study multi-ion-fluid shocks while incorporating some collisional and diffusion physics. The DGM is a high-order shock capturing scheme used extensively for the modeling of hyperbolic equation systems. The gradients at cell interfaces in the DGM are not defined hence recovery methods will be used to accurately and efficiently model diffusion terms using the DGM. This method allows for more accurate capture of species separation in the presence of multiple ion species. Using the newly developed PHORCE (Package for High ORder simulations of Convection-diffusion Equations) code, the physical interactions of multiple ion species within plasma shocks are simulated and compared to kinetic results. PHORCE has been developed at Virginia Tech for multi-dimensional simulations using DGM to solve for high order convection-diffusion equations using unstructured grids for various different equation systems. The numerical algorithm will be described in this work.   

Electronic correlation effects in the stopping power of ions in 2D materials

The energy loss of charged projectiles in correlated materials is of prime relevance for plasma-surface interaction for which we have developed a nonequilibrium Green functions (NEGF) approach. A particularly interesting effect is the \textit{correlation induced increase} of stopping power at low velocities\footnote{Balzer \textit{et al.}, \textit{Phys. Rev. Lett.} {\bf 121}, 267602 (2018)}. However, NEGF simulations are possible only for short time durations, due to the unfavorable $N_t^3$ scaling with the number of discretization time steps. The situation has changed radically with the recently developed G1-G2 scheme\footnote{Schluenzen \textit{et al.}, \textit{Phys. Rev. Lett.} {\bf 124}, 076601 (2020)}, which is based on the generalized Kadanaoff-Baym ansatz in combination with Hartree-Fock propagators, and allows to \textit{achieve linear scaling} with $N_t$. This enhancement enables us to improve previous simulations by using better selfenergies\footnote{Joost \textit{et al.}, Phys. Rev. B \textbf{101}, 245101 (2020)}, studying larger systems and by extending the simulation duration which gives access to slower projectiles. Finally, we will report further improvements of the G1-G2 scheme itself, by taking into account three-particle correlations.   

Fields and Particle Dynamics in the Vicinity of a MITL Load

The fields and associated electron flow for magnetically insulated transmission lines (MITLs) have been studied in great detail in previous works. However, much of the previous work has focused attention on electron flow in locations away from a load, i.e. where the circuit voltage goes to zero. In this work, we compute the vacuum electric fields within cylindrically symmetric MITLs for specific geometries containing a load. The field calculations are performed in the limit that the speed of light is infinite, but that the time-dependent MITL current is arbitrary. We show that the field calculations are in excellent agreement with the code EMPIRE developed at Sandia National Laboratories. Additionally, we simulate the motion of charged particles in a MITL due to the vacuum fields using a Runge-Kutta solver, and compare these simulations to results from EMPIRE. In general, we find that while the motion of charged particles in a MITL near a load can be highly nonlinear, using guiding center drift theory in conjunction with relativistic adiabatic invariants can be an excellent tool for approximating a particle's motion.   

Forecast-Enhanced Multiscale Modeling for High Energy-Density Matter

Microscopic data in the form of collision times, transport coefficients, reaction rates, and equations of state are needed to provide crucial material properties to macroscopic models. In many cases, this data can be precomputed and stored in tables or fits. In some cases, however, this is inconvenient because of the required span and dimensionality of the table. In such cases, it is more efficient and accurate to compute the closure data on the fly, alternating between microscopic and macroscopic model solvers. This scheme, known as the Heterogenous Multiscale Method (HMM), suffers from (1) the data becoming stale at an unknown time during the macroscale evolution and (2) reinitializing the microscale model requires more data than the macroscale model contains. Here, we propose the use of machine learning, specifically vector auto-regression (VAR), to solve the HMM problem by recasting it as a Bayesian forecasting problem. That is, we learn the microscopic behavior with VAR and evolve it alongside the macroscopic solver such that we can assess its uncertainty before we reinitialize the microscale model. We numerically explore this idea using a hydrodynamic model with various numbers of moments.   

Improvements to Quasineutral, Kinetic-Ion Hybrid Particle-in-Cell Modeling

In many physical situations, single-fluid magnetohydrodynamic codes do not adequately capture all relevant physics, yet fully-kinetic simulations are computationally prohibitive. For instance, in the acceleration of high-energy, $>$100 keV, ion beams in dense plasma focus Z-pinch implosions, where instabilities generate strong electric fields leading to ion acceleration and neutron production. Other situations include the interpenetration of fast, $>$100 km/s, ion streams that may have mean-free-paths much longer than relevant scale lengths. In such situations, a hybrid model with kinetic ions and fluid electrons can capture the relevant ion physics, while still operating at fast computational speeds. In this work, we detail recent improvements and benchmarks implemented in the framework of the particle-in-cell code Chicago [Thoma et al. PoP 24, 062707 (2017)]. The method follows the motion of kinetic ions, and models electrons using a quasi-neutral, magnetized Ohm’s law. The importance of proper treatment of ion-electron collisions will be discussed. Prepared by LLNL under Contract DE-AC52-07NA27344.   

A Conservative Multiscale 2D3V Curvilinear Phase-Space Moving Grid Algorithm for the Vlasov Equation with Applications to High Energy Density Systems

In high energy density systems such as inertial confinement and magnetic liner fusion experiments, the relevant length and velocity scales span many orders of magnitude from the initially preheated fuel to compressed burning plasmas, making kinetic simulations challenging. To deal with these challenges, we have developed a conservative 2D3V moving phase-space grid strategy for the hybrid Vlasov ion and fluid electron system. The configuration space grid is evolved to track macroscopic features such as shocks while the velocity space grid is expanded/contracted with the heating/cooling of the plasma. We transform the equations in terms of logical coordinates in phase-space with corresponding inertial terms (i.e., mesh motion terms). In configuration space, these inertial terms are cast in a particular form that lends the discretized equations to automatically satisfy the geometric conservation law. In velocity space, the grid is normalized and evolved in terms of the thermal speed and the resulting inertial terms are discretized using the technique of discrete nonlinear constraints to satisfy the underlying conservation symmetries. We will demonstrate the new algorithm's capability on several challenging benchmark implosion problems in cylindrical and spherical coordinates.