Session BO7: HED: Laboratory Astrophysics I: Atomic Physics

Density jump as a function of magnetic field strength for parallel collisionless shocks in pair plasmas.

Collisionless shocks follow the Rankine--Hugoniot jump conditions to a good approximation. However, for a shock propagating parallel to a magnetic field, magnetohydrodynamics states that the shock properties are independent of the field strength, whereas recent particle-in-cell simulations reveal a significant departure from magnetohydrodynamics behavior for such shocks in the collisionless regime [1]. This departure is found to be caused by a field-driven anisotropy in the downstream pressure, but the functional dependence of this anisotropy on the field strength is yet to be determined. Here, we present a non-relativistic model of the plasma evolution through the shock front, allowing for a derivation of the downstream anisotropy in terms of the field strength [2]. Our scenario assumes double adiabatic evolution of a pair plasma through the shock front. As a result, the perpendicular temperature is conserved. If the resulting downstream is firehose stable, then the plasma remains in this state. If unstable, it migrates towards the firehose stability threshold. In both cases, the conservation equations, together with the relevant hypothesis made on the temperature, allows a full determination of the downstream anisotropy in terms of the field strength. Similar results can be obtained for perpendicular shocks [3]. [1] Journal of Plasma Physics, vol. 83, 715830201 (2017). [2] Journal of Plasma Physics, vol. 84, 905840604 (2018). [3] Phys. Plasmas 26, 062108 (2019).   

Collisionless shock formation and electron acceleration in conditions relevant for NIF experiments

Collisionless shocks are ubiquitous in astrophysics and are known to be important in magnetic field amplification and acceleration of high-energy radiating electrons and cosmic rays. While diffusive shock acceleration (DSA) is well established, the details of how particle injection into the DSA phase depends on the shock structure are not yet fully clear. Recently laser-driven high-energy-density (HED) experiments at the National Ignition Facility (NIF) have observed for the first time the formation of collisionless shocks mediated by electromagnetic instabilities and nonthermal electron acceleration, opening a path for the detailed study of the shock acceleration physics in the laboratory. We will present particle-in-cell (PIC) simulations of counterstreaming inhomogeneous plasmas for the conditions of NIF experiments and discuss the associated shock formation and electron acceleration physics. We show that the inhomogeneous plasma profiles lead to efficient formation of a turbulent shock mediated by the Weibel instability and that electrons can be injected/accelerated to nonthermal energies via a Fermi-like mechanism occurring within the finite, turbulent shock transition. Our results suggest that high Mach number astrophysical shocks can be efficient electron accelerators.   

The Weibel instability beyond the scaling expected for bi-Maxwellian distribution functions: typical anisotropic electron velocity distribution functions in high energy density plasma physics and laser-plasma instabilities

The Weibel instability is a universal plasma instability driven by velocity distribution function anisotropy. It is inherently a kinetic effect wherein current perturbations in plasmas with anisotropic temperatures will pinch in a way to reinforce the perturbations, leading to an exponentially growing magnetic field. This instability plays a key role in the dynamics of astrophysical objects and also could be part of the seeding mechanism for the magnetic field of the universe. It is widely believed that the temperature (second moment of the distribution function) anisotropy is necessary to observe this instability. In this work, we solve the linear theory dispersion relation and use particle-in-cell simulations with the code OSIRIS to show that an anisotropy of the higher moments of the distribution function can also seed magnetic field growth. We explore the effects of the higher moments of typical distribution functions and parameters that occur in astrophysical and laboratory scenarios. Our emphasis will be on parametric instability modified and non-local heat transport modified electron velocity distribution functions.   

\textbf{Experimental exploration of the transition between the Biermann battery and Weibel instability magnetic field generation mechanisms in laser-driven plasmas }

The generation and amplification of magnetic fields throughout the universe is currently not fully understood. The Biermann battery and Weibel instability are two mechanisms for magnetic field growth. In laser-generated plasmas, where there are large misaligned temperature and density gradients, the Biermann battery typically dominates other mechanisms. PIC simulations (Schoeffler \textit{et al.} PRL 2014) showed that the Biermann battery gives way to the Weibel instability under the right conditions, which could be reached in a laser-generated plasma bubble expanding from a foil. Data from beam-on-foil experiments at OMEGA in the last few years have had hints of this transition to the Weibel instability. Characteristic Weibel filaments appear in the periphery of experiments with the appropriate conditions. We present this data in the context of understanding this Biermann-Weibel transition, as well as outline requirements for future experiments aimed at exploring the transition further. This work was supported in part by the U.S. DOE, NLUF and LLE.   

Simulations of photoionization fronts on the Z-machine using a well-characterized radiation flux input

In the early universe at the end of the dark ages, the first galaxies and stars started forming. This introduced a sustained ionizing photon flux into the intergalactic medium (IGN) in photoionization (PI) fronts, re-ionizing the universe. PI fronts are heat fronts where PI dominates the energy deposition at the interface. The Z-machine at Sandia is a very bright source of x-rays, emitting over 1 MJ of soft x-ray energy. This is an attractive platform to make measurements of photoionization fronts. We discuss a study performed with the Helios-CR code for a N gas cell for a potential Z experiment. The radiation-hydrodynamic simulations included inline, self-consistent non-equilibrium atomic physics and photon-energy resolved radiation transport. They were driven with the time-history of a spectrally resolved x-ray flux obtained from VISRAD view factor modeling of the Z radiation environment constrained with power and monochromatic image measurements of the z-pinch. A parameter study over gas pressure and atomic model complexity explores the front propagation with Z as a driving source. A resolution study shows the importance of capturing the photon mean free path in PI front calculations.   

Radiation cooling of laboratory photoionized plasmas

In separate experiments performed at the Z facility of Sandia National Laboratories two different samples were employed to produce and characterize photoionized plasmas. One was a gas cell filled with neon, and the other was a thin silicon-oxygen layer tamped with plastic. Both samples were driven by the broadband, intense x-ray flux produced at the collapse of a wire array z-pinch implosion. Transmission spectroscopy of a narrowband portion of the x-ray flux was used to diagnose the plasma. A method was used to extract the electron temperature that is independent of atomic kinetics modeling. To interpret the measurement, we performed Boltzmann electron kinetics and radiation-hydrodynamics modeling. The simulations of both experiments emphasized the critical interplay between atomic physics and plasma heating, and demonstrated the dramatic impact of photoexcitation on excited state populations, line emissivity, and radiation cooling.   

Utilizing Gas Jets to study Laboratory Photoionized Plasmas

Photoionized plasmas are important for astrophysical objects such as x-ray binary systems, active galactic nuclei, and planetary nebulae. Laboratory photoionized plasmas enable systematic studies and provide data to test plasma theory and benchmark modeling codes. Supersonic gas jets represent an attractive platform for photoionized plasma experiments. Experiments, at the 1MA Zebra pulsed power accelerator of the University of Nevada Reno, photoionize supersonic gas jets by a 25ns-duration broadband x-ray flux of a z-pinch. In this short time gas jet motion is negligible, and the x-ray flux drives the gas without undergoing attenuation through a window or tamper material. Neon, argon, and nitrogen gases have been investigated. Mach-Zehnder interferometry at 266nm, dual-color air wedge interferometry at 266 and 532nm, multi-color shadowgraphy at 266, 532, and 1064nm were used to probe the neutral gas jet as well as the photoionized plasma. A cylindrically bent KAP crystal spectrometer was employed for x-ray transmission spectroscopy. Interferometry and x-ray spectroscopy showed electron areal densities of 1-3.5x10$^{\mathrm{18}}$ cm$^{\mathrm{-2}}$, and electron number densities of 1-4x10$^{\mathrm{19}}$cm$^{\mathrm{-3}}$.   

Improving accuracy of stellar opacity experiments using calibration statistics and Monte-Carlo error propagation

Opacity quantifies photon absorption in matter and is an important quantity for accurately predicting plasma evolution for astrophysical objects (e.g., stars) and laboratory experiments (e.g., inertial confinement fusion). However, calculated opacities have never been extensively tested. Benchmark opacity measurements at stellar interior temperature are recently available from experiments at Sandia National Laboratories [Bailey Nature (2015), Nagayama PRL (2019)], providing invaluable clues on opacity-model accuracy and suggesting necessary opacity-model refinements. To realize benchmark opacity measurements, the experimental platform must satisfy many challenging criteria, including accurate data analysis and measurement reproducibility. We present recently improved analysis method; this relies on extensive statistics of calibration experiments and formal propagation of multiple sources of uncertainties using Monte-Carlo technique. Opacities inferred from repeated experiments agree within the inferred uncertainties, supporting the validity of the analysis method and reliability of experiments. The idea behind this analysis is general and can be applied to many other experiments. Sandia National Laboratories is a multimission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. DOE's NNSA under contract DE- NA0003525.   

Revised laboratory measurements of iron opacity for stellar interiors.

Models for stars, including our Sun, require accurate opacities. Thus, the persistent discrepancy between opacity model predictions and published measurements from Z experiments poses a dilemma for stellar astrophysics [Bailey et al, Nature (2015)]. Recent systematic measurements as a function of atomic number showed that either opacity theories are missing physics that has nonmonotonic dependence on the number of bound electrons or there is an experimental flaw unique to iron measurements at temperatures above 180 eV. The supposed flaw is not present at lower temperatures and densities [Nagayama et al. PRL (2019)]. To resolve this issue, we are performing new iron opacity experiments that replicate the high temperature conditions and extend the measurement to even higher temperature values. Furthermore, we are re-analyzing the opacity data using refined analysis methods that provide higher accuracy and reliability. The goal is benchmark opacity measurements with the highest possible confidence. Sandia National Laboratories is a multimission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. DOE's NNSA under contract DE- NA0003525.   

First experimental assessment of the Z opacity sample evolution using time-resolved spectroscopy with a gated hybrid CMOS detector.

The discrepancy between opacity models and laboratory experiments injects uncertainty into stellar interior models. The model/data discrepancy in Fe opacity measurements at high temperature (T \textgreater 180 eV) and high electron density 3x10$^{\mathrm{22}}$ cm$^{\mathrm{-3}}$ [Bailey et al, \textit{Nature} (2015), Nagayama et al. \textit{PRL} (2019)] have yet to be resolved. Systematic errors from unresolved temporal gradients are one possible hypothesis, despite evidence that such errors are unimportant. Past data recorded on x-ray film provide spectral measurements over a time determined by the backlighter time history, but direct sequential time-resolved measurements didn't exist; until now. The novel hCMOS Ultra-fast X-ray Imager (UXI) camera developed at Sandia National Laboratories and implemented in the opacity spectrometers allows such tests for the first time. Mg K-shell absorption was recorded to measure the opacity sample evolution. These measurements enable further evaluation of possible temporal gradient effects, test simulation predictions, and to optimize future opacity experiment designs.   

\textbf{Multiconfiguration Opacities from an Average Atom Model}

Average-Atom models based on density functional theory provide a self-consistent picture of electronic structure in high energy density plasmas, providing essential input for equation of state and conductivity tables. While opacities derived from average-atom wavefunctions (e.g. by Kubo-Greenwood) have the attractive feature of natively including dense plasma effects such as continuum lowering and pressure ionization, they lack both the accuracy in transition energies and the detailed structure necessary to calculate reliable opacities. Here we demonstrate a technique to efficiently generate detailed multiconfiguration atomic structure and spectra through a Taylor expansion of Slater coefficients. The basis wavefunction include both bound states and the ``scars'' of pressure-ionized bound states, ensuring smooth changes under density variations and extending self-consistent dense plasma effects from the average atom to a multiconfiguration model.   

Collisional-radiative modeling applied to post-disruption fusion plasmas with a runaway electron component

Relativistic runaway electrons generated in post-disruption tokamak discharges have the capacity to cause significant damage. A primary disruption mitigation approach currently being considered for ITER is to inject large amounts of high-Z impurities, such as neon or argon. Interaction between runaway electrons and high-Z impurities can set both the runaway evolution and the plasma cooling rate, through impurity ion charge state distribution and radiative power loss. In order to generate greater understanding of these properties, we have extended upon the popular FLYCHK collisional-radiative model, with focus on modeling fusion plasmas where high-Z impurities are introduced and a minority relativistic energy electron population is present. Novel to our CR model is inclusion of relativistic effects for electron impact inelastic cross-sections integrated over an arbitrary electron energy distribution. It is shown that significantly different predictions are produced when full relativistic effects are present, highlighting the importance of accurate atomic data in improving our understanding of fusion science. By accounting for these vital phenomena with the help of uncertainty quantification, we demonstrate a much improved predictive capability for CR modeling of fusion plasmas.   

Numerical Simulation of Marshak Wave Propagation in Stochastic Media on the OMEGA-60

Radiation flows in stochastic media have direct and analogous applications in astrophysics and nuclear engineering. Advances in experimental techniques, meanwhile, have enabled the adoption of scaled platforms on HED facilities to recreate the Marshak wave physics underlying these radiation flows. In preparation for near-future experiments on the OMEGA-60 laser, we use the radiation-hydrodynamics code Cassio to model Marshak wave propagation in a $\rho <$100 g/cc, nanoporous silica aerogel foam medium. To break the homogeneity of this medium, thereby imposing stochasticity, we include a number of resolved microscale grains in regular or random distributions. These grains are either TiO$_2$ inclusions in a pure SiO$_2$ background, or vanadium inclusions in a TiO$_2$(SiO$_2$)$_5$ background as prepared for the COAX radiation temperature diagnostic. We find that, while the effect of grains on the bulk speed of the Marshak wave is below the COAX threshold of discrimination, the presence of grains themselves can be conspicuous in COAX spectra even in locations far downstream of the Marshak wavefront. Consequently, we are compelled by this analysis to reinterpret results from past COAX experiments that saw clumping of nominally nanoscale TiO$_2$ dopant grains into microscale plaques.