Session CO5: HEDP Laboratory Astrophysics

Experimental results of astrophysical collisionless shock experiments from NIF

We discuss our laboratory experiments using the Omega and NIF lasers to investigate the dynamics of high Mach number collisionless shock formation in two interpenetrating plasma streams. It is believed that in astrophysical environments such shocks are the sites where seed magnetic fields are generated on a cosmologically fast timescale via the Weibel instability. Particle-in-cell (PIC) numerical simulations$^{\mathrm{\thinspace }}$generate magnetic fields whose magnitude and scale are consistent with this concept. We will present recent experimental results [1,2] as well as simulations and theoretical interpretations of these observations. The NIF experiments were able to observe the counter-streaming flow interactions through the transition from collisional to collisionless regimes. The latest proton radiography results will be presented. [1] C. M. Huntington, et al., Nature Physics, 11, 173 (2015). [2] J. S. Ross et al., Phys. Rev. Lett., 118, 185003 (2017).   

Abstract Withdrawn

Radiative opacity of CH plasmas with doped Si and Ge is calculated in the framework of detailed-level-accounting (DLA) model. A complete atomic database including energy level, oscillator strength and photoionization cross section is established for H, C, Si and Ge. For ground and lowly excited states, large-scale multi-configuration Dirac Fock (MCDF) calculations are implemented with effects of configuration interaction included. For highly excited ones, calculations of a single configuration approximation are carried out. Radiative opacity of CH plasmas with doped Si and Ge at variable conditions are obtained and compared with the results of pure CH plasmas.   

First Iron Opacity Experiments on the National Ignition Facility

Iron opacity experiments on the Sandia National Laboratories Z machine have shown up to factors of two discrepancies between theory and experiment. To help resolve these discrepancies an experimental platform for doing comparable opacity experiments is being developed on the National Ignition Facility (NIF). Initial iron data has been taken at a temperature of \textasciitilde 150 eV and an electron density of \textasciitilde 6x10$^{\mathrm{21}}$/cm$^{\mathrm{3}}$, but higher temperatures and densities will be required to address the discrepancies that have been observed in the Z experiments. The plans to go to higher temperatures and densities and how to deal with current issues with instrumental backgrounds will be discussed.   

Simulations of the National Ignition Facility Opacity Sample

A platform to study the opacity of high temperature materials at the National Ignition Facility has been developed[1]. Experiments to study the opacity of materials relevant to inertial confinement fusion and stellar astrophysics are being conducted. The initial NIF experiments are focused on reaching the same plasma conditions (T \textgreater 150 eV and Ne $\ge $ 7 x 10$^{\mathrm{21}}$ cm$^{\mathrm{-3}})$, for iron, as those achieved in previous experiments at Sandia National Laboratories' (SNL) Z-facility which have shown discrepancies between opacity theory and experiment. We developed a methodology, using 1D HYDRA simulations, to study the effects of tamper thickness on the conditions of iron-magnesium samples. We heat the sample using an x-ray drive from 2D LASNEX hohlraum simulations. We also use this methodology to predict sample uniformity and expansion for comparison with experimental data. [1] T. S. Perry \textit{et al.}, HEDP 23, 223 (2017)   

Initial experiments to understand the interaction of stellar radiation with molecular clouds

Enhanced star formation triggered by local O and B type stars is an astrophysical problem of interest. O and B type stars are massive, hot stars that emit an enormous amount of radiation. This radiation acts to either compress or blow apart gas clumps in the interstellar media. For example, in the optically thick limit, when the radiation in the gas clump has a short mean free path, radiation is absorbed near the clump edge and compresses the clump. In the optically thin limit, when the mean free path is long, the radiation is absorbed throughout, acting to heat the clump. This heating explodes the gas clump. Careful selection of parameters, such as foam density or source temperature, allow the experimental platform to access different hydrodynamic regimes. 2D CRASH simulations guide our parameter selection. A stellar radiation source is mimicked by a laser-irradiated, thin, gold foil, providing a source of thermal x-rays around 100 eV. The gas clump is mimicked by low-density CRF foam. We plan to show the preliminary experimental results of this platform in the optically thick limit, from experiments scheduled in August. This work is funded by the~U.S. DOE, through the~NNSA-DS and SC-OFES Joint Program in HEDPLP, grant No.~DE-NA0002956, and the NLUF Program, grant No.~DE-NA0002719,~and~through LLE, University of Rochester~by the NNSA/OICF under Cooperative Agreement No.~DE-NA0001944. This work is funded by the Lawrence Livermore National Laboratory under subcontract B614207.   

A Benchmark Experiment for Photoionized Plasma Emission from Accretion-Powered X-ray Sources

Accretion-powered emission from X-ray binaries or black-hole accretion in Active Galactic Nuclei is a powerful diagnostic for their behavior and structure. Interpretation of x-ray emission from these objects requires a spectral synthesis model for \textit{photoionized} plasma. Models must predict the photoionized charge state distribution, the photon emission processes, and the radiation transport influence on the observed emission. At the Z facility, we have measured simultaneously emission and absorption from a photoionized silicon plasma suitable to benchmark photoionization and spectrum formation models with \textpm 5{\%} reproducibility and E/dE \textgreater 2500 spectral resolution. Plasma density, temperature, and charge state distribution are determined with absorption spectroscopy. Self-emission measured at adjustable column densities tests radiation transport effects. Observation of 14 transitions in He-like silicon will help understand population mechanisms in a photoionized plasma. First observation of radiative recombination continuum in a photoionized plasma will be presented. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.   

Atomic kinetics of a neon photoionized plasma experiment at Z

We discuss an experimental effort to study the atomic kinetics in neon photoionized plasmas via K-shell line absorption spectroscopy. The experiment employs the intense x-ray flux emitted at the collapse of a Z-pinch to heat and backlight a photoionized plasma contained within a cm-scale gas cell placed at various distances from the Z-pinch and filled with neon gas pressures in the range from 3.5 to 120 Torr. The experimental platform affords an order of magnitude range in the ionization parameter characterizing the photoionized plasma from about 5 to 80 erg*cm/s. Thus, the experiment allows for the study of trends in ionization distribution as a function of the ionization parameter. An x-ray crystal spectrometer capable of collecting both time-integrated and time-gated data is used to collect absorption spectra. The spectra show line absorption by several ionization stages of neon, including Be-, Li-, He-, and H-like ions. Analysis of these spectra yields ion areal-densities and charge state distributions, which can be compared with results from atomic kinetics codes. In addition, the electron temperature is extracted from level population ratios of nearby energy levels in Li- and Be-like ions, which can be used to test heating models of photoionized plasmas.   

X-ray heating of laboratory photoionized plasmas at Z

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 layer coated with plastic. Both samples were driven by the broadband 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 charge state distribution, and the electron temperature was extracted from a Li-like ion level population ratio. To interpret the temperature measurement, we performed Boltzmann kinetics and radiation-hydrodynamic simulations. We found that non-equilibrium atomic physics and the coupling of the radiation flux to the level population kinetics play a critical role in modeling the x-ray heating of photoionized plasmas. In spite of being driven by similar x-ray drives, differences of ionization and charged state distributions in the neon and silicon plasmas are reflected in the plasma heating and observed temperatures.   

First Successes and Modifications of the NIF Opacity Spectrometer

The NIF Opacity Spectrometer (OpSpec) began returning X-ray spectra on its first NIF shot in September 2016. In May 2017, OpSpec recorded the first X-ray transmission data for iron-magnesium plasmas on NIF, at ``Anchor 1'' sample conditions (150 eV and 7E21 e-/cc). OpSpec diffracts X-rays in the 540-2100 eV range off a KAP or RbAP crystal onto either image plate or X-ray film. Modifications to further improve OpSpec's performance are underway, with the largest improvements expected in resolving power (E/dE up to 1000) and reduction of background levels. Implementation is planned for NIF shots in August and December 2017. This presentation will discuss the OpSpec data and design improvements, and also future goals.   

Helium at white dwarf photosphere conditions: experimental line widths and shifts

We present preliminary results of an experimental study exploring He line shapes, widths, and shifts photospheric conditions of white dwarf stars. These data were collected as part of the Z Astrophysical Plasma Properties (ZAPP) collaboration on Sandia National Laboratories' Z-machine, the largest x-ray source on earth. Our helium results could have many applications ranging from validating current DB white dwarf atmospheric models to providing accurate He pressure shifts at varying temperatures and density. In a much broader context, these helium data can also be used to guide theoretical developments in new continuum lowering models for two electron atoms.   

Study of photoionization of supersonic gas jets at the pulsed power generator

Supersonic, nitrogen, neon and argon, gas jets photoionized by a broadband x-ray flux were studied at the University of Nevada, Reno. The x-ray flux was produced by the collapse of a wire-array z-pinch implosion on the 1MA Zebra pulsed power accelerator, with photons mostly under 1keV and photon-energy integrated energy between 12kJ -16kJ. A Mach-Zehnder interferometer at 266 nm was set up to extract the atom number density profile of the jet before the Zebra shot. Air-wedge interferometers, at 266 and 532 nm, were used to determine the electron number density of the plasma during the Zebra shot. The ratio of electron to atom number densities provide the average ionization state of the plasma. A program has been developed to automate the extraction of phase shift maps from both types of interferometers. Preliminary results from the experiment are promising and show that a photoionized plasma has been created in the gas jet, thus demonstrating a new experimental platform to study photoionized plasmas in the laboratory.   

Simulation and Modeling of Magnetized Jet Creation using a Hollow Ring of Laser Beams

Using 20 OMEGA beams to form a ring pattern to irradiate a flat plastic target, we have created strongly magnetized, highly collimated jets. The density, temperature, flow velocity and magnetic field of the supersonic outflows were diagnosed using Thomson scattering, proton radiography, and x-ray imaging. We present 3D FLASH full-physics magneto-hydrodynamics simulations of the experiments, which are in good agreement with the experimental data. These results demonstrate that our experimental configuration of a hollow ring of laser beams can become a versatile new platform to study magnetized jets in the context of laboratory astrophysics.   

Observing Magnetized Shocks Using the OMEGA Laser

Results from a campaign to generate and study magnetized bow shocks using the OMEGA laser are presented in which optical imaging Thomson scattering and proton radiography diagnostics were used to make measurements of magnetized shocks in a sufficiently low $\beta_{ram}$ regime. The system consisted of a slow, low-density plasma flow impinging on the azimuthal magnetic field produced by a current-carrying wire. Data collected at multiple times captured dynamical features of shock formation for two levels of the current in the wire. The proton images show regions of magnetic compression, and sharp increases in density and temperature are observed by the Thomson scattering diagnostic, all evidence of shock formation. Combining measurements from both diagnostics, some shock characteristics can be determined. This work is funded by the U.S. Department of Energy, through the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-NA0002956, and the National Laser User Facility Program and William Marsh Rice University, grant number, R19071, and through the Laboratory for Laser Energetics, University of Rochester by the NNSA/OICF under Cooperative Agreement No. DE-NA0001944.