Session PO8: Laboratory Astrophysics

Same-Shot X-Ray Thomson Scattering and Streaked Imaging of Radiative Shock Experiments at Omega

A shock system undergoing radiative cooling is able to form a collapsed layer behind the shock that is significantly denser than the simple strong shock limit would predict. Using a Be pusher to drive a shock in excess of 100 km/s in a xenon-filled shock tube creates such a dense layer, which is preceded down the tube by a radiation-heated precursor region and followed by a downstream layer of expanding Be. In experiments on the OMEGA Laser, streaked x-ray radiography and x-ray Thomson scattering diagnostics were employed. We detail how this diagnostic combination allows for several measurements of the different regions of this system. For each region, x-ray Thomson scattering may provide information on electron temperature, while streaked radiography yields shock velocity and acceleration. These measurements complement previous radiative shock experiments and suggest future directions.   

Measurement of Emission from a Radiative Shock

Radiative shocks are shock waves whose structure has been altered by radiation transport from the shock-heated matter. Such shocks are present in numerous astrophysical systems, including supernova remnants, supernovae, and accretion disks. Recent experiments have used the Omega laser to study radiative shock systems that are optically thin upstream and optically thick downstream. In these systems, a radiative precursor and high density cooling layer are formed in response to radiation lost in the upstream region. A thin slab of low-Z material is driven into a 1.1 atm. cylinder of high-Z gas at speeds $>$ 100 km/s, producing strong radiative effects.. Measurements of radiative emission from the shocked region and the precursor region have been made using a streaked optical pyrometer. From these measurements, the temperature of the system can be inferred. Details of the experiment and results will be discussed. This work is funded by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, by the National Laser User Facility Program in NNSA-DS and by the Predictive Sciences Academic Alliances Program in NNSA-ASC.~ The corresponding grant numbers are DE-FG52-09NA29548, DE-FG52-09NA29034, and DE-FC52-08NA28616.   

High energy density radiative blast wave experiments in clustered gas targets

Using the efficient absorption of laser energy by clustered gases allows the study of high energy density systems at low average density with table-top scale lasers. Thus generation of radiative shocks at moderate drive energies in high Z materials can be achieved for the purpose of laboratory astrophysics. Using the Vulcan laser we have performed radiative shock experiments. We observe radiative shell thinning in Ar at high shock velocities, increasing the shocks susceptibility to instabilities. Utilising streaked Schlieren measurements, we measure shock velocity oscillations in Kr for the first time, indicative of the thermal cooling instability [1]. We also report on advanced diagnostics for characterisation of blast wave propagation. These include temporally resolved temperature measurement, proton radiography and the use of a second, perpendicular blast wave to probe the primary shock ambient medium. \\[4pt] [1] M. Hohenberger et al., \textit{submitted to Phys. Rev. Lett.}   

High-power laser experiments to study collisionless shock in counter-streaming plasmas

In this paper we investigate laboratory experiments to study collisionless shock generation in counter-streaming plasmas using Gekko XII HIPER laser system (352 nm (3$\omega )$, 500 ps, $<$ 1015 W/cm2) at ILE, Osaka University. 60 $\mu $m thick CH double-plane targets with the separation of 4.5 mm were used. Beams were irradiated on the 1st CH plane, and an ablation plasma was formed. Plasma from the 2nd CH was created by radiation and/or plasma from the 1st CH. The plasmas and shocks were diagnosed transverse to the main laser propagation direction using shadowgraphy, interferometrym, and SOP, etc. Counter-streaming plasmas were produced, and shock waves were observed. A particle-in-cell simulation predicted generation of an electrostatic shock. We also investigate an experimental proposal to demonstrate the formation of collisionless shocks through the self-generated magnetic fields due to the beam-Weibel instability.   

Magnetized spherical couette flow

Spherical Couette flow, i.e. the flow between differentially rotating concentric spheres, has attracted a revival of interest in recent years. It has been shown that despite the simplicity of the problem,the flow undergoes many bifurcations depending on the value of the Reynolds number and of the aspect ratio. When one considers an electrically conducting fluid and imposes an external magnetic field, magnetohydrodynamic effects can significantly change the purely hydrodynamical problem and lead to new instabilities. We will present numerical simulations of magnetic spherical Couette flow, comparing the effects of different magnetic boundary conditions. We will also show that for some parameters, this problem yields interesting non-linear dynamics, including interaction with dynamo action. Finally, this work can be investigated in the framework of the magnetorotational instability. In particular, we will present a comparison between our numerical simulations and the experimental results obtained by Sisan et al, who used a similar configuration and observed non-axisymmetric MHD instabilities.   

Elasto-Rotational Instability in Couette-Taylor Flows of Visco-Elastic Fluids

We discuss the so-called ``elasto-rotational'' instability of visco-elastic polymer fluids in a Couette-Taylor geometry. We study under what conditions (viscosity and relaxation time of the polymer fluid, velocity profile of the Couette-Taylor flow) this instability is analogous to the magnetorotational instability that plays a fundamental role in astrophysical accretion disks.   

Analog of Astrophysical Magnetorotational Instability in Couette-Taylor Experiments Using Polymer Fluids

We report the experimental observations and numerical simulations of an instability in a Couette-Taylor flow of a polymer fluid in a narrow gap between two rotating concentric cylinders with a Keplerian-like velocity profile, where the angular velocity decreases radially outward while the specific angular momentum increases radially outward. Under these conditions, the inertial Rayleigh instability and the purely elastic instability are not possible. It is proposed that this observed instability is analogous to the magnetorotational instability which plays a fundamental role in astrophysical Keplerian accretion disks.   

Radiation Spectral Synthesis during the Relativistic Filamentation Instability

Radiation from high-energy density plasmas in astro observations and lab experiments can be used to diagnose complex field topologies and non-thermal particle distributions. Simulating spectral emission and spectral evolution numerically in various relativistic shock scenarios is then the only viable method to determine the detailed physical origin of the emitted spectra. We present synthetic radiation spectra obtained from the development of the filamentation (streaming) instability in a periodic box, which is relevant for collisionless shocks and laser plasma experiments. They were obtained using an \textit{in situ} diagnostics collection method, based on detailed particle-in-cell modeling of collisionless plasmas. The synthetic spectra are compared with those predicted by a semi-analytical model for jitter radiation from the filamentation instability. The spectra exhibit dependence on the plasma composition, the viewing angle wrt the plasma anisotropy direction and are variable in time. The results also illustrate that considerable care should be taken when using lower-dimensional (2D) models of radiation emission to obtain information about magnetic turbulence on sub-Larmor scales.   

Verification of FLASH Implementations of the 3T Approximation for Plasma Hydrodynamics

FLASH is a highly capable, fully modular, professionally managed Eulerian code with a wide user base. We are adding capabilities to FLASH to make it a n open code for the academic HEDP community. A key need is to provide the ability to model single-fluid plasmas as consisting of three coupled compone nts -- electrons, ions, and radiation in the diffusion approximation --- that can be described by 3 temperatures (3T). We discuss the challenges of im plementing the 3T approximation in an Eulerian code. We report the results of verification tests of the solvers we have implemented in FLASH for elect ron, ion, and radiation thermal diffusion, including flux limiting. We also report the results of a new hydrodynamics plus thermal conduction verifica tion test, and various 2T and 3T Su-Olson-type verification tests.   

Making FLASH an Open Code for the Academic High-Energy Density Physics Community

High-energy density physics (HEDP) is an active and growing field of research. DOE has recently decided to make FLASH a code for the academic HEDP community. FLASH is a modular and extensible compressible spatially adaptive hydrodynamics code that incorporates capabilities for a broad range of physical processes, performs well on a wide range of existing advanced computer architectures, and has a broad user base. A rigorous software maintenance process allows the code to operate simultaneously in production and development modes. We summarize the work we are doing to add HEDP capabilities to FLASH. We are adding (1) Spitzer conductivity, (2) super time-stepping to handle the disparity between diffusion and advection time scales, and (3) a description of electrons, ions, and radiation (in the diffusion approximation) by 3 temperatures (3T) to both the hydrodynamics and the MHD solvers. We are also adding (4) ray tracing, (5) laser energy deposition, and (6) a multi-species equation of state incorporating ionization to the hydrodynamics solver; and (7) Hall MHD, and (8) the Biermann battery term to the MHD solver.   

An Eddy-Based Model and Measurements of the Ekman-induced Turbulent Transport of Momentum and Magnetic Flux in the Liquid Sodium a?-Dynamo Experiment:

The two coherent motions, rotational shear, the ``$w$-effect,'' and pulsed unidirectional plume-driven helicity, the ``$a$-effect,'' of the Liquid Sodium $aw$-Dynamo Experiment at NMIMT depends upon the two orthogonal instability-constrained, low turbulent flows. The stability of the $w$-effect is achieved by stable Couette flow, $dw/dr >0$, (that of the ``$a$-effect by the transient nature of the plumes.) The effective ``$w$-gain'' of the Couette shear flow, (experimentally measured $\times 8$) is limited by both the magnetic diffusivity of liquid sodium, $h$~750 cm$^2/s$, $Rm \sim 120$, and the diffusivity of the turbulence induced by the Ekman flow. We measure the torque induced by the Ekman flow, thickness, $h\sim r Re^{-1/2}$, $Re\sim 10^7$ and infer the velocity distribution from pressure measurements vs radius. A comparison is then made with an eddy-based theory of turbulence, 1) a laminar sub-layer, 2) log-law of the walls eddy size distribution, and 3) an eddy size truncated at the scale of the Couette shear stability. With this eddy size and stress distribution a turbulent velocity distribution is compared to the measured pressure distribution, and the $w$-gain. Supported by the DOE.   

Experimental study of a supersonic plasma jet interacting with an ambient gas

The dynamics of the interaction of a supersonic, radiatively cooled plasma jet with an ambient gas are presented. The experimental setup consists of a radial foil, a $\mu $m-thick aluminum disc held between two concentric electrodes and subjected to a 1.4 MA, 250 ns current pulse from the MAGPIE generator. The plasma flow, with typical velocities of $\sim $70-90 km/s, is produced by the JxB force acting on the plasma ablated from the foil. A jet is formed from the convergence of this ablated plasma on the axis of the system. The jet interacts with an argon ambient (N$\sim $10$^{16-17}$ cm$^{-3})$ from a supersonic gas nozzle (Mach$\sim $9). The formation of several shock structures from the interaction of the jet with the gas will be presented and discussed.   

Laboratory Measurements of Astrophysical Magnetic Fields

It has been proposed that high Mach number collisionless shocks propagating in an initially unmagnetized plasma play a major role in the magnetization of large scale structures in the Universe. A detailed study of the experimental configuration necessary to scale such environments down to laboratory dimensions will be presented. We will show initial results from preliminary experiments conducted at the Phoenix laser (UCLA) and the LULI laser (Ecole Polytechnique) where collisionless shocks are generated by the expansion of exploding foils driven by energetic laser beams. The time evolution of the magnetic field is probed with induction coils placed at 10 cm from the laser focus. We will discuss various mechanisms of magnetic field generation and compare them with the experimental results.   

Transmission spectroscopy and atomic kinetics of neon photoionized plasma experiments at Z

We discuss a series of experiments performed at the Z facility in which photoionized plasmas were produced by driving a neon-filled gas cell with the intense x-ray flux emitted at the collapse of a z-pinch. The broad-band radiation flux from the z-pinch is used to both create the neon photoionised plasma and provide a source of backlighting photons to study the atomic kinetics through K-shell transmission spectroscopy. The plasma is contained in cm-scale gas cell and the filling pressure is carefully monitored in situ all the way to shot time since it is the particle number density diagnostic of the plasma. Time-integrated and gated transmission spectra are recorded with a TREX spectrometer equipped with two KAP elliptically-bent crystals and a set of slits to record up to six spatially-resolved spectra per crystal in the same shot. The transmission data shows line absorption transitions in several ionization stages of neon. Detailed modeling calculations are used to interpret the data.   

Progress in iron transmission measurements relevant to the solar convection/radiation boundary

Iron plasma opacity influences the internal structure of the sun. However, opacity models have never been experimentally tested at stellar interior conditions. Initial experiments at the Sandia Z facility reached temperatures high enough to investigate the iron charge states that exist near the convection/radiation zone (CZ) boundary. In these experiments the density was an order of magnitude lower than at the CZ boundary, preventing studies of important effects such as line broadening. New experiments have reached higher densities and temperatures. Progress to solidify these results and use them to examine opacity models will be described.++Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.