Phase contrast imaging of irradiated foils through Talbot Lau X ray Deflectometry on OMEGA EP.
Abstract not provided.
Valdivia, M P↗
Engineering topics
Drake, R P
Publications and source records attributed to Drake, R P.
Measurement of high-dynamic range x-ray Thomson scattering spectra for the characterization of nano-plasmas at LCLS
Atomic clusters can serve as ideal model systems for exploring ultrafast (∼100 fs) laser-driven ionization dynamics of dense matter on the nanometer scale. Resonant absorption of optical laser pulses enables heating to temperatures on the order of 1 keV at near solid density conditions. To date, direct probing of transient states of such nano-plasmas was limited to coherent x-ray imaging. Here we present the first measurement of spectrally resolved incoherent x-ray scattering from clusters, enabling measurements of transient temperature, densities, and ionization. Single shot x-ray Thomson scattering signals were recorded at 120 Hz using a crystal spectrometer in combination with a single-photon counting and energy-dispersive pnCCD. A precise pump laser collimation scheme enabled recording near background-free scattering spectra from Ar clusters with an unprecedented dynamic range of more than 3 orders of magnitude. Such measurements are important for understanding collective effects in laser-matter interactions on femtosecond time scales, opening new routes for the development of schemes for their ultrafast control.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Collisionless shock experiments with lasers and observation of Weibel instabilities
Astrophysical collisionless shocks are common in the universe, occurring in supernova remnants, gamma ray bursts, and protostellar jets. They appear in colliding plasma flows when the mean free path for ion-ion collisions is much larger than the system size. It is believed that such shocks could be mediated via the electromagnetic Weibel instability in astrophysical environments without pre-existing magnetic fields. Here, we present laboratory experiments using high-power lasers and investigate the dynamics of high-Mach-number collisionless shock formation in two interpenetrating plasma streams. Our recent proton-probe experiments on Omega show the characteristic filamentary structures of the Weibel instability that are electromagnetic in nature with an inferred magnetization level as high as ∼1% [C. M. Huntington et al., “Observation of magnetic field generation via the weibel instability in interpenetrating plasma flows,” Nat. Phys. 11, 173–176 (2015)]. These results imply that electromagnetic instabilities are significant in the interaction of astrophysical conditions.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Laboratory astrophysical collisionless shock experiments on Omega and NIF
Abstract not provided
42 ENGINEERING↗
Direct observation of electromagnetic Weibel filamentation in counter-streaming plasma flows
Abstract not provided
42 ENGINEERING↗
REVERSE RADIATIVE SHOCK LASER EXPERIMENTS RELEVANT TO ACCRETING STREAM-DISK IMPACT IN INTERACTING BINARIES
We present the first results from high-energy-density laboratory astrophysics experiments that explore the hydrodynamic and radiative properties of a reverse shock relevant to a cataclysmic variable system. A reverse shock is a shock wave that develops when a freely flowing, supersonic plasma is impeded. In our experiments, performed on the Omega Laser Facility, a laser pulse is used to accelerate plasma ejecta into a vacuum. This flow is directed into an Al plate in front of which a shock forms in the rebounding plasma. The plasma flow is moving fast enough that it is shocked to high enough temperatures that radiative cooling affects the shock structure. These are the first experiments to produce a radiative reverse shock wave.
79 ASTRONOMY AND ASTROPHYSICS↗
Imaging x-ray Thomson scattering spectrometer design and demonstration (invited)
In many laboratory astrophysics experiments, intense laser irradiation creates novel material conditions with large, one-dimensional gradients in the temperature, density, and ionization state. X-ray Thomson scattering is a powerful technique for measuring these plasma parameters. However, the scattered signal has previously been measured with little or no spatial resolution, which limits the ability to diagnose inhomogeneous plasmas. We report on the development of a new imaging x-ray Thomson spectrometer (IXTS) for the Omega laser facility. The diffraction of x-rays from a toroidally curved crystal creates high-resolution images that are spatially resolved along a one-dimensional profile while spectrally dispersing the radiation. This focusing geometry allows for high brightness while localizing noise sources and improving the linearity of the dispersion. Preliminary results are presented from a scattering experiment that used the IXTS to measure the temperature profile of a shocked carbon foam.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Experimental observations of turbulent mixing due to Kelvin-Helmholtz instability on the OMEGA Laser Facility
Shear-flow, Kelvin-Helmholtz (KH) turbulent mixing experiments were performed on the OMEGA Laser Facility [Boehly et al., Opt. Commun. 133, 495 (1997)] in which laser-driven shock waves propagated through a low-density plastic foam placed on top of a higher-density plastic foil. The plastic foil was comprised a thin iodine-doped plastic tracer layer bonded on each side to an undoped density-matched polyamide-imide plastic. Behind the shock front, lower-density foam plasma flowed over the higher-density plastic plasma, such that the interface between the foam and plastic was KH unstable. The initial perturbations consisted of pre-imposed, sinusoidal 2D perturbations, and broadband 3D perturbations due to surface roughness at the interface between the plastic and foam. KH instability growth was measured using side-on radiography with a point-projection 5-keV vanadium backlighter. Time-integrated images were captured on D-8 x-ray film. Spatial density profiles of iodine-doped plastic mixed with foam were inferred using x-ray radiographs. The mixing layer ensuing from the KH instability with layer width up to {approx}100 {mu}m was observed at a location {approx}1 mm behind the shock front. The measured mixing layer width was in good agreement with predictions based on a simple self-similar model of KH instability growth using an estimate of the shear velocity obtained from numerical simulations of the experiments.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Three-dimensional modeling and analysis of a high energy density Kelvin-Helmholtz experiment
A recent series of experiments on the OMEGA laser provided the first controlled demonstration of the Kelvin-Helmholtz (KH) instability in a high-energy-density physics context [E. C. Harding et al., Phys. Rev. Lett. 103, 045005, (2009); O. A. Hurricane et al., Phys. Plasmas 16, 056305, (2009)]. We present 3D simulations which resolve previously reported discrepancies between those experiments and the 2D simulation used to design them. Our new simulations reveal a three-dimensional mechanism behind the low density 'bubble' structures which appeared in the experimental x-ray radiographs at late times but were completely absent in the 2D simulations. We also demonstrate that the three-dimensional expansion of the walls of the target is sufficient to explain the 20% overprediction by 2D simulation of the late-time growth of the KH rollups. The implications of these results for the design of future experiments are discussed.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Characterizing counter-streaming interpenetrating plasmas relevant to astrophysical collisionless shocks
A series of Omega experiments have produced and characterized high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks. Single and double CH{sub 2} foils have been irradiated with a laser intensity of {approx} 10{sup 16} W/cm{sup 2}. The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2000 km/s, an electron temperature of {approx} 110 eV, an ion temperature of {approx} 30 eV, and a density of {approx} 10{sup 18} cm{sup -3} were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured single foil plasma conditions were used to calculate the ion skin depth, c/{omega}{sub pi}{approx}0.16 mm, the interaction length, Script-Small-L {sub int}, of {approx} 8 mm, and the Coulomb mean free path, {lambda}{sub mfp}{approx}27mm. With c/{omega}{sub pi} Much-Less-Than Script-Small-L {sub int} Much-Less-Than {lambda}{sub mfp}, we are in a regime where collisionless shock formation is possible.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DESIGN CONSIDERATIONS FOR UNMAGNETIZED COLLISIONLESS-SHOCK MEASUREMENTS IN HOMOLOGOUS FLOWS
The subject of this paper is the design of practical laser experiments that can produce collisionless shocks mediated by the Weibel instability. Such shocks may be important in a wide range of astrophysical systems. Three issues are considered. The first issue is the implications of the fact that such experiments will produce expanding flows that are approximately homologous. As a result, both the velocity and the density of the interpenetrating plasma streams will be time dependent. The second issue is the implications of the linear theory of the Weibel instability. For the experiments, the instability is in a regime where standard simplifications do not apply. It appears feasible but non-trivial to obtain adequate growth. The third issue is collisionality. The need to keep resistive magnetic-field dissipation small enough implies that the plasmas should not be allowed to cool substantially.
79 ASTRONOMY AND ASTROPHYSICS↗
Design of experiments to observe radiation stabilized Rayleigh-Taylor instability growth at an embedded decelerating interface
Using a hohlraum produced thermal x-ray drive at the National Ignition Facility (NIF) to create pressure by material ablation, a shock exceeding 200 Mbar can be driven through a planar, solid-density target and into a lower-density foam material. The shock driven through the foam is strongly radiative, and this radiation significantly alters the dynamics of the system, including those of the Rayleigh-Taylor (RT) fluid instability at the interface between the two materials. We discuss here the design of experiments that can produce such radiative conditions. One will be able to compare the observed growth rates with an extensive body of hydrodynamic experiments performed previously. In this paper, we describe a set of 1D simulations performed to understand the mechanisms of stabilization in a strongly radiative Rayleigh-Taylor unstable system. Simulation results are used to calculate modified analytic RT growth rates which have been proposed in the literature. Calculations predict reduced RT spike growth as a result of increases in density gradient scale length and mass ablation from the unstable interface. This work has direct applicability to the observable features in upcoming NIF experiments.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
CRASH: A BLOCK-ADAPTIVE-MESH CODE FOR RADIATIVE SHOCK HYDRODYNAMICS-IMPLEMENTATION AND VERIFICATION
We describe the Center for Radiative Shock Hydrodynamics (CRASH) code, a block-adaptive-mesh code for multi-material radiation hydrodynamics. The implementation solves the radiation diffusion model with a gray or multi-group method and uses a flux-limited diffusion approximation to recover the free-streaming limit. Electrons and ions are allowed to have different temperatures and we include flux-limited electron heat conduction. The radiation hydrodynamic equations are solved in the Eulerian frame by means of a conservative finite-volume discretization in either one-, two-, or three-dimensional slab geometry or in two-dimensional cylindrical symmetry. An operator-split method is used to solve these equations in three substeps: (1) an explicit step of a shock-capturing hydrodynamic solver; (2) a linear advection of the radiation in frequency-logarithm space; and (3) an implicit solution of the stiff radiation diffusion, heat conduction, and energy exchange. We present a suite of verification test problems to demonstrate the accuracy and performance of the algorithms. The applications are for astrophysics and laboratory astrophysics. The CRASH code is an extension of the Block-Adaptive Tree Solarwind Roe Upwind Scheme (BATS-R-US) code with a new radiation transfer and heat conduction library and equation-of-state and multi-group opacity solvers. Both CRASH and BATS-R-US are part of the publicly available Space Weather Modeling Framework.
79 ASTRONOMY AND ASTROPHYSICS↗
Oblique radiative shocks, including their interactions with nonradiative polytropic shocks
A theory of shocks dominated by radiation energy flux in optically mixed thin-upstream thick-downstream systems, in which the temperature immediately ahead and some short distance behind the shock front are equilibrated by radiation transport, is presented. This theory is applied to determine properties of the normal and oblique radiative shock, followed by applications to interactions when radiative and polytropic shocks are present in the same system. Comparison with experimental data is presented.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Current Filamentation Instability in Laser Wakefield Accelerators
Experiments using an electron beam produced by laser-wakefield acceleration have shown that varying the overall beam-plasma interaction length results in current filamentation at lengths that exceed the laser depletion length in the plasma. Three-dimensional simulations show this to be a combination of hosing, beam erosion, and filamentation of the decelerated beam. This work suggests the ability to perform scaled experiments of astrophysical instabilities. Additionally, understanding the processes involved with electron beam propagation is essential to the development of wakefield accelerator applications.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Electronic measurement of microchannel plate pulse height distributions
Microchannel plates are a central component of the x-ray framing cameras used as analog imagers in many plasma experiment diagnostic systems. The microchannel plate serves as an amplifying element, increasing the electronic signal from incident radiation by factors of 10{sup 3}-10{sup 5}, with a broad pulse-height distribution. Seeking to optimize the photon-to-electron conversion efficiency and noise distribution of x-ray cameras, we will characterize the pulse-height distribution of the electron output from a single microchannel plate. Replacing the framing camera's phosphor-coated fiber optic screen with a charge-collection plate and coupling to a low-noise multichannel analyzer, we quantified the distribution in the total charge generated per photon event. The electronically measured pulse height distribution is used to estimate the signal-to-noise ratio of radiographic images from framing cameras.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Development of a short duration backlit pinhole for radiography on the National Ignition Facility
Experiments on the National Ignition Facility (NIF) will require bright, short duration, near-monochromatic x-ray backlighters for radiographic diagnosis of many high-energy density systems. This paper details a vanadium pinhole backlighter producing (1.8{+-}0.5)x10{sup 15} x-ray photons into 4{pi} sr near the vanadium He-like characteristic x-ray energy of 5.18 keV. The x-ray yield was quantified from a set of Ross filters imaged to a calibrated image plate, with the Dante diagnostic used to confirm the quasimonochromatic nature of the spectrum produced. Additionally, an x-ray film image shows a source-limited image resolution of 26 {mu}m from a 20 {mu}m diameter pinhole.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Laser driven supersonic flow over a compressible foam surface on the Nike laser
A laser driven millimeter-scale target was used to generate a supersonic shear layer in an attempt to create a Kelvin-Helmholtz (KH) unstable interface in a high-energy-density (HED) plasma. The KH instability is a fundamental fluid instability that remains unexplored in HED plasmas, which are relevant to the inertial confinement fusion and astrophysical environments. In the experiment presented here the Nike laser [S. P. Obenschain et al., Phys. Plasmas 3, 2098 (1996)] was used to create and drive Al plasma over a rippled foam surface. In response to the supersonic Al flow (Mach=2.6+-1.1) shocks should form in the Al flow near the perturbations. The experimental data were used to infer the existence and location of these shocks. In addition, the interface perturbations show growth that has possible contributions from both KH and Richtmyer-Meshkov instabilities. Since compressible shear layers exhibit smaller growth, it is important to use the KH growth rate derived from the compressible dispersion relation.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗