C. C. Kuranz

Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows

Astrophysical processes are often driven by collisionless plasma shock waves. The Weibel instability, a possible mechanism for developing such shocks, has now been generated in a laboratory set-up with laser-generated plasmas.

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 CH2 foils have been irradiated with a laser intensity of ∼ 1016 W/cm2. 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 ∼ 110 eV, an ion temperature of ∼ 30 eV, and a density of ∼ 1018 cm−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/ωpi∼0.16 mm, the interaction length, lint, of ∼ 8 mm, and the Coulomb mean free path, λmfp∼27mm. With c/ωpi≪lint≪λmfp, we are in a regime where collisionless shock formation is possible.

TWO-DIMENSIONAL BLAST-WAVE-DRIVEN RAYLEIGH-TAYLOR INSTABILITY: EXPERIMENT AND SIMULATION

This paper shows results from experiments diagnosing the development of the Rayleigh-Taylor instability with two-dimensional initial conditions at an embedded, decelerating interface. Experiments are performed at the Omega Laser and use ~5 kJ of energy to create a planar blast wave in a dense, plastic layer that is followed by a lower density foam layer. The single-mode interface has a wavelength of 50 μm and amplitude of 2.5 μm. Some targets are supplemented with additional modes. The interface is shocked then decelerated by the foam layer. This initially produces the Richtmyer-Meshkov instability followed and then dominated by Rayleigh-Taylor growth that quickly evolves into the nonlinear regime. The experimental conditions are scaled to be hydrodynamically similar to SN1987A in order to study the instabilities that are believed to occur at the He/H interface during the blast-wave-driven explosion phase of the star. Simulations of the experiment were performed using the FLASH hydrodynamics code.

Nonlinear mixing behavior of the three-dimensional Rayleigh–Taylor instability at a decelerating interface

Results are reported from the first experiments to explore the evolution of the Rayleigh–Taylor (RT) instability from intentionally three-dimensional (3D) initial conditions at an embedded, decelerating interface in a high-Reynolds-number flow. The experiments used ∼5 kJ of laser energy to produce a blast wave in polyimide and/or brominated plastic having an initial pressure of ∼50 Mbars. This blast wave shocked and then decelerated the perturbed interface between the first material and lower-density C foam. This caused the formation of a decelerating interface with an Atwood number ∼2/3, producing a long-term positive growth rate for the RT instability. The initial perturbations were a 3D perturbation in an “egg-crate” pattern with feature spacings of 71 μm in two orthogonal directions and peak-to-valley amplitudes of 5 μm. The resulting RT spikes appear to overtake the shock waves, moving at a large fraction of the predeceleration, “free-fall” velocity. This result was unanticipated by prior simulations...

Dual, orthogonal, backlit pinhole radiography in OMEGA experiments

Backlit pinhole radiography used with ungated film as a detector creates x-ray radiographs with increased resolution and contrast. Current hydrodynamics experiments on the OMEGA Laser use a three-dimensional sinusoidal pattern as a seed perturbation for the study of instabilities. The structure of this perturbation makes it highly desirable to obtain two simultaneous orthogonal backlighting views. We accomplished this using two backlit pinholes each mounted 12mm from the target. The pinholes, of varying size and shape, were centered on 5mm square foils of 50μm thick Ta. The backlighting is by K-alpha emission from a 500μm square Ti or Sc foil mounted 500μm from the Ta on a plastic substrate. Four laser beams overfill the metal foil, so that the expanding plastic provides radial tamping of the expanding metal plasma. The resulting x-rays pass through the target onto (ungated) direct exposure film (DEF). Interference between the two views is reduced by using a nose cone in front of the DEF, typically with a...

A high energy density shock driven Kelvin-Helmholtz shear layer experiment

Radiographic data from a novel and highly successful high energy density Kelvin–Helmholtz (KH) instability experiment is presented along with synapses of the theory and simulation behind the target design. Data on instability growth are compared to predictions from simulation and theory. The key role played by baroclinic vorticity production in the functioning of the target and the key design parameters are also discussed. The data show the complete evolution of large distinct KH eddies, from formation to turbulent break-up. Unexpectedly, low density bubbles comparable to the vortex size are observed forming in the free-stream region above each vortex at late time. These bubbles have the appearance of localized shocks, possibly supporting a theoretical fluid dynamics conjecture about the existence of supersonic bubbles over the vortical structure [transonic convective Mach numbers, D. Papamoschou and A. Roshko, J. Fluid Mech. 197, 453 (1988)] that support localized shocks (shocklets) not extending into th...

Transition to turbulence and effect of initial conditions on three-dimensional compressible mixing in planar blast-wave-driven systems

Perturbations on an interface driven by a strong blast wave grow in time due to a combination of Rayleigh–Taylor, Richtmyer–Meshkov, and decompression effects. In this paper, results from three-dimensional (3D) numerical simulations of such a system under drive conditions to be attainable on the National Ignition Facility [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] are presented. Using the multiphysics, adaptive mesh refinement, higher order Godunov Eulerian hydrocode, Raptor [L. H. Howell and J. A. Greenough, J. Comput. Phys. 184, 53 (2003)], the late nonlinear instability evolution, including transition to turbulence, is considered for various multimode perturbation spectra. The 3D post-transition state differs from the 2D result, but the process of transition proceeds similarly in both 2D and 3D. The turbulent mixing transition results in a reduction in the growth rate of the mixing layer relative to its pretransition value and, in the case of the bubble front, relative to the 2D result. The post...

Wall shocks in high-energy-density shock tube experiments

The radiative precursor of a sufficiently fast shock has been observed to drive the vaporization of shock tube material ahead of the shock. The resulting expansion drives a converging blast wave into the gas volume of the tube. The effects of this wall shock may be observed and correlated with primary shock parameters. We demonstrate this process in experiments performed on the Omega Laser Facility, launching shocks propagating through xenon with speeds above 100 km/s driven by ablation pressures of approximately 50 Mbars. Wall shocks in laser experiments, in which the principal shock waves themselves should not be radiative, are also reported—in which the wall shocks have been launched by some other early energy source.

Three-dimensional blast-wave-driven Rayleigh-Taylor instability and the effects of long-wavelength modes

This paper describes experiments exploring the three-dimensional (3D) Rayleigh–Taylor instability at a blast-wave-driven interface. This experiment is well scaled to the He/H interface during the explosion phase of SN1987A. In the experiments,  ∼5 kJ of energy from the Omega laser was used to create a planar blast wave in a plastic disk, which is accelerated into a lower-density foam. These circumstances induce the Richtmyer–Meshkov instability and, after the shock passes the interface, the system quickly becomes dominated by the Rayleigh–Taylor instability. The plastic disk has an intentional pattern machined at the plastic/foam interface. This perturbation is 3D with a basic structure of two orthogonal sine waves with a wavelength of 71 μm and an amplitude of 2.5 μm. Additional long-wavelength modes with a wavelength of either 212 or 424 μm are added onto the single-mode pattern. The addition of the long-wavelength modes was motivated by the results of previous experiments where material penetrated unex...

Prediction and Computer Model Calibration Using Outputs From Multifidelity Simulators

Computer simulators are widely used to describe and explore physical processes. In some cases, several simulators are available, each with a different degree of fidelity, for this task. In this work, we combine field observations and model runs from deterministic multifidelity computer simulators to build a predictive model for the real process. The resulting model can be used to perform sensitivity analysis for the system, solve inverse problems, and make predictions. Our approach is Bayesian and is illustrated through a simple example, as well as a real application in predictive science at the Center for Radiative Shock Hydrodynamics at the University of Michigan. The Matlab code that is used for the analyses is available from the online supplementary materials.