E. C. Harding

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.

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...

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.

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.

Electronic measurement of microchannel plate pulse height distributions.

MicroChannel plates are a central component to the x-ray framing cameras used in many plasma experiment diagnostic systems. The microchannel plate serves as an amplifying element, increasing the electronic signal from incident radiation by a factor of

Performance of Au transmission photocathode on a microchannel plate detector

10^3-10^5

Approaches to turbulence in high-energy-density experiments

, 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 cameras phosphor-coated fiber optic screen with a charge-collection plate and coupling to a low-noise multichannel analyzer, we will quantify the total charge generated per photon event over a range of x-ray energies and incident fluxes. The electronically-measured pulse height distribution will be compared to the same data collected via a purely-optical method, as described previously1

Electronic measurement of microchannel plate pulse height distributions

X-ray framing cameras, employing microchannel plates (MCPs) for detection and signal amplification, play a key role in research in high-energy-density physics. These instruments convert radiographic x-rays into electrons produced by plasma during such experiments into electrons that are amplified in the channels and then detected by a phosphor material. The separation of detection from signal amplification offers potential improvements in sensitivity and noise properties. We have implemented a suspended Au transmission photocathode (160 A thick) on a MCP and are evaluating it using a 1.5 keV Al K alpha x-ray source. We find an approximately twofold increase in the ratio of detected events to incident photons when the photocathode-to-MCP voltage difference is sufficiently large. Our calculations indicate that this increase is probably caused by a combination of signal produced by the photocathode and an increase in the efficiency of detection of x-rays that reach the MCP surface through modification of the local electric field.

Understanding the implications of the data from recent high-energy-density Kelvin-Helmholtz shear layer experiments

This paper provides a discussion of the properties of hydrodynamic systems at high energy density, discusses the methods of doing hydrodynamic experiments and discusses studies to date of the three primary instabilities?Richtmyer?Meshkov (RM), Rayleigh?Taylor (RT) and Kelvin?Helmholtz (KH). The first two of these have been systematically observed, but have not yet produced a system with a clear transition to turbulence. The KH instability remains to be systematically observed in its pure form, although some related effects such as spike tip broadening have been seen. However, the KH effects seen in some simulations of RT systems and supersonic jets have not been seen to date in experiments. We note that the time-dependent condition for turbulence of Zhou et al (2003 Phys. Plasmas 10 1883) is roughly equivalent to the Reynolds-number threshold of Dimotakis in that eddies will dissipate by turbulence in about one eddy-turnover time. We suggest that a plausible explanation of the absence of KH in several experimental systems may be that finite velocity gradients have quenched the instability. Finally, we argue that despite the smearing of the shear layer caused by viscous diffusion, KH instabilities have the potential to contribute to the generation of fluctuations at all scales, but only if the local shear layers are initially formed with a sufficiently steep velocity gradient.

Subsonic and supersonic shear flows in laser driven high-energy-density plasmas

MicroChannel plates are a central component to the x-ray framing cameras used in many plasma experiment diagnostic systems. The microchannel plate serves as an amplifying element, increasing the electronic signal from incident radiation by a factor of