Observation and analysis of emergent coherent structures in a high-energy-density shock-driven planar mixing layer experiment

Abstract

Coherent emergent structures have been observed in a high-energy-density supersonic mixing layer experiment. A millimeter-scale shock tube uses lasers to drive Mbar shocks into the tube volume. The shocks are driven into initially solid foam (60 mg/cm^{3}) hemicylinders separated by an Al or Ti metal tracer strip; the components are vaporized by the drive. Before the experiment disassembles, the shocks cross at the tube center, creating a very fast (ΔU> 200 km/s) shear-unstable zone. After several nanoseconds, an expanding mixing layer is measured, and after 10+ ns we observe the appearance of streamwise-periodic, spanwise-aligned rollers associated with the primary Kelvin-Helmholtz instability of mixing layers. We additionally image roller pairing and spanwise-periodic streamwise-aligned filaments associated with secondary instabilities. New closures are derived to connect length scales of these structures to estimates of fluctuating velocity data otherwise unobtainable in the high-energy-density environment. This analysis indicates shear-induced specific turbulent energies 10^{3}-10^{4} times higher than the nearest conventional experiments. Because of difficulties in continuously driving systems under these conditions and the harshness of the experimental environment limiting the usable diagnostics, clear evidence of these developing structures has never before been observed in this regime.

Evolution of surface structure in laser-preheated perturbed materials

“…One can construct a nondimensional quantity from these parameters, in which allowing T to enter the expression through the sound speed for a plasma gives further insight for how the effect might change if we were to substitute other elements for aluminum. (Though this has not been done with the present experiment, it has been done for the NIF analog [45].) The easiest way to visualize the result is to approximate the EOS of the Al plasma as that of a polytropic ideal gas, with the resulting expression being…”

Modeling hydrodynamics, magnetic fields, and synthetic radiographs for high-energy-density plasma flows in shock-shear targets

“…pressure larger than 1Mbar, has been used to investigate the turbulent mixing [26,27] at material interfaces when subject to multiple shocks and reshocks or high-speed shear [23,28]. The experiments [29][30][31][32][33] using the shockshear platform has been carried out on the OMEGA Laser Facility and National Ignition Facility (NIF). These experiments provide quantitative measurements to assist in validation efforts [34][35][36] for mix models, such as Besnard-Harlow-Rauenzahn (BHR) model [37,38].…”

Late-Time Mixing Sensitivity to Initial Broadband Surface Roughness in High-Energy-Density Shear Layers

“…The overall mixing layer Reynolds number, calculated conventionally at late times using the overall dynamic mixing-layer width (∼300 μm) as the scale length [11,35], remains over 10 6 , and is expected to be post-transition in both cases. Reference [8] has details about how the tracer mix layer evolves on the surface and in the bulk under KH shear, and can be thought of as a half-layer for the energy and mix analysis [25].…”

“…However, KH systems driven single sidedly by impulsive decaying shock waves have been studied previously and typically in only one of the two mixing materials or in the presence of imposed initial sinusoidal seed perturbations [17][18][19][20][21][22][23]. This platform [8,24,25] uses a 10 ns indirect drive from two sides to support the shocks during the experiment.…”

“…However, these experiments did not last long enough for a full determination of the late-time asymptotic behavior of the system. Thus, a similar experimental platform was developed for the NIF [8,24,25] using indirectly (soft x-ray) driven ablators. The platform is shown in Fig.…”