ABSTRACT. Magnetized Kelvin-Helmholtz instability (KHI) develops between two magnetized fluids flowing pass one another, producing a shear layer. The stabilizing effect of a tangential magnetic field along the shear velocity is well-known since Chandrasekhar in 1961, and supersonic stabilization for non-magnetized KHI was conjectured by Landau in 1944. Compressible magnetized KHI encompasses both space and fusion applications, from magnetosphere and solar wind physics [1], to edge tokamak plasma and hohlraum-gas interface [2] in indirect-drive inertial confinement fusion implosions. Yet, there has been sparse experimental data in this compressible, magnetized regime.

By combining the stabilization effects of compressibility and a strong external magnetic field in the laboratory, we aim to demonstrate mitigation of instability growth in a magnetized KHI scenario. At the OMEGA laser facility, we built upon the previous successful non-magnetized Kelvin-Helmholtz platform [3,4], adding a pre-imposed magnetic field, driving a shock across a plastic-foam interface, and imaging the instability growth using a point-projection X-ray backlighter. At the National Ignition Facility (NIF), the much larger amount of available laser energy enables a new ongoing experiment platform with robust post-shock flow at late time, and also hotter plasma conditions to mitigate the diffusion of magnetic fields across the mixing region. We will discuss the development of and data from these two experimental platforms on OMEGA and the NIF, and comparison to magnetohydrodynamic simulations.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

[1] V. G. Merkin, et al., J. Geophys. Res. Space Physics, 118, 5478–5496. (2013) [2] M. Vandenboomgaerde, et al, Physics of Plasmas 23, 052704 (2016). [3] O. A. Hurricane, et al., Phys. Rev. Lett. 109, 155004 (2012) [4] V. A. Smalyuk, et al., High Energy Density Phys. 9, 47-51 (2013)

ABSTRACT. Hydrodynamic instabilities are ubiquitous in inertial confinement fusion implosion scenarios, leading to loss of energy for compression and to mix of dissimilar materials. In order to study them and assess their impact, dedicated instability experiments have been performed using the Z Machine at Sandia National Laboratories. Complementary to laser-driven instability experiments, pinch-driven experiments naturally drive interfaces in their light-to-heavy configuration and include the effects of cylindrical convergence. We present experimental results and simulations of a suite of platforms investigating the Richtmyer-Meshkov (RM) process and interfacial feedthrough. Liners filled with liquid deuterium are magnetically imploded, driving a converging shock to the central axis and creating a magnetically isolated region suitable for studying hydrodynamic processes. The first platform investigates the interaction of this shock with a solid beryllium rod machined with sinusoidal perturbations that then grow under RM. The second replaces the on-axis rod with another cylindrical liner, enabling investigation of the feedthrough of these instabilities to the inner surface.

*This work conducted for the U.S. DOE by LANL under contract 89233218CNA000001 and by SNL under contract DE-NA0003525

ABSTRACT. As the shock passes through the layers of a collapsing star during type II supernovae, baroclinic vorticity generated at interfaces between layers stimulates mixing via the Richtmyer-Meshkov instability (RMI). Previous research ties the RMI to the ejection of high-velocity projectiles thought to be responsible for the early detection of stellar core elements following Supernova 1987A. Predicting and characterizing these projectiles, however, remains challenging. Recent improvements in experimental diagnostics and numerical simulations reveal that such projectiles share key characteristics with classical fluid vortex rings, thus enabling a path to understand their dynamics. Our objective is to isolate the ejection of vortex rings from shocked interfaces and determine their scaling through numerical simulations and experiments at the Omega EP Laser facility. We generate an isolated vortex ring by passing a shock through an interface between a heavy and light fluid along which there is a protrusion of heavy fluid into the light. After shock passage, the protrusion inverts, generating a jet led by the vortex ring. Our theoretical and computational results show that the strength of the vortex rings expectedly scales with the intensity of density and pressure gradients but saturates beyond a critical protrusion size, enabling an a priori prediction of the energy transported by vortex rings in RMI flows.

ABSTRACT. Shock propagation across the interface between regions of different acoustic impedance leads to the unbounded amplification of any perturbations initially present on the interface (the Richtmyer-Meshkov instability, RMI) which eventually leads to the development of a turbulent mixing region. Shock propagation through a molecular cloud of a supernova remnant (SNR) has been proposed as a mechanism for triggering star formation, with possible relevance to star-burst galaxies.

In the past, experimental campaigns have been performed on the NIF and OMEGA facilities to study the scalability of the Rayleigh-Taylor instability (RTI) across length scale and time scale factors of 3. Experimental studies of the RMI have taken place in shock tube facilities (at energy densities far below those of an astrophysical event) and on laser-driven facilities (where the energy density is much closer to prototypical) but appropriate scaling between the two regimes is still an open question and represents the motivation for the present work.

Experiments will be performed on NIF to study the RMI of a shock-accelerated material interface in a light-to-heavy geometry, following its evolution to long post-shock times, well into the highly non-linear regime. The results will be compared in a scaled sense to shock tube experiments at spatial and time scales that are factors of 10-100 longer. The main objectives are (1) to assess scalability of the results across large ranges of length scales and shock strengths, and (2) quantify the influence of strong compression effects (adjustable on NIF but not present in the shock tube) on the RMI. Both these issues are of significant relevance to astrophysical settings. The results will also provide a new database that can be used in benchmarking and calibrating computer codes (like ARES) developed specifically to describe this type of physics. The NIF is the only facility capable of delivering the required energy densities for these high Mach number RMI experiments, the time scales to reach the deep nonlinear RMI regimes and the ability to shape the laser pulse to ensure minimal interface deceleration after the initial shock-induced step in velocity.

A planar shock wave will be generated in a halfraum using a CH(I) ablator. The shock will propagate into a CRF foam layer followed by a CH(I) layer, with a perturbed interface between the two. Side-on imaging will be performed to measure the perturbation amplitude time history. One of the main novelties of this setup is the direction of propagation of the shock, from the light to the heavy material. Because of this, the post-shock deceleration of the interface will not lead to the RTI (with exponential amplitude growth rates that would overtake the RMI growth) but to steady, bounded oscillations whose effect on the RMI is predicted to be small.

The central question is how the RMI growth rates scale between the shock tube and NIF. For moderate shock strengths, it is expected that the hydrodynamics of the shock-interface interactions are decoupled from compression effects at least over the range of Mach numbers covered in NIF experiments at moderate laser intensities. Conversely, at high laser intensities, our experiment would offer a first measure of the degree of coupling between the RMI hydrodynamics and compression effects at an embedded interface (far from the ablation front).

ABSTRACT. Inertial Confinement Fusion (ICF) and High-Energy Density Physics (HEDP) experiments experience complicated forcing for instability growth and mix, due to the ubiquitous presence of multiple shocks interacting with perturbations on multiple material interfaces, including successive shocks from the same direction. There is a severe lack of analytic work and modeling validation for same-sided successive shocks since they are extremely difficult to achieve with conventional (non-HED) drivers. Successive shocks access a large instability parameter space; idealized fluid theory [Mikaelian, Physical Review A 31(1), 410 (1985)] predicts 15 different interface evolution scenarios for just a single mode, corresponding to three different vorticity competition cases. Growth becomes more complex for multi-mode, compressible HED systems. The Mshock campaign is the first experiment in any fluid regime to probe a wide portion of successive shock parameter space and deliver data capable of rigorously challenging our models and their ability to accurately capture Richtmyer-Meshkov growth under successive shocks.

Single-mode experiments have successfully demonstrated the ability to access and control the various growth states of the shocked interface, including re-inversion, freeze-out, and continued growth [Merritt et al., Physics of Plasmas 30, 072108 (2023)]. Data agrees among experiment, theory, and simulation in the linear growth phase, giving us confidence in our ICF/HED design codes (Fig. 1). Multi-mode experiments demonstrate distinct growth scenarios for the different modes and form a first basis of tests for initialization parameters of non-linear instability and turbulence models [Braun and Gore, Physica D 404 (2020] in this regime. *This work conducted under the auspices of the U.S. DOE by LANL under contract 89233218CNA000001 and by LLNL under Contract Nos. DE-AC52-07NA27344.

ABSTRACT. The high-energy density properties of iron are largely unconstrained and require complicated laser-driven experiments to probe. The design of such experiments involves the use of multimaterial fluid codes to model and predict laser absorption and material dynamics. The Rayleigh-Taylor (RT) instability can be used to infer iron strength or viscosity at multi-megabar pressures and thousands of Kelvin temperatures. The target and laser pulse shape are carefully designed using 1- and 2D hydrodynamic simulations to compress the iron to planetary relevant conditions of 150-300 GPa and 4000-6000 K. Strength/viscosity is then inferred from cross-comparison of experimental and simulated growth of RT unstable ripples [1]. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [1] G. Righi, et al., JAP 131 (2022) 145902.

ABSTRACT. Iron (Fe) is one of the most abundant elements in the universe and is found in a wide range of astronomical objects. Understanding its strength at extreme pressure-temperature conditions is crucial for better understanding the interior structures and processes of celestial bodies, such as planetary core formation, seismic wave propagation, magnetic field generation, and impact event. Here, we present laser-driven Rayleigh-Taylor instability experiments for measuring the strength of iron. A physics package, consisting of a ripple interface between lighter epoxy and heavier iron, is directly accelerated by a high-power laser. The Rayleigh-Taylor growth of the ripple amplitude is measured using face-on x-ray radiography. Hydrodynamic simulations are performed to extract the pressure, temperature, flow stress, and strain profiles during acceleration. Our results provide dynamic strength data for two single-crystalline irons ([100] and [111] orientations) and a FeNi alloy (35 at% Ni in Fe) up to 350 GPa and 6 kK at strain rates of 10^-7-10^-8 /s. In summary, this study contributes to our understanding of iron's behavior under extreme conditions, especially the effects of crystalline orientation and alloying, with implications for various fields of geophysics, planetary science, and astrophysics.

* This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

ABSTRACT. Investigating how solid matter behaves at enormous pressures, such as those found in the deep interiors of giant planets, is a great experimental challenge. Over the past decade, computational predictions have revealed that compression to terapascal pressures may bring about counter-intuitive changes in the structure and bonding of solids as quantum mechanical forces grow in influence. Although this behaviour has been observed at modest pressures in the highly compressible light alkali metals it has not been established whether it is commonplace among high-pressure solids more broadly. We used shaped laser pulses at the National Ignition Facility to compress elemental Mg up to 1.3 TPa, which is approximately four times the pressure at the Earth’s core. By directly probing the crystal structure using nanosecond-duration X-ray diffraction, we found that Mg changes its crystal structure several times with non-close-packed phases emerging at the highest pressures. Our results demonstrate that phase transformations of extremely condensed matter, previously only accessible through theoretical calculations, can now be experimentally explored.

ABSTRACT. Dynamic compression is now a widespread technique for investigating material properties at extraordinary pressure, density, and temperature. However, there is a nearly complete lack of temperature measurements across the full scope of this field, leaving thermal effects as a large source of uncertainty. Extended X-ray Absorption Fine Structure (EXAFS) is is sensitive to density, temperature, and crystal structure in the range 100s-10000 K, where most materials form a solid at high pressure. Here we present results of experiments at the National Ignition Facility (NIF) that measured EXAFS from both copper [1] and tantalum compressed to multi-Mbar pressures along both shock and ramp compression paths. These measurements are made possible by the high flux x-ray source [2] and high fidelity laser pulse shaping available at the NIF, as well as the high-resolution x-ray spectrometer design [3]. L-edge EXAFS, which is required for high atomic number materials, is particularly challenging due to intrinsically small amplitude EXAFS oscillations compared to K-edge. We discuss these results in the context of predicted thermal states, thermal diffusion on nanosecond timescale, detailed strength models, and design of future experiments.

1. Sio et al. “Extended X-ray absorption fine structure of dynamically-compressed copper up to 1 terapascal” Nature Communications 14, 7046 (2023) 2. Krygier et al. “Optimized continuum x-ray emission from laser-generated plasma” Appl. Phys. Lett. 117, 251106 (2020) 3. Stoupin et al. “The multi-optics high-resolution absorption x-ray spectrometer (HiRAXS) for studies fo materials under extreme conditions” Rev. Sci. Instrum. 92, 053102 (2021)

This work was performed under the auspices of U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.