The solid-solid phase transition between the bcc (alpha) and hcp (epsilon) lattice structures in iron is known to occur as the material is compressed. When kept below its melting point, an effective increase in the macroscopic strength of the material accompanies this phase transition. Understanding the material strength of iron throughout the deformation process presents a significant computational challenge, but is important for improving models of planetary structure, including interpretation of seismic measurements taken through our own Earth's core.To explore the strength of iron at high pressures and strain rates, we have developed the IronRT campaign at the OMEGA laser [1]. This laser-driven platform produces pressure greater than 1 Mbar on a thin Fe disk with a sinusoidal ripple pattern imposed on its face. These ripples seed the Rayleigh-Taylor (RT) instability, the growth of which is suppressed by the material strength of the sample. The amplitude of the ripples is diagnosed with high-energy x-ray radiography, and the measured growth is compared to simulations performed with different strength models. By matching the simulations to the low level of growth measured, we infer an average flow stress of greater than 40 GPa over the course of the experiment. This value is in agreement with other dynamic iron strength experiments at pressures greater than 1 Mbar [2].

We are performing scaled astrophysics experiments on Omega and on NIF. Laser driven counter-streaming interpenetrating supersonic plasma flows can be studied to understand astrophysical electromagnetic plasma phenomena in a controlled laboratory setting. In our Omega experiments, the counter-streaming flow plasma state is measured using Thomson scattering diagnostics, demonstrating the plasma flows are indeed super-sonic and in the collisionless regime. We observe a surprising additional electron and ion heating from ion drag force in the double flow experiments that are attributed to the ion drag force and electrostatic instabilities. [1] A proton probe is used to image the electric and magnetic fields. We observe unexpected large, stable and reproducible electromagnetic field structures that arise in the counter-streaming flows [2]. The Biermann battery magnetic field generated near the target plane, advected along the flows, and recompressed near the midplane explains the cause of such self-organizing field structures [3]. A (DHe)-He-3 implosion proton probe image showed very clear filamentary structures; three-dimensional Particle-In-Cell simulations and simulated proton radiography images indicate that these filamentary structures are generated by Weibel instabilities and that the magnetization level (ratio of magnetic energy over kinetic energy in the system) is similar to 0.01 [4]. These findings have very high astrophysical relevance and significant implications. We expect to observe true collisionless shock formation when we use >100 kJ laser energy on NIF.

We report the measurements of electrostatic field structures associated with an electrostatic shock formed in laser-produced counter-streaming plasmas with proton imaging. The thickness of the electrostatic structure is estimated from proton images with different proton kinetic energies from 4.7 MeV to 10.7 MeV. The width of the transition region is characterized by electron scale length in the laser-produced plasma, suggesting that the field structure is formed due to a collisionless electrostatic shock.

A basic tenet of material science is that the flow stress of a metal increases as its grain size decreases, an effect described by the Hall-Petch relation. This relation is used extensively in material design to optimize the hardness, durability, survivability, and ductility of structural metals. This Letter reports experimental results in a new regime of high pressures and strain rates that challenge this basic tenet of mechanical metallurgy. We report measurements of the plastic flow of the model body-centered-cubic metal tantalum made under conditions of high pressure (>100  GPa) and strain rate (∼10(7)  s(-1)) achieved by using the Omega laser. Under these unique plastic deformation ("flow") conditions, the effect of grain size is found to be negligible for grain sizes >0.25  μm sizes. A multiscale model of the plastic flow suggests that pressure and strain rate hardening dominate over the grain-size effects. Theoretical estimates, based on grain compatibility and geometrically necessary dislocations, corroborate this conclusion.

We report direct lattice-level measurements of plastic relaxation kinetics through time-resolved, in situ Laue diffraction of shock-compressed single-crystal [001] Ta at pressures of 27-210 GPa. For a 50-GPa shock, a range of shear strains is observed extending up to the uniaxial limit for early data points (<0.6 ns), and the average shear strain relaxes to a near steady state over similar to 1 ns. For 80- and 125-GPa shocks, the measured shear strains are fully relaxed already at 200 ps, consistent with rapid relaxation associated with the predicted threshold for homogeneous nucleation of dislocations occurring at shock pressure similar to 65 GPa. The relaxation rate and shear stresses are used to estimate the dislocation density, and these quantities are compared to the results of other high-pressure work, flow stress models, and molecular dynamics simulations.

Collisionless shocks can be produced as a result of strong magnetic fields in a plasma flow, and therefore are common in many astrophysical systems. The Weibel instability is one candidate mechanism for the generation of sufficiently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability efficiently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. This result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts.

Experiments are being developed on high power lasers and intense x--‐ray sources, such as the Omega laser at LLE, the Janus and NIF lasers at LLNL, and LCLS at SLAC to study lattice dynamics and solid--‐state plasticity at high pressures and strain rates. These experiments probe the plastic response of materials to high pressure (50--‐500 GPa), high strain rate (10 6 – 10 10 s -1 ) deformation. Three classes of experiments in tantalum, a model bcc metal, will be described. Dynamic diffraction experiments with a time resolution of ~0.1 ns or shorter are being developed to probe the microscopic lattice response of samples to a strong shock. [1,2] In particular, the time scale for the onset of plasticity and the rate of the 1D to 3D lattice relaxation are a direct measure of how rapidly dislocations can be generated and transported on sub--‐nanosecond time scales. [3] Macroscopic plastic flows at high pressure and strain rate can be generated that span a few tens of nanoseconds by using the Rayleigh--‐Taylor or Richtmyer-‐Meshkov fluid instabilities. [4,5] Finally, shock driven samples are also recovered so that the residual microstructure caused by the shock and subsequent plastic flow can be examined by SEM, TEM, and other laboratory techniques. [6,7] Results from all three classes of experiments will be compared with simulations using various models of flow stress, and a multi--‐scale model for bcc strength. [8,9] The mechanisms for plastic deformation and flow vary, depending on the sample pressure, temperature, and strain rate due to the shock or ramp compression. Given the small sample sizes and short time scales of the experiments, MD simulations also provide a valuable guide to the lattice level dynamics, and exhibit significant sensitivity in the predicted results to the interatomic potential. [10,11] Results from the three classes of experiments summarized above will be described, comparisons to theory and simulations given, and a path forward suggested.

EP shots) supported the Indirect Drive Inertial Confinement Fusion Campaign. The remaining 67% (175 OMEGA-only shots and 150 EP-only shots) were dedicated to experiments for High-Energy-Density Physics. Highlights of the various HED and ICF-ID campaigns are summarized in the following reports. In addition to these experiments, LLNL Principal Investigators (PIs) led a variety of Laboratory Basic Science campaigns using OMEGA and EP, including 80 target shots using just OMEGA, 68 shots using just EP, and 7 Joint shots using both lasers together. The highlights of these are also summarized, following the ICF and HED campaigns. Overall, LLNL PIs led a total of 636 shots at LLE in FY 2019. In addition, LLNL PIs supported 7 NLUF shots on Omega and 32 NLUF shots on EP in collaboration with the academic community.

Experiments investigating the physics of interpenetrating, collisionless, ablated plasma flows have become an important area of research in the high-energy-density field. In order to evaluate the feasibility of designing experiments that will generate a collisionless shock mediated by the Weibel instability on the National Ignition Facility (NIF) laser, computer simulations using the Center for Radiative Shock Hydrodynamics (CRASH) radiation-hydrodynamics model have been carried out. This paper reports assessment of whether the experiment can reach the required scale size while maintaining the low interflow collisionality necessary for the collisionless shock to form. Comparison of simulation results with data from Omega experiments shows the ability of the CRASH code to model these ablated systems. The combined results indicate that experiments on the NIF are capable of reaching the regimes necessary for the formation of a collisionless shock in a laboratory experiment.

Collisionless shocks are often observed in fast-moving astrophysical plasmas, formed by non-classical viscosity that is believed to originate from collective electromagnetic fields driven by kinetic plasma instabilities. However, the development of small-scale plasma processes into large-scale structures, such as a collisionless shock, is not well understood. It is also unknown to what extent collisionless shocks contain macroscopic fields with a long coherence length. For these reasons, it is valuable to explore collisionless shock formation, including the growth and self-organization of fields, in laboratory plasmas. The experimental results presented here show at a glance with proton imaging how macroscopic fields can emerge from a system of supersonic counter-streaming plasmas produced at the OMEGA EP laser. Interpretation of these results, plans for additional measurements, and the difficulty of achieving truly collisionless conditions are discussed. Future experiments at the National Ignition Facility are expected to create fully formed collisionless shocks in plasmas with no pre-imposed magnetic field.