Solid state experiments at extreme pressures (10-100 GPa) and strain rates (10
6
-10
8
s
−1
) are being developed on high energy laser facilities, and offer the possibility for exploring new regimes ...of materials science. These extreme solid state conditions can be accessed with either shock loading or with a quasi-isentropic ramped pressure drive. Velocity interferometer measurements establish the high pressure conditions. Constitutive models for solid state strength under these conditions are tested by comparing 2D continuum simulations with experiments measuring perturbation growth from the Rayleigh-Taylor instability in solid state samples. Lattice compression, phase and temperature are deduced from extended X-ray absorption fine structure (EXAFS) measurements, from which the shock induced α-ω phase transition in Ti and the α-ϵ phase transition in Fe, are inferred to occur on subnanosec time scales. Time resolved lattice response and phase can also be measured with dynamic X-ray diffraction measurements, where the elastic-plastic (1D-3D) lattice relaxation in shocked Cu is shown to occur promptly (<1 ns). Subsequent large scale molecular dynamics (MD) simulations elucidate the microscopic dislocation dynamics that underlies this 1D-3D lattice relaxation. Deformation mechanisms are identified by examining the residual microstructure in recovered samples. The slip-twinning threshold in single crystal Cu shocked along the 001 direction is shown to occur at shock strengths of ∼20 GPa, whereas the corresponding transition for Cu shocked along the 134 direction occurs at higher shock strengths. This slip twinning threshold also depends on the stacking fault energy (SFE), being lower for low SFE materials. Designs have been developed for achieving much higher pressures, P>1000 GPa, in the solid state on the National Ignition Facility (NIF) laser.
Targets intended to produce ignition on the National Ignition Facility are being simulated, and the simulations are used to set specifications for target fabrication. Recent design work has focused ...on refining designs that use 1.3 MJ of laser energy, with an ablator of Be(Cu) or CH(Ge). The mainline hohlraum design now has a He-H gas fill and a wall of U-Au layers. The emphasis in this paper is on changes in the requirements over the last year. Complete tables of specifications are regularly updated for all of the targets. All the specifications are rolled together into an error budget indicating adequate margin for ignition with all of the designs.
Solid-state dynamics experiments at very high pressures and strain rates are becoming possible with high-power laser facilities, albeit over brief intervals of time and spatially small scales. To ...achieve extreme pressures in the solid state requires that the sample be kept cool, with T^sub sample^ > T^sub melt^. To this end, a shockless, plasma-piston "drive" has been developed on the Omega laser, and a staged shock drive was demonstrated on the Nova laser. To characterize the drive, velocity interferometer measurements allow the high pressures of 10 to 200 GPa (0.1 to 2 Mbar) and strain rates of 10^sup 6^ to 10^sup 8^ s^sup ^-1 to be determined. Solid-state strength in the sample is inferred at these high pressures using the Rayleigh-Taylor (RT) instability as a "diagnostic." Lattice response and phase can be inferred for single-crystal samples from time-resolved X-ray diffraction. Temperature and compression in polycrystalline samples can be deduced from extended X-ray absorption fine-structure (EXAFS) measurements. Deformation mechanisms and residual melt depth can be identified by examining recovered samples. We will briefly review this new area of laser-based materials-dynamics research, then present a path forward for carrying these solid-state experiments to much higher pressures, P > 10^sup 3^ GPa (10 Mbar), on the National Ignition Facility (NIF) laser at Lawrence Livermore National Laboratory. PUBLICATION ABSTRACT
Collisions of high Mach number flows occur frequently in astrophysics, and the resulting shock waves are responsible for the properties of many astrophysical phenomena, such as supernova remnants, ...Gamma Ray Bursts and jets from Active Galactic Nuclei. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are “collisionless”, since they form due to plasma instabilities and self-generated magnetic fields. Laboratory experiments at the laser facilities can achieve the conditions necessary for the formation of collisionless shocks, and will provide a unique avenue for studying the nonlinear physics of collisionless shock waves. We are performing a series of experiments at the Omega and Omega-EP lasers, in Rochester, NY, with the goal of generating collisionless shock conditions by the collision of two high-speed plasma flows resulting from laser ablation of solid targets using ∼10
16 W/cm
2 laser irradiation. The experiments will aim to answer several questions of relevance to collisionless shock physics: the importance of the electromagnetic filamentation (Weibel) instabilities in shock formation, the self-generation of magnetic fields in shocks, the influence of external magnetic fields on shock formation, and the signatures of particle acceleration in shocks. Our first experiments using Thomson scattering diagnostics studied the plasma state from a single foil and from double foils whose flows collide “head-on”. Our data showed that the flow velocity and electron density were 10
8 cm/s and 10
19 cm
−3, respectively, where the Coulomb mean free path is much larger than the size of the interaction region. Simulations of our experimental conditions show that weak Weibel mediated current filamentation and magnetic field generation were likely starting to occur. This paper presents the results from these first Omega experiments.
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