S. M. Pollaine

Accessing ultrahigh-pressure, quasi-isentropic states of matter

A new approach to the study of material strength of metals at extreme pressures has been developed on the Omega laser, using a ramped plasma piston drive. The laser drives a shock through a solid plastic reservoir that unloads at the rear free surface, expands across a vacuum gap, and stagnates on the metal sample under study. This produces a gently increasing ram pressure, compressing the sample nearly isentropically. The peak pressure on the sample, inferred from interferometric measurements of velocity, can be varied by adjusting the laser energy and pulse length, gap size, and reservoir density, and obeys a simple scaling relation [J. Edwards et al., Phys. Rev. Lett. 92, 075002 (2004)]. In an important application, using in-flight x-ray radiography, the material strength of solid-state samples at high pressure can be inferred by measuring the reductions in the growth rates (stabilization) of Rayleigh–Taylor unstable interfaces. This paper reports the first attempt to use this new laser-driven, quasi-i...

Strong stabilization of the Rayleigh-Taylor instability by material strength at megabar pressures

Experimental results showing significant reductions from classical in the Rayleigh–Taylor (RT) instability growth rate due to high pressure effective lattice viscosity in metal foils are presented. Stabilization of RT instability (RTI) by ablation and density gradients has been studied for decades. The regime of stabilized RTI due to material strength at high pressure is new. On the Omega Laser in the Laboratory for Laser Energetics, University of Rochester, target samples of polycrystalline vanadium are compressed and accelerated quasi-isentropically at ∼1 Mbar pressures, while maintaining the samples in the solid-state. Provided strong shocks are avoided, the higher the applied peak pressure, the higher the predicted foil strength, and hence, the higher the degree of strength stabilization of RTI. Several experiments were conducted where the amount of RT growth is measured by face-on radiography. The vanadium samples are probed by a laser driven He-α x-ray backlighter which produced 5.2 keV radiation. C...

Collisional current drive in two interpenetrating plasma jets

The magnetic field generation in two interpenetrating, weakly collisional plasma streams produced by intense lasers is considered. The generation mechanism is very similar to the neutral beam injection current drive in toroidal fusion devices, with the differences related to the absence of the initial magnetic field, short interaction time, and different geometry. Spatial and temporal characteristics of the magnetic field produced in two counterstreaming jets are evaluated; it is shown that the magnetic field of order of 1 T can be generated for modest jet parameters. Conditions under which this mechanism dominates that of the “Biermann battery” are discussed. Other settings where the mechanism of the collisional current drive can be important for the generation of seed magnetic fields include astrophysics and interiors of hohlraums.

Optimization of the NIF Ignition Point Design Hohlraum

In preparation for the start of NIF ignition experiments, we have designed a porfolio of targets that span the temperature range that is consistent with initial NIF operations: 300 eV, 285 eV, and 270 eV. Because these targets are quite complicated, we have developed a plan for choosing the optimum hohlraum for the first ignition attempt that is based on this portfolio of designs coupled with early NIF experiements using 96 beams. These early experiments will measure the laser plasma instabilities of the candidate designs and will demonstrate our ability to tune symmetry in these designs. These experimental results, coupled with the theory and simulations that went into the designs, will allow us to choose the optimal hohlraum for the first NIF ignition attempt.

Quasi-isentropic material property studies at extreme pressures: from Omega to NIF

We are developing an experimental platform that can compress materials quasi-isentropically to very high pressures at ultrahigh strain rates. This laser driven, ramped (shockless) drive is used to study material properties such as strength, equation of state, phase, and phase transition kinetics under extreme conditions. We have achieved a ramped, shockless drive up to 2 Mbar on the Omega laser using both direct laser illumination and indirect x-ray illumination. In order to probe high-Z materials under extreme pressures, we are also developing high energy x-ray backlighters, 17 to 100 keV, created by high intensity (>1018 W/cm2) short pulse lasers (1 to 50 ps) such as the Titan laser at LLNL. Using a micro-wire embedded in a low-Z substrate, we have obtained radiographs with better than 10 ?m spatial resolution. This paper will show designs of isentropic platforms that can reach >10 Mbar on the NIF laser, using both direct and indirect drive configurations.

Accessing High Pressure States Relevant to Core Conditions in the Giant Planets

We have designed an experimental technique to use on the National Ignition Facility (NIF) laser to achieve very high pressure (P max > 10 Mbar = 1000 GPa), dense states of matter at moderate temperatures (T < 0.5 eV = 6000 K), relevant to the core conditions of the giant planets. A discussion of the conditions in the interiors of the giant planets is given, and an experimental design that can approach those conditions is described.

Interpretation of laser-driven V and TA Rayleigh-Taylor strength experiments

We present theoretical and computational analysis of the deformation regimes accessed by recent Rayleigh-Taylor (RT) material strength experiments in vanadium (V) and tantalum (Ta) done at the Omega laser at high pressures (>1 Mbar) and high strain rates (106 - 108 sec-1). Within the context of the LLNL multiscale models, the V-RT experiment appears to be dominated by deformation in the drag regime, whereas the Ta-RT experiment resides largely within the thermal activation regime.

Designfor solid-state Rayleigh-Taylor experiments in tantalum at Omega

We have designed an experiment for the Omega – EP laser facility to measure the Rayleigh-Taylor (RT) growth rate of solid-state Ta samples at ~1 Mbar pressures and very high strain rates, 107–108 s−1. A thin walled, hohlraum based, ramp-wave, quasi-isentropic drive has been developed for this experiment. Thick samples (~50 um) of Ta, with a pre-imposed sinusoidal rippled on the driven side, will be accelerated. The ripple growth due to the RT instability is greatly reduced due to the dynamic material strength. We will show detailed designs, and a thorough error analysis used to optimize the experiment and minimize uncertainty.

Materials Response under Extreme Conditions

Solid state experiments at extreme pressures (0.1 – 1 Mbar) and strain rates (106–108 s−1) are being developed on high‐energy laser facilities. The goal is a capability to test constitutive models for high‐pressure, solid‐state strength of materials. Relevant constitutive models are discussed, and our progress in developing a ramped‐pressure, shockless drive is given. Designs to test the constitutive models with experiments measuring perturbation growth due to the Rayleigh‐Taylor instability ‐in solid‐state samples are presented. Results from dynamic diffraction and EXAFS lattice diagnostics are given, showing that compression, phase, and temperature can be inferred on sub‐nsec time scales.

Calculations and measurements of x‐ray Thomson scattering spectra in warm dense matter

We present analytical expressions for the dynamic structure factor, or form factor S(k, ω), which is the quantity describing the inelastic x‐ray cross section from a dense plasma or a simple liquid. Our results, based on the random phase approximation (RPA) for the treatment on the charged particle coupling, can be applied to describe scattering from either weakly coupled classical plasmas or degenerate electron liquids. Our form factor correctly reproduces the Compton energy downshift and the usual Fermi‐Dirac electron velocity distribution for S(k, ω) in the case of a cold degenerate plasma. The results shown in this work can be applied to interpreting x‐ray scattering in warm dense plasmas occurring in inertial confinement fusion experiments. We show that electron density, electron temperature and ionization state can be directly inferred from such measurements. Specifically, we present as an example, use the results of experiments performed at the Vulcan laser facility at the Rutherford Appleton Labor...