On the Properties of Plastic Ablators in Laser-Driven Material Dynamics Experiments

Radiation hydrodynamics simulations were used to study the effect of plastic ablators in laser-driven shock experiments. The sensitivity to composition and equation of state was found to be 5-10% in ablation pressure. As was found for metals, a laser pulse of constant irradiance gave a pressure history which decreased by several percent per nanosecond. The pressure history could be made more constant by adjusting the irradiance history. The impedance mismatch with the sample gave an increase o(100%) in the pressure transmitted into the sample, for a reduction of several tens of percent in the duration of the peak load applied to the sample, and structured the release history by adding a release step to a pressure close to the ablation pressure. Algebraic relations were found between the laser pulse duration, the ablator thickness, and the duration of the peak pressure applied to the sample, involving quantities calculated from the equations of state of the ablator and sample using shock dynamics.Comment: Typos fixe

Shock formation and the ideal shape of ramp compression waves

We derive expressions for shock formation based on the local curvature of the flow characteristics during dynamic compression. Given a specific ramp adiabat, calculated for instance from the equation of state for a substance, the ideal nonlinear shape for an applied ramp loading history can be determined. We discuss the region affected by lateral release, which can be presented in compact form for the ideal loading history. Example calculations are given for representative metals and plastic ablators. Continuum dynamics (hydrocode) simulations were in good agreement with the algebraic forms. Example applications are presented for several classes of laser-loading experiment, identifying conditions where shocks are desired but not formed, and where long duration ramps are desired

Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions

The high-pressure behavior of Fe alloys governs the interior structure and dynamics of super-Earths, rocky extrasolar planets that could be as much as 10 times more massive than Earth. In experiments reaching up to 1300 GPa, we combine laser-driven dynamic ramp compression with in situ x-ray diffraction to study the effect of composition on the crystal structure and density of Fe-Si alloys, a potential constituent of super-Earth cores. We find that Fe-Si alloy with 7 weight % (wt %) Si adopts the hexagonal close-packed structure over the measured pressure range, whereas Fe-15wt%Si is observed in a body-centered cubic structure. This study represents the first experimental determination of the density and crystal structure of Fe-Si alloys at pressures corresponding to the center of a ~3–Earth mass terrestrial planet. Our results allow for direct determination of the effects of light elements on core radius, density, and pressures for these planets

Efficacy of a Manualized and Workbook-Driven Individual Treatment for Social Anxiety Disorder

Social anxiety disorder is a prevalent and impairing disorder for which viable cognitive-behavioral therapies exist. However, these treatments have not been easily packaged for dissemination and may be underutilized as a result. The current study reports on the findings of a randomized controlled trial of a manualized and workbook-driven individual cognitive-behavioral treatment for social anxiety disorder (Hope, Heimberg, Juster, & Turk, 2000; Hope, Heimberg, & Turk, 2006). This treatment package was derived from an empirically supported group treatment for social anxiety disorder and intended for broad dissemination, but it has not previously been subjected to empirical examination on its own. As a first step in that examination, 38 clients seeking treatment for social anxiety disorder at either the Adult Anxiety Clinic of Temple University or the Anxiety Disorders Clinic of the University of Nebraska–Lincoln were randomly assigned to receive either immediate treatment with this cognitive-behavioral treatment package or treatment delayed for 20 weeks. Evaluation at the posttreatment/postdelay period revealed substantially greater improvements among immediate treatment clients on interviewer-rated and self-report measures of social anxiety and impairment. Three-month follow-up assessment revealed maintenance of gains. Clinical implications and directions for future research are discussed

A Review of the N-bound and the Maximal Mass Conjectures Using NUT-Charged dS Spacetimes

The proposed dS/CFT correspondence remains an intriguing paradigm in the context of string theory. Recently it has motivated two interesting conjectures: the entropic N-bound and the maximal mass conjecture. The former states that there is an upper bound to the entropy in asymptotically de Sitter spacetimes, given by the entropy of pure de Sitter space. The latter states that any asymptotically de Sitter spacetime cannot have a mass larger than the pure de Sitter case without inducing a cosmological singularity. Here we review the status of these conjectures and demonstrate their limitation. We first describe a generalization of gravitational thermodynamics to asymptotically de Sitter spacetimes, and show how to compute conserved quantities and gravitational entropy using this formalism. From this we proceed to a discussion of the N-bound and maximal mass conjectures. We then illustrate that these conjectures are not satisfied for certain asymptotically de Sitter spacetimes with NUT charge. We close with a presentation of explicit examples in various spacetime dimensionalities.Comment: 49 pages, 17 figures, a few typos corrected, addendum added with regard to some references that were later brought to our attentio

The characteristics of cognitive neuroscience tests in a schizophrenia cognition clinical trial: Psychometric properties and correlations with standard measures.

In comparison to batteries of standard neuropsychological tests, cognitive neuroscience tests may offer a more specific assessment of discrete neurobiological processes that may be aberrant in schizophrenia. However, more information regarding psychometric properties and correlations with standard neuropsychological tests and functional measures is warranted to establish their validity as treatment outcome measures. The N-back and AX-Continuous Performance Task (AX-CPT) are two promising cognitive neuroscience tests designed to measure specific components of working memory and contextual processing respectively. In the current study, we report the psychometric properties of multiple outcome measures from these two tests as well as their correlations with standard neuropsychological measures and functional capacity measures. The results suggest that while the AX-CPT and N-back display favorable psychometric properties, they do not exhibit greater sensitivity or specificity with functional measures than standard neurocognitive tests

Cosmological expansion on a dilatonic brane-world

In this paper we study brane-world scenarios with a bulk scalar field, using a covariant formalism to obtain a 4D Einstein equation via projection onto the brane. We discuss, in detail, the effects of the bulk on the brane and how the scalar field contribute to the gravitational effects. We also discuss choice of conformal frame and show that the frame selected by the induced metric provides a natural choice. We demonstrate our formalism by applying it to cosmological scenarios of Randall-Sundrum and Horava-Witten type models. Finally we consider the cosmology of models where the scalar field couples non-minimally to the matter on the brane. This gives rise to a novel scenario where the universe expands from a finite scale factor with an initial period of accelerated expansion, thus avoiding the singularity and flatness problem of the standard big bang model.Comment: 20 pages - Version to appear in Classical and Quantum Gravity. New section added on conformal rescaling of the metric. Some other minor changes made and references adde