The Dynamics of Brane-World Cosmological Models

Brane-world cosmology is motivated by recent developments in string/M-theory and offers a new perspective on the hierarchy problem. In the brane-world scenario, our Universe is a four-dimensional subspace or {\em brane} embedded in a higher-dimensional {\em bulk} spacetime. Ordinary matter fields are confined to the brane while the gravitational field can also propagate in the bulk, leading to modifications of Einstein's theory of general relativity at high energies. In particular, the Randall-Sundrum-type models are self-consistent and simple and allow for an investigation of the essential non-linear gravitational dynamics. The governing field equations induced on the brane differ from the general relativistic equations in that there are nonlocal effects from the free gravitational field in the bulk, transmitted via the projection of the bulk Weyl tensor, and the local quadratic energy-momentum corrections, which are significant in the high-energy regime close to the initial singularity. In this review we discuss the asymptotic dynamical evolution of spatially homogeneous brane-world cosmological models containing both a perfect fluid and a scalar field close to the initial singularity. Using dynamical systems techniques it is found that, for models with a physically relevant equation of state, an isotropic singularity is a past-attractor in all orthogonal spatially homogeneous models (including Bianchi type IX models). In addition, we describe the dynamics in a class of inhomogeneous brane-world models, and show that these models also have an isotropic initial singularity. These results provide support for the conjecture that typically the initial cosmological singularity is isotropic in brane-world cosmology.Comment: Einstein Centennial Review Article: to appear in CJ

Target diagnostic system for the National Ignition Facility (NIF)

A review of recent progress on the design of a diagnostic system proposed for ignition target experiments on the National Ignition Facility (NIF) will be presented. This diagnostic package contains an extensive suite of optical, x-ray, gamma-ray, and neutron diagnostics that enable measurements of the performance of both direct and indirect driven NIF targets. The philosophy used in designing all of the diagnostics in the set has emphasized redundant and independent measurement of fundamental physical quantities relevant to the operation of the NIF target. A unique feature of these diagnostics is that they are being designed to be capable of operating, in the high radiation, EMP, and debris backgrounds expected on the NIF facility. The diagnostic system proposed can be categorized into three broad areas: laser characterization, hohlraum characterization, and capsule performance diagnostics. The operating principles of a representative instrument from each class of diagnostic employed in this package will be summarized and illustrated with data obtained in recent prototype diagnostic tests

DIAGNOSTICS PROGRAM FOR A MAGNETICALLY INSULATED ION DIODE FOR INERTIAL CONFINEMENT FUSION

No abstract availabl

Measuring neutron yield and ρR anisotropies with activation foils at the National Ignition Facility

Neutron yields at the National Ignition Facility (NIF) are measured with a suite of diagnostics, including activation of ∼20–200 g samples of materials undergoing a variety of energy-dependent neutron reactions. Indium samples were mounted on the end of a Diagnostic Instrument Manipulator (DIM), 25–50 cm from the implosion, to measure 2.45 MeV D-D fusion neutron yield. The 336.2 keV gamma rays from the 4.5 hour isomer of 115mIn produced by (n,n′) reactions are counted in high-purity germanium detectors. For capsules producing D-T fusion reactions, zirconium and copper are activated via (n,2n) reactions at various locations around the target chamber and bay, measuring the 14 MeV neutron yield to accuracies on order of 7%. By mounting zirconium samples on ports at nine locations around the NIF chamber, anisotropies in the primary neutron emission due to fuel areal density asymmetries can be measured to a relative precision of 3%

Production of thermonuclear neutrons from deuterium-filled capsule implosion experiments driven by Z-Pinch dynamic hohlraums at Sandia National Laboratories' Z facility

Deuterium-filled capsule implosion experiments that employ the dynamic hohlraum are presently being conducted on the Z facility at Sandia National Laboratories. This paper will address the evidence for thermonuclear neutron production in the initial series and subsequent series of experiments that have been conducted to date employing Be, plastic, and glass capsules. The novelty of this approach motivated using several techniques to determine that the neutrons were thermonuclear in origin. The diagnostic techniques employed consist of measuring the average neutron energy and yield isotropy in two directions that were separated by a polar angle of 102 degrees. Additional “null” experiments were also employed that used the addition of Xe gas to the deuterium gas fill to suppress fusion neutron yields from the capsules by an order of magnitude. Use of these techniques are of particular importance because alternative, nonthermonuclear neutron processes were previously found to exist in Z-pinch and dense plasma focus plasmas. Such processes typically involved the creation of directed energetic ions leading to the production of nonthermal, “ion beam” generated neutrons. If not properly diagnosed, neutrons produced by these nonthermal processes could be misinterpreted as thermonuclear in origin