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Welch, D

Publications and source records attributed to Welch, D.

12 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Fully kinetic simulations of megajoule-scale dense plasma focus

Dense plasma focus (DPF) Z-pinch devices are sources of copious high energy electrons and ions, x-rays, and neutrons. Megajoule-scale DPFs can generate 10{sup 12} neutrons per pulse in deuterium gas through a combination of thermonuclear and beam-target fusion. However, the details of the neutron production are not fully understood and past optimization efforts of these devices have been largely empirical. Previously, we reported on the first fully kinetic simulations of a kilojoule-scale DPF and demonstrated that both kinetic ions and kinetic electrons are needed to reproduce experimentally observed features, such as charged-particle beam formation and anomalous resistivity. Here, we present the first fully kinetic simulation of a MegaJoule DPF, with predicted ion and neutron spectra, neutron anisotropy, neutron spot size, and time history of neutron production. The total yield predicted by the simulation is in agreement with measured values, validating the kinetic model in a second energy regime.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Twinning and Dislocation Pileups in Heavily Implanted LiNbO3

The nature of a striking pattern of extended defects formed by very high dose implantation of helium in a complex oxide, for example, ferroelectricz-cut LiNbO{sub 3}, is studied. After irradiation, a high concentration of defects is found to collect and create a network of thick prismatic planar defects, which have typical dimensions of {approx}1.5 {micro}m and 200 nm parallel and perpendicular to the z axis, respectively. This defect network and its effect on the lattice were studied using a set of x-ray and electron microscopy probes of the lattice structure and spatial variation. Optical microscopy shows that there is strong temperature dependence for forming the network; the density of these extended defects reaches a maximum value for an annealing temperature of 250 C but is fully eliminated by a temperature of 380 C. High-resolution transmission electron microscopy studies indicate that these extended defects are probably localized twinning and dislocation pileups due to plastic deformation of the lattice to relieve He-implantation-induced stress. During this deformation, He accumulates at the twin boundaries. The study also shows that the He interstitials evolve into bubbles causing high stress and resulting in a formation of thick prismatic planar defects. Finally, a mechanism is proposed for defect creation and elimination.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

RF Plasma Heating in the PFRC-2 Device: Motivation, Goals and Methods

The motivation for using radio frequency, odd-parity rotating magnetic fields for heating field-reversed-configuration (FRC) plasmas is explained. Calculations are presented of the expected electron and ion temperatures in the PFRC-2 device, currently under construction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Oxygen Reordering Near Room temperature in YBa2Cu3O6+x: A Thermodynamic Model

We propose a thermodynamic model to explain an unusual phase transformation occurring near room temperature in YBa{sub 2}Cu{sub 3}O{sub 6+x} that greatly affects properties of the superconductor. Based on our model, the material's thermodynamic response functions, specific heat, thermal-expansion coefficient, and elastic compliances are deduced at the critical temperature of the phase transformation. We discuss the change of critical temperature with stress, and analyze the anomaly of specific heat in critical temperature of the phase transformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Spherical Solid He Nanometer Bubbles in an Anisotropic Complex Oxide

We show, using room temperature, high-resolution electron microscopy studies, that implanted He in LiNbO{sub 3} nucleates and accumulates as bubbles. These He inclusions are at {approx} 20 GPa pressure and most probably in the solid phase. In addition, the energetically favored shape of the inclusions in their as-implanted form is spherical and not oblate; this spherical shape is due to the fact that their diameter is below a critical radius for balancing the surface and elastic energies as predicted by elastic theory. When annealed, the characteristic length scale of the He inclusions increases, forming faceted bubbles. Annealing also causes the He inclusions to migrate and accumulate into strings due to the preferred {l_brace}101{bar 4}{r_brace}-pyramidal-twinning planes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗