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A particle within an ion beam is subjected to positive and negative currents. These currents deposit a net charge on the particle which interacts with the potential of the beam. A model is presented which describes this charging, the time required to attain an arbitrary charge, and the resulting coulomb force. Confinement by the beam is investigated through comparison of the electric force to the opposing force of gravity. To quantify this comparison, a normalized force is defined which, when negative, predicts those spatial regions where particle entrapment can occur. Utilizing a specially written VAX-Fortran program, the behavior of this force was characterized as a function of beam parameters. Regions were predicted in which particle confinement can arise and it was found that the magnitude of the trapping force varied significantly with those parameters that affect the beam-ion density. Moreover, calculations of the charging time revealed that the time to attain the minimum trapping charge was sufficiently short so as not to preclude entrapment.

acase@tulane.edu

acase@tulane.edu

This paper matches four different classes of axisymmetric solutions to Einstein's Field equations (Weyl and Levi-Civita static solutions, Kerr solution, Papapetrou solutions or Lewis and Van Stockum solutions) across a thin axisymmetric rotating shell to a flat interior metric. No assumptions about the behavior of the gravitational field at infinity are imposed, including the usual assumptions of asymptotic flatness or compactness. The purpose is to see how changing the boundary conditions at infinity affects frame dragging inside the shell The first case discussed is that of a spherical rigidly rotating shell of uniform density. Excluding the limit of the shell radius equal to the gravitational radius, the only matching exterior solutions finite on the rotation axis are trivial flat and Schwarzschild metrics. The freedom gained by not imposing boundary conditions at infinity is not sufficient to allow other matchings across such a shell to a flat interior metric Allowing the shell to be any of a class of axisymmetric prolate spheroidal shells with rotation and density dependent on latitude, I find matchings in cases other than the gravitational limit for both the Weyl and Levi-Civita static case and the Papapetrou case. Both contain asymptotically flat solutions as well as solutions with other behavior at infinity. The component of the stress energy tensor for time and angle of rotation as seen in an inertial frame is zero in the Weyl and Levi-Civita static case for all boundary conditions. It may however be nonzero in the Papapetrou case depending upon the conditions at infinity imposed. The interpretation is that changing the boundary conditions in this case changes the frame dragging of the interior inertial frames / acase@tulane.edu

acase@tulane.edu

acase@tulane.edu

Explicit expressions for the location of trapped, and anti-trapped surfaces of cosmological origin in Friedmann type universes are obtained. Friedmann universes dominated by energy-momentum tensors of matter with an equation of state of the form $p = (\gamma - 1)\rho$ (where $\gamma > {2\over3},\ p$ is the pressure, and $\rho$ is the energy density), a decoupled mixture of matter and radiation, and a positive cosmological constant are considered. The first form includes the physically significant special cases of pressureless matter or dust ($\gamma$ = 1), electromagnetic radiation ($\gamma = {4\over3}$), and a non-interacting massless scalar field ($\gamma$ = 2). A model closed Friedmann universe with trapped surfaces in the contracting phase but with no event horizons is also considered in this study, in order to emphasize the distinction between horizons and trapped surfaces. Penrose diagrams for special cases are drawn, showing the location and nature of trapped/anti-trapped, and marginally trapped/anti-trapped surfaces. The computational results for the marginally trapped and marginally anti-trapped surfaces and the trapping horizons foliated by marginally trapped/anti-trapped surfaces are analyzed in the context of available classification schemes for such surfaces. It is demonstrated by means of counterexamples that the current classification schemes fail to properly distinguish between trapping horizons of cosmological vs. non-cosmological origin, and that event horizons and trapped surfaces need not occur together. This demonstrates that an adequate definition for black holes in closed universes does not yet exist. A joint between a dust dominated closed Friedmann universe and a horizonless closed universe is also studied. Since the latter universe has no future event horizons, such a matching will enable a closed Friedmann universe, which has future event horizons, to evolve into a universe without such horizons. Although a physical process which could accomplish this has not been postulated here, the junction conditions and the energy conditions do not preclude such a transition / acase@tulane.edu

Computational proposal for locating local defects in superconducting tapes

The use of the long-lived radioisotope ¹²⁹I as an environmental tracer is increasing in popularity. An application of nuclear physics that is known as Accelerator Mass Spectrometry (AMS) can be used to measure ¹²⁹I in various sample matrices at a level of a few parts in 10¹⁴. The system employs a 3 MV electrostatic accelerator as one component of a mass spectrometer. As accelerated ions have kinetic energies of several million electron volts, nuclear physics techniques can be used to remove interferences from the ions of interest. Techniques for the extraction of iodine from a variety of samples including calcium carbonate, seawater, plants and animals have been developed. Results from many of these samples have been submitted as part of a laboratory inter-comparison exercise. As an application of oceanographic tracer studies, ¹²⁹I has been measured in a banded coral that lived in the southern Pacific Ocean for more than 50 years. The results of this study confirm that iodine is fixed within the coral skeleton upon deposition, and that these measurements record the history of iodine levels in the surface waters of the ocean over time. ¹²⁹I may also be a good indicator of past changes in the galactic cosmic ray flux and the magnitude and orientation of Earth's magnetic field. A 40 kyr iodine record extracted from a stalagmite in the Bahama Islands shows varying ¹²⁹I concentrations that may be linked to such changes. This record is in agreement with other published records of ¹⁰Be and ³⁶Cl. As ¹²⁹I has a much longer half-life than other cosmogenically produced isotopes, there is a potential for extending these records much further back in time.