5,838 research outputs found
Overlay copy technique to provide high-contrast electron micrographs for automatic metallographic analysis
Overlay copy technique to provide high-contrast electron micrographs for automatically determining quantitative microstructure of multiphase specimen
Comparison of selected submicron powder blending methods for dispersion alloys
Wet and dry blending nickel-aluminum oxide submicron powders for dispersion-strengthened alloy
A wider audience: Turning VLBI into a survey instrument
Radio observations using the Very Long Baseline Interferometry (VLBI)
technique typically have fields of view of only a few arcseconds, due to the
computational problems inherent in imaging larger fields. Furthermore,
sensitivity limitations restrict observations to very compact and bright
objects, which are few and far between on the sky. Thus, while most branches of
observational astronomy can carry out sensitive, wide-field surveys, VLBI
observations are limited to targeted observations of carefully selected
objects. However, recent advances in technology have made it possible to carry
out the computations required to target hundreds of sources simultaneously.
Furthermore, sensitivity upgrades have dramatically increased the number of
objects accessible to VLBI observations. The combination of these two
developments have enhanced the survey capabilities of VLBI observations such
that it is now possible to observe (almost) any point in the sky with
milli-arcsecond resolution. In this talk I review the development of wide-field
VLBI, which has made significant progress over the last three years.Comment: Invited review at the General Assembly of the Astronomische
Gesellschaf
Hysteretic ac losses in a superconductor strip between flat magnetic shields
Hysteretic ac losses in a thin, current-carrying superconductor strip located
between two flat magnetic shields of infinite permeability are calculated using
Bean's model of the critical state. For the shields oriented parallel to the
plane of the strip, penetration of the self-induced magnetic field is enhanced,
and the current dependence of the ac loss resembles that in an isolated
superconductor slab, whereas for the shields oriented perpendicular to the
plane of the strip, penetration of the self-induced magnetic field is impaired,
and the current dependence of the ac loss is similar to that in a
superconductor strip flanked by two parallel superconducting shields. Thus,
hysteretic ac losses can strongly augment or, respectively, wane when the
shields approach the strip.Comment: 9 pages, 5 figures, submitted to PR
Local Government Risk Management Handbook
This handbook originated in a series of risk management workshops developed by the authors in 1980 as part of a special project in local government technology innovation at the University of Tennessee\u27s Municipal Technical Advisory Service. The workshops, funded in part by a grant from the National Science Foundation, were sufficiently well-received that the authors decided to expand workshop materials into a practical guide to assist local governments to establish and operate risk management programs. Initially, each of the three authors drafted specific chapters or sections for the Handbook. Dr. Dotterweich was responsible for writing sections on the concepts and principles of risk management, Dr. Sinclair for sections on insurance and departmental exposures and for many of the items in the Appendices, and Dr. Norris for sections on local government functions and departmental exposures. Dr. Norris also had overall responsibility for compiling and editing all sections into a single, hopefully coherent, manuscript. The manuscript was then reviewed by the authors with changes made where required. The result is a fully cooperative product which bears the imprimatur of all three authors, regardless of initial assignment
ac Losses in a Finite Z Stack Using an Anisotropic Homogeneous-Medium Approximation
A finite stack of thin superconducting tapes, all carrying a fixed current I,
can be approximated by an anisotropic superconducting bar with critical current
density Jc=Ic/2aD, where Ic is the critical current of each tape, 2a is the
tape width, and D is the tape-to-tape periodicity. The current density J must
obey the constraint \int J dx = I/D, where the tapes lie parallel to the x axis
and are stacked along the z axis. We suppose that Jc is independent of field
(Bean approximation) and look for a solution to the critical state for
arbitrary height 2b of the stack. For c<|x|<a we have J=Jc, and for |x|<c the
critical state requires that Bz=0. We show that this implies \partial
J/\partial x=0 in the central region. Setting c as a constant (independent of
z) results in field profiles remarkably close to the desired one (Bz=0 for
|x|<c) as long as the aspect ratio b/a is not too small. We evaluate various
criteria for choosing c, and we show that the calculated hysteretic losses
depend only weakly on how c is chosen. We argue that for small D/a the
anisotropic homogeneous-medium approximation gives a reasonably accurate
estimate of the ac losses in a finite Z stack. The results for a Z stack can be
used to calculate the transport losses in a pancake coil wound with
superconducting tape.Comment: 21 pages, 17 figures, accepted by Supercond. Sci. Techno
Source amplitudes for active exterior cloaking
The active cloak comprises a discrete set of multipole sources that
destructively interfere with an incident time harmonic scalar wave to produce
zero total field over a finite spatial region. For a given number of sources
and their positions in two dimensions it is shown that the multipole amplitudes
can be expressed as infinite sums of the coefficients of the incident wave
decomposed into regular Bessel functions. The field generated by the active
sources vanishes in the infinite region exterior to a set of circles defined by
the relative positions of the sources. The results provide a direct solution to
the inverse problem of determining the source amplitudes. They also define a
broad class of non-radiating discrete sources.Comment: 21 pages, 17 figure
Further Evidence for a Merger Origin for the Thick Disk: Galactic Stars Along Lines-of-sight to Dwarf Spheroidal Galaxies
The history of the Milky Way Galaxy is written in the properties of its
stellar populations. Here we analyse stars observed as part of surveys of local
dwarf spheroidal galaxies, but which from their kinematics are highly probable
to be non-members. The selection function -- designed to target metal-poor
giants in the dwarf galaxies, at distances of ~100kpc -- includes F-M dwarfs in
the Milky Way, at distances of up to several kpc. Thestars whose motions are
analysed here lie in the cardinal directions of Galactic longitude l ~ 270 and
l ~ 90, where the radial velocity is sensitive to the orbital rotational
velocity. We demonstrate that the faint F/G stars contain a significant
population with V_phi ~ 100km/s, similar to that found by a targeted, but
limited in areal coverage, survey of thick-disk/halo stars by Gilmore, Wyse &
Norris (2002). This value of mean orbital rotation does not match either the
canonical thick disk or the stellar halo. We argue that this population,
detected at both l ~ 270 and l ~ 90, has the expected properties of `satellite
debris' in the thick-disk/halo interface, which we interpret as remnants of the
merger that heated a pre-existing thin disk to form the thick disk.Comment: Accepted, Astrophysical Journal Letter
- …