1,336 research outputs found
The epe-d two year timer
Solid state timing apparatus to disconnect solar cells from batteries after two year perio
Navigational and Environmental Measurement System (NEMS)
The NEMS concept and design were initiated from the need to measure and record positional and environmental information during aircraft flights of developmental science research instrumentation. The unit was designed as a stand-alone system which could serve the needs of instruments whose developmental nature did not justify the cost and complexity of including these measurements within the instrument data system. Initially, the system was comprised of a Loran-C receiver and a portable IBM compatible computer recording position and time. Later, the system was interfaced with the Wallops aircraft inertial navigation system (INS), and various other sensors were supplied and shared by the Goddard science users. Real-time position mapping on video monitors was added for investigator's use and information. In 1987, the use of a Global Positioning System (GPS) receiver was included in some missions. A total configuration of the system and the various sensors which can be incorporated are shown
Self-field effects upon the critical current density of flat superconducting strips
We develop a general theory to account self-consistently for self-field
effects upon the average transport critical current density Jc of a flat
type-II superconducting strip in the mixed state when the bulk pinning is
characterized by a field-dependent depinning critical current density Jp(B),
where B is the local magnetic flux density. We first consider the possibility
of both bulk and edge-pinning contributions but conclude that bulk pinning
dominates over geometrical edge-barrier effects in state-of-the-art YBCO films
and prototype second-generation coated conductors. We apply our theory using
the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba)
as a function of a perpendicular applied magnetic induction Ba and show how
Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to
JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux
density zero at the strip's edge. However, Jc(Ba) is suppressed relative to
JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring
when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference
Flux domes in superconducting films without edges
Domelike magnetic-flux-density distributions previously have been observed
experimentally and analyzed theoretically in superconducting films with edges,
such as in strips and thin plates. Such flux domes have been explained as
arising from a combination of strong geometric barriers and weak bulk pinning.
In this paper we predict that, even in films with bulk pinning, flux domes also
occur when vortices and antivortices are produced far from the film edges
underneath current-carrying wires, coils, or permanent magnets placed above the
film. Vortex-antivortex pairs penetrating through the film are generated when
the magnetic field parallel to the surface exceeds H_{c1}+K_c, where H_{c1} is
the lower critical field and K_c = j_c d is the critical sheet-current density
(the product of the bulk critical current density j_c and the film thickness
d). The vortices and antivortices move in opposite directions to locations
where they join others to create separated vortex and antivortex flux domes. We
consider a simple arrangement of a pair of current-carrying wires carrying
current I_0 in opposite directions and calculate the magnetic-field and
current-density distributions as a function of I_0 both in the
bulk-pinning-free case (K_c = 0) and in the presence of bulk pinning,
characterized by a field-independent critical sheet-current density (K_c > 0).Comment: 15 pages, 23 figure
Josephson junctions in thin and narrow rectangular superconducting strips
I consider a Josephson junction crossing the middle of a thin rectangular
superconducting strip of length L and width W subjected to a perpendicular
magnetic induction B. I calculate the spatial dependence of the gauge-invariant
phase difference across the junction and the resulting B dependence of the
critical current Ic(B).Comment: 4 pages, 6 figures, revised following referee's comment
TeV Gamma Rays from Geminga and the Origin of the GeV Positron Excess
The Geminga pulsar has long been one of the most intriguing MeV-GeV gamma-ray
point sources. We examine the implications of the recent Milagro detection of
extended, multi-TeV gamma-ray emission from Geminga, finding that this reveals
the existence of an ancient, powerful cosmic-ray accelerator that can plausibly
account for the multi-GeV positron excess that has evaded explanation. We
explore a number of testable predictions for gamma-ray and electron/positron
experiments (up to ~100 TeV) that can confirm the first "direct" detection of a
cosmic-ray source.Comment: 4 pages and 3 figures; Minor revisions, accepted for publication in
Physical Review Letter
Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements
In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova
Science Publishers, Inc., NY, 2003
Individualized initiation of statin therapy determined by baseline LDL-C: Are you more likely to achieve goal LDL-C?
Cardiovascular disease remains the leading cause of death in the world. A significant amount of clinical data are available to demonstrate the positive influence that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin) therapy has on slowing the progression of cardiovascular disease and improving clinical outcomes. Achieving the treatment goals for cholesterol in cardiovascular disease continues to present challenges. Recent clinical trial information is available assessing the use of more aggressive initial doses of statin therapy based on initial low-density lipoprotein cholesterol (LDL-C) measurements in an attempt to reach treatment goals sooner. Six clinical trials assessed low-, moderate- and high-risk individuals as well as those with type 2 diabetes mellitus to determine if this treatment approach is both safe and effective. The studies concluded that initial dosing of statin therapy determined by a baseline LDL-C measurement demonstrates good achievement in reaching treatment goals and does not result in a higher rate of adverse effects
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