31,486 research outputs found
Temperature effect on space charge dynamics in XLPE insulation
This paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kVdc/mm under four temperature 25 oC, 50 oC, 70 oC and 90 oC for 24 hours. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The dc threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown
The effect of degassing on morphology and space charge
It is believed that space charge buildup in cross-linked polyethylene (XLPE) insulation is the main cause for premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is one of the material treatment process commonly employ in cable manufacturing to improve insulation performance. In this paper, investigation on the effect of degassing period has on the morphology and space charge was carried out. Planar XLPE samples of the same composite were subjected to different degassing time. It is discovered that apart from removing volatile by-products, degassing also anneal XLPE material; changing the morphology as a result
Evaluation of materials and design modifications for aircraft brakes
A test program is described which was carried out to evaluate several proposed design modifications and several high-temperature friction materials for use in aircraft disk brakes. The evaluation program was carried out on a specially built test apparatus utilizing a disk brake and wheel half from a small het aircraft. The apparatus enabled control of brake pressure, velocity, and braking time. Tests were run under both constant and variable velocity conditions and covered a kinetic energy range similar to that encountered in aircraft brake service. The results of the design evaluation program showed that some improvement in brake performance can be realized by making design changes in the components of the brake containing friction material. The materials evaluation showed that two friction materials show potential for use in aircraft disk brakes. One of the materials is a nickel-based sintered composite, while the other is a molybdenum-based material. Both materials show much lower wear rates than conventional copper-based materials and are better able to withstand the high temperatures encountered during braking. Additional materials improvement is necessary since both materials show a significant negative slope of the friction-velocity curve at low velocities
Multi-wavelength and black hole mass properties of Low Luminosity Active Nuclei
We investigate the relation between the X-ray nuclear emission, optical
emission line, radio luminosity and black hole mass for a sample of nearby
Seyfert galaxies. Strong linear correlations between the 2-10 keV and [OIII],
radio luminosities have been found, showing the same slopes found in quasars
and luminous Seyfert galaxies, thus implying independence from the level of
nuclear activity displayed by the sources. Moreover, despite the wide range of
Eddington ratios (L/L(Edd)) tested here (six orders of magnitude, from 0.1 down
to 10^(-7), no correlation is found between the X-ray, optical emission lines,
radio luminosities and the black hole mass. These results suggest that low
luminosity Seyfert galaxies are a scaled down version of luminous AGN and
probably are powered by the same physical processes.Comment: 6 pages, 3 figures, contributed talk presented at the Workshop "The
multicoloured landscape of compact objects and their explosive origin",
Cefalu' (Sicily), 11-24 June 2006, to be published by AI
Circumbinary Molecular Rings Around Young Stars in Orion
We present high angular resolution 1.3 mm continuum, methyl cyanide molecular
line, and 7 mm continuum observations made with the Submillimeter Array and the
Very Large Array, toward the most highly obscured and southern part of the
massive star forming region OMC1S located behind the Orion Nebula. We find two
flattened and rotating molecular structures with sizes of a few hundred
astronomical units suggestive of circumbinary molecular rings produced by the
presence of two stars with very compact circumstellar disks with sizes and
separations of about 50 AU, associated with the young stellar objects 139-409
and 134-411. Furthermore, these two circumbinary rotating rings are related to
two compact and bright {\it hot molecular cores}. The dynamic mass of the
binary systems obtained from our data are 4 M for 139-409 and
0.5 M for 134-411. This result supports the idea that
intermediate-mass stars will form through {\it circumstellar disks} and
jets/outflows, as the low mass stars do. Furthermore, when intermediate-mass
stars are in multiple systems they seem to form a circumbinary ring similar to
those seen in young, multiple low-mass systems (e.g., GG Tau and UY Aur).Comment: Accepted by Astronomy and Astrophysic
VLA Imaging of the Disk Surrounding the Nearby Young Star TW Hya
The TW Hya system is perhaps the closest analog to the early solar nebula. We
have used the Very Large Array to image TW Hya at wavelengths of 7mm and 3.6 cm
with resolutions 0.1 arcseconds (about 5 AU) and 1.0 arcseconds (about 50 AU),
respectively. The 7mm emission is extended and appears dominated by a dusty
disk of radius larger than 50 AU surrounding the star. The 3.6 cm emission is
unresolved and likely arises from an ionized wind or gyrosynchrotron activity.
The dust spectrum and spatially resolved 7mm images of the TW Hya disk are
fitted by a simple model with temperature and surface density described by
radial power laws, and . These
properties are consistent with an irradiated gaseous accretion disk of mass
with an accretion rate and viscosity parameter . The estimates of
mass and mass accretion rates are uncertain as the gas-to-dust ratio in the TW
Hya disk may have evolved from the standard interstellar value.Comment: 13 pages, 3 figures, accepted by ApJ Letter
A large accretion disk of extreme eccentricity in the TDE ASASSN-14li
In the canonical model for tidal disruption events (TDEs), the stellar debris
circularizes quickly to form an accretion disk of size about twice the orbital
pericenter of the star. Most TDEs and candidates discovered in the optical/UV
have broad optical emission lines with complex and diverse profiles of puzzling
origin. Liu et al. recently developed a relativistic elliptical disk model of
constant eccentricity in radius for the broad optical emission lines of TDEs
and well reproduced the double-peaked line profiles of the TDE candidate
PTF09djl with a large and extremely eccentric accretion disk. In this paper, we
show that the optical emission lines of the TDE ASASSN-14li with radically
different profiles are well modelled with the relativistic elliptical disk
model, too. The accretion disk of ASASSN-14li has an eccentricity 0.97 and
semimajor axis of 847 times the Schwarzschild radius (r_S) of the black hole
(BH). It forms as the consequence of tidal disruption of a star passing by a
massive BH with orbital pericenter 25r_S. The optical emission lines of
ASASSN-14li are powered by an extended X-ray source of flat radial distribution
overlapping the bulk of the accretion disk and the single-peaked asymmetric
line profiles are mainly due to the orbital motion of the emitting matter
within the disk plane of inclination about 26\degr and of pericenter
orientation closely toward the observer. Our results suggest that modelling the
complex line profiles is powerful in probing the structures of accretion disks
and coronal X-ray sources in TDEs.Comment: 10 pages, 8 figures, accepted for publication in the MNRA
Efficient out-coupling and beaming of Tamm optical states via surface plasmon polariton excitation
We present evidence of optical Tamm states to surface plasmon polariton (SPP) coupling. We experimentally demonstrate that for a Bragg stack with a thin metal layer on the surface, hybrid Tamm-SPP modes may be excited when a grating on the air-metal interface is introduced. Out-coupling via the grating to free space propagation is shown to enhance the transmission as well as the directionality and polarization selection for the transmitted beam. We suggest that this system will be useful on those devices, where a metallic electrical contact as well as beaming and polarization control is needed
Multiplpe Choice Minority Game With Different Publicly Known Histories
In the standard Minority Game, players use historical minority choices as the
sole public information to pick one out of the two alternatives. However,
publishing historical minority choices is not the only way to present global
system information to players when more than two alternatives are available.
Thus, it is instructive to study the dynamics and cooperative behaviors of this
extended game as a function of the global information provided. We numerically
find that although the system dynamics depends on the kind of public
information given to the players, the degree of cooperation follows the same
trend as that of the standard Minority Game. We also explain most of our
findings by the crowd-anticrowd theory.Comment: Extensively revised, to appear in New J Phys, 7 pages with 4 figure
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