27,251 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
Atmospheres and radiating surfaces of neutron stars with strong magnetic fields
We review the current status of the theory of thermal emission from the
surface layers of neutron stars with strong magnetic fields G, including formation of the spectrum in a partially ionized
atmosphere and at a condensed surface. In particular, we describe recent
progress in modeling partially ionized atmospheres of central compact objects
in supernova remnants, which may have moderately strong fields G. Special attention is given to polarization of thermal
radiation emitted by a neutron star surface. Finally, we briefly describe
applications of the theory to observations of thermally emitting isolated
neutron stars.Comment: 27 pages, 5 figures, invited review at the conference "The Modern
Physics of Compact Stars 2015" (Yerevan, Armenia, Sept. 30 - Oct. 3, 2015),
edited by R. Avagyan, A. Saharian, and A. Sedrakian. In v.2, a citation
(Ref.114) is correcte
Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars
There is observational evidence that central compact objects (CCOs) in
supernova remnants have moderately strong magnetic fields G.
Meanwhile, available models of partially ionized hydrogen atmospheres of
neutron stars with strong magnetic fields are restricted to
G. We extend the equation of state and radiative opacities, presented in
previous papers for 10^{12}\mbox{ G}\lesssim B \lesssim 10^{15} G, to weaker
fields. An equation of state and radiative opacities for a partially ionized
hydrogen plasma are obtained at magnetic fields , temperatures , and
densities typical for atmospheres of CCOs and other isolated neutron
stars with moderately strong magnetic fields. The first- and second-order
thermodynamic functions, monochromatic radiative opacities, and Rosseland mean
opacities are calculated and tabulated, taking account of partial ionization,
for 3\times10^{10}\mbox{ G}\lesssim B\lesssim 10^{12} G, K K, and a wide range of densities. Atmosphere models and spectra
are calculated to verify the applicability of the results and to determine the
range of magnetic fields and effective temperatures where the incomplete
ionization of the hydrogen plasma is important.Comment: 11 pages, 7 figures, accepted for publication in A&
Two--Electron Atoms in Short Intense Laser Pulses
We discuss a method of solving the time dependent Schrodinger equation for
atoms with two active electrons in a strong laser field, which we used in a
previous paper [A. Scrinzi and B. Piraux, Phys. Rev. A 56, R13 (1997)] to
calculate ionization, double excitation and harmonic generation in Helium by
short laser pulses. The method employs complex scaling and an expansion in an
explicitly correlated basis. Convergence of the calculations is documented and
error estimates are provided. The results for Helium at peak intensities up to
10^15 W/cm^2 and wave length 248 nm are accurate to at least 10 %. Similarly
accurate calculations are presented for electron detachment and double
excitation of the negative hydrogen ion.Comment: 14 pages, including figure
Investigation of defect cavities formed in three-dimensional woodpile photonic crystals
We report the optimisation of optical properties of single defects in
three-dimensional (3D) face-centred-cubic (FCC) woodpile photonic crystal (PC)
cavities by using plane-wave expansion (PWE) and finite-difference time-domain
(FDTD) methods. By optimising the dimensions of a 3D woodpile PC, wide photonic
band gaps (PBG) are created. Optical cavities with resonances in the bandgap
arise when point defects are introduced in the crystal. Three types of single
defects are investigated in high refractive index contrast (Gallium
Phosphide-Air) woodpile structures and Q-factors and mode volumes ()
of the resonant cavity modes are calculated. We show that, by introducing an
air buffer around a single defect, smaller mode volumes can be obtained. We
demonstrate high Q-factors up to 700000 and cavity volumes down to
. The estimates of and are then used to
quantify the enhancement of spontaneous emission and the possibility of
achieving strong coupling with nitrogen-vacancy (NV) colour centres in diamond.Comment: 12 pages, 11 figure
Numerical analysis of the Iosipescu specimen for composite materials
A finite element analysis of the Iosipescu shear tests for unidirectional and cross-ply composites is presented. It is shown that an iterative analysis procedure must be used to model the fixture-specimen kinematics. The correction factors which are needed to compensate for the nonuniformity of stress distribution in calculating shear modulus are shown to be dependent on the material orthotropic ratio and the finite element loading models. Test section strain distributions representative of typical graphite-epoxy specimens are also presented
Micro Balloon Actuators for Aerodynamic Control
A robust, large-force, large-deflection micro balloon actuator for aerodynamic (manoeuvring) control of transonic aircraft has been developed. Using a novel process, high yield linear arrays of silicone balloons on a robust silicon substrate have been fabricated that can deflect vertically in excess of one mm. Balloon actuators have been tested under cyclic conditions to assess reliability. The actuators have been characterized in a wind tunnel to assess their suitability as aerodynamic control surfaces and flight-tested on a jet fighter to assess their resistance to varied temperatures and pressures at high velocity
A comparative study of the electronic and magnetic properties of BaFe_2As_2 and BaMn_2As_2 using the Gutzwiller approximation
To elucidate the role played by the transition metal ion in the pnictide
materials, we compare the electronic and magnetic properties of BaFe_{2}As_{2}
with BaMn_{2}As_{2}. To this end we employ the LDA+Gutzwiller method to analyze
the mass renormalizations and the size of the ordered magnetic moment of the
two systems. We study a model that contains all five transition metal 3d
orbitals together with the Ba-5d and As-4p states (ddp-model) and compare these
results with a downfolded model that consists of Fe/Mn d-states only (d-model).
Electronic correlations are treated using the multiband Gutzwiller
approximation. The paramagnetic phase has also been investigated using
LDA+Gutzwiller method with electron density self-consistency. The
renormalization factors for the correlated Mn 3d orbitals in the paramagnetic
phase of BaMn_{2}As_{2} are shown to be generally smaller than those of
BaFe_{2}As_{2}, which indicates that BaMn_{2}As_{2} has stronger electron
correlation effect than BaFe_{2}As_{2}. The screening effect of the main As 4p
electrons to the correlated Fe/Mn 3d electrons is evident by the systematic
shift of the results to larger Hund's rule coupling J side from the ddp-model
compared with those from the d-model. A gradual transition from paramagnetic
state to the antiferromagnetic ground state with increasing J is obtained for
the models of BaFe_{2}As_{2} which has a small experimental magnetic moment;
while a rather sharp jump occurs for the models of BaMn_{2}As_{2}, which has a
large experimental magnetic moment. The key difference between the two systems
is shown to be the d-level occupation. BaMn_{2}As_{2}, with approximately five
d-electrons per Mn atom, is for same values of the electron correlations closer
to the transition to a Mott insulating state than BaFe_{2}As_{2}. Here an
orbitally selective transition, required for a system with close to six
electrons only occurs at significantly larger values for the Coulomb
interactions
Atmospheres and Spectra of Strongly Magnetized Neutron Stars -- III. Partially Ionized Hydrogen Models
We construct partially ionized hydrogen atmosphere models for magnetized
neutron stars in radiative equilibrium with surface fields B=10^12-5 \times
10^14 G and effective temperatures T_eff \sim a few \times 10^5-10^6 K. These
models are based on the latest equation of state and opacity results for
magnetized, partially ionized hydrogen plasmas that take into account various
magnetic and dense medium effects. The atmospheres directly determine the
characteristics of thermal emission from isolated neutron stars. For the models
with B=10^12-10^13 G, the spectral features due to neutral atoms lie at extreme
UV and very soft X-ray energy bands and therefore are difficult to observe.
However, the continuum flux is also different from the fully ionized case,
especially at lower energies. For the superstrong field models (B\ga 10^14 G),
we show that the vacuum polarization effect not only suppresses the proton
cyclotron line as shown previously, but also suppresses spectral features due
to bound species; therefore spectral lines or features in thermal radiation are
more difficult to observe when the neutron star magnetic field is \ga 10^14 G.Comment: 12 pages, 10 figures; ApJ, accepted (v599: Dec 20, 2003
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