113,684 research outputs found
Acceleration Rates and Injection Efficiencies in Oblique Shocks
The rate at which particles are accelerated by the first-order Fermi
mechanism in shocks depends on the angle, \teq{\Tbone}, that the upstream
magnetic field makes with the shock normal. The greater the obliquity the
greater the rate, and in quasi-perpendicular shocks rates can be hundreds of
times higher than those seen in parallel shocks. In many circumstances
pertaining to evolving shocks (\eg, supernova blast waves and interplanetary
traveling shocks), high acceleration rates imply high maximum particle energies
and obliquity effects may have important astrophysical consequences. However,
as is demonstrated here, the efficiency for injecting thermal particles into
the acceleration mechanism also depends strongly on obliquity and, in general,
varies inversely with \teq{\Tbone}. The degree of turbulence and the resulting
cross-field diffusion strongly influences both injection efficiency and
acceleration rates. The test particle \mc simulation of shock acceleration used
here assumes large-angle scattering, computes particle orbits exactly in
shocked, laminar, non-relativistic flows, and calculates the injection
efficiency as a function of obliquity, Mach number, and degree of turbulence.
We find that turbulence must be quite strong for high Mach number, highly
oblique shocks to inject significant numbers of thermal particles and that only
modest gains in acceleration rates can be expected for strong oblique shocks
over parallel ones if the only source of seed particles is the thermal
background.Comment: 24 pages including 6 encapsulated figures, as a compressed,
uuencoded, Postscript file. Accepted for publication in the Astrophysical
Journa
Immunity of information encoded in decoherence-free subspaces to particle loss
We demonstrate that for an ensemble of qudits, subjected to collective
decoherence in the form of perfectly correlated random SU(d) unitaries, quantum
superpositions stored in the decoherence free subspace are fully immune against
the removal of one particle. This provides a feasible scheme to protect quantum
information encoded in the polarization state of a sequence of photons against
both collective depolarization and one photon loss, and can be demonstrated
with photon quadruplets using currently available technology.Comment: to appear in Phys. Rev. A; 5 pages, 2 figures; content changed a bit
(the property demonstrated explicitly on a 4 qubit state
The rich cluster of galaxies ABCG 85. III. Analyzing the ABCG 85/87/89 complex
We present a combined X-ray and optical analysis of the ABCG 85/87/89 complex
of clusters of galaxies, based on the ROSAT PSPC image, optical photometric
catalogues (Slezak et al. 1998), and an optical redshift catalogue (Durret et
al. 1998). From this combined data set, we find striking alignments at all
scales at PA160\deg. At small scales, the cD galaxy in ABCG 85 and the
brightest galaxies in the cluster are aligned along this PA. At a larger scale,
X-ray emission defines a comparable PA south-southeast of ABCG 85 towards ABCG
87, with a patchy X-ray structure very different from the regular shape of the
optical galaxy distribution in ABCG 87. The galaxy velocities in the ABCG 87
region show the existence of subgroups, which all have an X-ray counterpart,
and seem to be falling onto ABCG 85 along a filament almost perpendicular to
the plane of the sky. To the west of ABCG 85, ABCG 89 appears as a significant
galaxy density enhancement, but is barely detected at X-ray wavelengths. The
galaxy velocities reveal that in fact this is not a cluster but two groups with
very different velocities superimposed along the line of sight. These two
groups appear to be located in intersecting sheets on opposite sides of a large
bubble. These data and their interpretation reinforce the cosmological scenario
in which matter, including galaxies, groups and gas, falls onto the cluster
along a filament.Comment: accepted for publication in Astronomy & Astrophysic
Salt-gradient Solar Ponds: Summary of US Department of Energy Sponsored Research
The solar pond research program conducted by the United States Department of Energy was discontinued after 1983. This document summarizes the results of the program, reviews the state of the art, and identifies the remaining outstanding issues. Solar ponds is a generic term but, in the context of this report, the term solar pond refers specifically to saltgradient solar pond. Several small research solar ponds have been built and successfully tested. Procedures for filling the pond, maintaining the gradient, adjusting the zone boundaries, and extracting heat were developed. Theories and models were developed and verified. The major remaining unknowns or issues involve the physical behavior of large ponds; i.e., wind mixing of the surface, lateral range or reach of horizontally injected fluids, ground thermal losses, and gradient zone boundary erosion caused by pumping fluid for heat extraction. These issues cannot be scaled and must be studied in a large outdoor solar pond
Leptogenesis in the two right-handed neutrino model revisited
We revisit leptogenesis in the minimal non-supersymmetric type I see-saw
mechanism with two right-handed (RH) neutrinos, including flavour effects and
allowing both RH neutrinos N_1 and N_2 to contribute, rather than just the
lightest RH neutrino N_1 that has hitherto been considered. By performing scans
over parameter space in terms of the single complex angle z of the orthogonal
matrix R, for a range of PMNS parameters, we find that in regions around z \sim
\pm \pi/2, for the case of a normal mass hierarchy, the N_2 contribution can
dominate the contribution to leptogenesis, allowing the lightest RH neutrino
mass to be decreased by about an order of magnitude in these regions, down to
M_1 \sim 1.3*10^11 GeV for vanishing initial N_2-abundance, with the numerical
results supported by analytic estimates. We show that the regions around z \sim
\pm \pi /2 correspond to light sequential dominance, so the new results in this
paper may be relevant to unified model building.Comment: 41 pages, 10 figures; v2 matches published version in PR
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The Only Eye Study (OnES): a qualitative study of surgeon experiences of only eye surgery and recommendations for patient safety
OBJECTIVE: Performing surgery on patients with only one seeing-eye, where complications may result in catastrophic vision loss, presents unique challenges for the ophthalmic care team. There is currently no evidence regarding how surgeons augment their care when treating only eye patients and no guidelines for how these patients should be managed in hospital eye services. This study aimed to explore ophthalmic surgeons' experiences of only eye surgery and perceptions of current practice.
DESIGN AND PARTICIPANTS: Ten ophthalmic surgeons were asked to relate their experiences and views on performing only eye surgery in indepth, semistructured interviews. Interviews were audio-recorded and transcribed. Qualitative data were subjected to thematic analysis to identify key themes.
SETTING: Hospital eye service.
RESULTS: Five key themes emerged relating to surgeons' experiences and perceptions of only eye surgery: (1) differences in approach to consent, (2) strategies for risk reduction, (3) unmet training needs, (4) value of surgical mentor and (5) emotional impact of unsuccessful outcomes. Recommendations for improving the surgical journey for both the patient and the surgeon related primarily to better recognition and understanding of the complexities inherent with only eye surgery.
CONCLUSIONS: Outcomes of only eye surgery may be improved through a number of methods, including development of purpose-designed training fellowships, adoption of stress-reducing strategies and enhancement of available support services. The findings identify emerging themes unique to only eye surgery and the need for guidelines on the provision of care for these high-stakes surgical patients
The non-linear transient behavior of second, third and fourth order phase-locked loops
Non-linear transient behavior of second, third, and fourth order phase-locked loop
Using a fuzzy inference system to control a pumped storage hydro plant
The paper discusses the development of a fuzzy inference system (FIS) based governor control for a pumped storage hydroelectric plant. The First Hydro Company's plant at Dinorwig in North Wales is the largest of its kind in Europe and is mainly used for frequency control of the UK electrical grid. In previous investigations, a detailed model of the plant was developed using MATLAB(R)/SIMULINK(R) and this is now being used to compare FIS governor operation with the proportional-integral-derivative (PID) controller currently used. The paper describes the development of an FIS governor, and shows that its response to a step increase in load is superior to the PID under certain conditions of load. The paper proceeds to discuss the implications of these results in view of the possible practical application of an FIS governor at the Dinorwig plant
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