259 research outputs found
Experimental investigation of a magnetically balanced arc in a transverse argon flow
"Reprinted from Journal of Engineering for Power January, 1966.""An experimental study of an electric arc in crossed convective and magnetic fields has been made. An electric arc was established across a rectangular test section through which argon gas was flowing at approximately atmospheric pressure and velocities up to 100 m/sec. Magnetic field strengths up to 3 webers/m2, oriented so that the Lorentz force opposed the convective force on the arc, were applied perpendicular lo both the arc and the direction of the argon gas flow. The test apparatus and the procedure used to obtain the experimental relationship between the velocity of the argon flow and the balancing magnetic field are described. An analysis which assumed the magnetically balanced arc lo be a gaseous cylinder positioned between the electrodes and with a diameter varying directly as the arc current satisfactorily explained the observed dependence of the balancing magnetic field on the gas velocity."--Page 1
Chemical telemetry of OH observed to measure interstellar magnetic fields
We present models for the chemistry in gas moving towards the ionization
front of an HII region. When it is far from the ionization front, the gas is
highly depleted of elements more massive than helium. However, as it approaches
the ionization front, ices are destroyed and species formed on the grain
surfaces are injected into the gas phase. Photodissociation removes gas phase
molecular species as the gas flows towards the ionization front. We identify
models for which the OH column densities are comparable to those measured in
observations undertaken to study the magnetic fields in star forming regions
and give results for the column densities of other species that should be
abundant if the observed OH arises through a combination of the liberation of
H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S.
Observations of these other species may help establish the nature of the OH
spatial distribution in the clouds, which is important for the interpretation
of the magnetic field results.Comment: 11 pages, 2 figures, accepted by Astrophysics and Space Scienc
Generalized harmonic formulation in spherical symmetry
In this pedagogically structured article, we describe a generalized harmonic
formulation of the Einstein equations in spherical symmetry which is regular at
the origin. The generalized harmonic approach has attracted significant
attention in numerical relativity over the past few years, especially as
applied to the problem of binary inspiral and merger. A key issue when using
the technique is the choice of the gauge source functions, and recent work has
provided several prescriptions for gauge drivers designed to evolve these
functions in a controlled way. We numerically investigate the parameter spaces
of some of these drivers in the context of fully non-linear collapse of a real,
massless scalar field, and determine nearly optimal parameter settings for
specific situations. Surprisingly, we find that many of the drivers that
perform well in 3+1 calculations that use Cartesian coordinates, are
considerably less effective in spherical symmetry, where some of them are, in
fact, unstable.Comment: 47 pages, 15 figures. v2: Minor corrections, including 2 added
references; journal version
Neural correlates of probabilistic category learning in patients with schizophrenia
Functional neuroimaging studies of probabilistic category learning in healthy adults report activation of cortical-striatal circuitry. Based on previous findings of normal learning rate concurrent with an overall performance deficit in patients with schizophrenia, we hypothesized that relative to healthy adults, patients with schizophrenia would display preserved caudate nucleus and abnormal prefrontal cortex activation during probabilistic category learning. Forty patients with schizophrenia receiving antipsychotic medication and 25 healthy participants were assessed on interleaved blocks of probabilistic category learning and control tasks while undergoing blood oxygenation level-dependent functional magnetic resonance imaging. In addition to the whole sample of patients with schizophrenia and healthy adults, a subset of patients and healthy adults matched for good learning was also compared. In the whole sample analysis, patients with schizophrenia displayed impaired performance in conjunction with normal learning rate relative to healthy adults. The matched comparison of patients and healthy adults classified as good learners revealed greater caudate and dorsolateral prefrontal cortex activity in the healthy adults and greater activation in a more rostral region of the dorsolateral prefrontal, cingulate, parahippocampal and parietal cortex in patients. These results demonstrate that successful probabilistic category learning can occur in the absence of normal frontal-striatal function. Based on analyses of the patients and healthy adults matched on learning and performance, a minority of patients with schizophrenia achieve successful probabilistic category learning and performance levels through differential activation of a circumscribed neural network which suggests a compensatory mechanism in patients showing successful learning. Copyright © 2009 Society for Neuroscience
Electromagnetic Dissociation of Nuclei in Heavy-Ion Collisions
Large discrepancies have been observed between measured Electromagnetic
Dissociation(ED) cross sections and the predictions of the semiclassical
Weiz\"acker-Williams-Fermi(WWF) method. In this paper, the validity of the
semiclassical approximation is examined. The total cross section for
electromagnetic excitation of a nuclear target by a spinless projectile is
calculated in first Born approximation, neglecting recoil. The final result is
expressed in terms of correlation functions and convoluted densities in
configuration space. The result agrees with the WWF approximation to leading
order(unretarded electric dipole approximation), but the method allows an
analytic evaluation of the cutoff, which is determined by the details of the
electric dipole transition charge density. Using the Goldhaber-Teller model of
that density, and uniform charge densities for both projectile and target, the
cutoff is determined for the total cross section in the nonrelativistic limit,
and found to be smaller than values currently used for ED calculations. In
addition, cross sections are calculated using a phenomenological momentum space
cutoff designed to model final state interactions. For moderate projectile
energies, the calculated ED cross section is found to be smaller than the
semiclassical result, in qualitative agreement with experiment.Comment: 28 page
A 750 mW, continuous-wave, solid-state laser source at 313 nm for cooling and manipulating trapped 9Be+ ions
We present a solid-state laser system that generates 750 mW of
continuous-wave single-frequency output at 313 nm. Sum-frequency generation
with fiber lasers at 1550 nm and 1051 nm produces up to 2 W at 626 nm. This
visible light is then converted to UV by cavity-enhanced second-harmonic
generation. The laser output can be tuned over a 495 GHz range, which includes
the 9Be+ laser cooling and repumping transitions. This is the first report of a
narrow-linewidth laser system with sufficient power to perform fault-tolerant
quantum-gate operations with trapped 9Be+ ions by use of stimulated Raman
transitions.Comment: 9 pages, 4 figure
Giant Gravitons - with Strings Attached (III)
We develop techniques to compute the one-loop anomalous dimensions of
operators in the super Yang-Mills theory that are dual to open
strings ending on boundstates of sphere giant gravitons. Our results, which are
applicable to excitations involving an arbitrary number of open strings,
generalize the single string results of hep-th/0701067. The open strings we
consider carry angular momentum on an S embedded in the S of the
AdSS background. The problem of computing the one loop anomalous
dimensions is replaced with the problem of diagonalizing an interacting Cuntz
oscillator Hamiltonian. Our Cuntz oscillator dynamics illustrates how the
Chan-Paton factors for open strings propagating on multiple branes can arise
dynamically.Comment: 66 pages; v2: improved presentatio
The index of the overlap Dirac operator on a discretized 2d non-commutative torus
The index, which is given in terms of the number of zero modes of the Dirac
operator with definite chirality, plays a central role in various topological
aspects of gauge theories. We investigate its properties in non-commutative
geometry. As a simple example, we consider the U(1) gauge theory on a
discretized 2d non-commutative torus, in which general classical solutions are
known. For such backgrounds we calculate the index of the overlap Dirac
operator satisfying the Ginsparg-Wilson relation. When the action is small, the
topological charge defined by a naive discretization takes approximately
integer values, and it agrees with the index as suggested by the index theorem.
Under the same condition, the value of the index turns out to be a multiple of
N, the size of the 2d lattice. By interpolating the classical solutions, we
construct explicit configurations, for which the index is of order 1, but the
action becomes of order N. Our results suggest that the probability of
obtaining a non-zero index vanishes in the continuum limit, unlike the
corresponding results in the commutative space.Comment: 22 pages, 8 figures, LaTeX, JHEP3.cls. v3:figures 1 and 2 improved
(all the solutions included),version published in JHE
Changes in synaptic transmission and protein expression in the brains of adult offspring after prenatal inhibition of the kynurenine pathway
During early brain development, N-methyl-d-aspartate (NMDA) receptors are involved in cell migration, neuritogenesis, axon guidance and synapse formation, but the mechanisms which regulate NMDA receptor density and function remain unclear. The kynurenine pathway of tryptophan metabolism includes an agonist (quinolinic acid) and an antagonist (kynurenic acid) at NMDA receptors and we have previously shown that inhibition of the pathway using the kynurenine-3-monoxygenase inhibitor Ro61-8048 in late gestation produces rapid changes in protein expression in the embryos and effects on synaptic transmission lasting until postnatal day 21 (P21). The present study sought to determine whether any of these effects are maintained into adulthood. After prenatal injections of Ro61-8048 the litter was allowed to develop to P60 when some offspring were euthanized and the brains removed for examination. Analysis of protein expression by Western blotting revealed significantly reduced expression of the GluN2A subunit (32%) and the morphogenetic protein sonic hedgehog (31%), with a 29% increase in the expression of doublecortin, a protein associated with neurogenesis. No changes were seen in mRNA abundance using quantitative real-time polymerase chain reaction. Neuronal excitability was normal in the CA1 region of hippocampal slices but paired-pulse stimulation revealed less inhibition at short interpulse intervals. The amount of long-term potentiation was decreased by 49% in treated pups and recovery after low-frequency stimulation was delayed. The results not only strengthen the view that basal, constitutive kynurenine metabolism is involved in normal brain development, but also show that changes induced prenatally can affect the brains of adult offspring and those changes are quite different from those seen previously at weaning (P21). Those changes may be mediated by altered expression of NMDAR subunits and sonic hedgehog
Typicality, Black Hole Microstates and Superconformal Field Theories
We analyze the structure of heavy multitrace BPS operators in N = 1
superconformal quiver gauge theories that arise on the worldvolume of D3-branes
on an affine toric cone. We exhibit a geometric procedure for counting heavy
mesonic operators with given U(1) charges. We show that for any fixed linear
combination of the U(1) charges, the entropy is maximized when the charges are
in certain ratios. This selects preferred directions in the charge space that
can be determined with the help of a piece of string. We show that almost all
heavy mesonic operators of fixed U(1) charges share a universal structure. This
universality reflects the properties of the dual extremal black holes whose
microstates they create. We also interpret our results in terms of typical
configurations of dual giant gravitons in AdS space.Comment: 40 pages + 3 appendices, 11 figure
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