1,611 research outputs found
Negative-resistance models for parametrically flux-pumped superconducting quantum interference devices
A Superconducting QUantum Interference Device (SQUID) modulated by a fast
oscillating magnetic flux can be used as a parametric amplifier, providing gain
with very little added noise. Here, we develop linearized models to describe
the parametrically flux-pumped SQUID in terms of an impedance. An unpumped
SQUID acts as an inductance, the Josephson inductance, whereas a flux-pumped
SQUID develops an additional, parallel element which we have coined the
``pumpistor.'' Parametric gain can be understood as a result of a negative
resistance of the pumpistor. In the degenerate case, the gain is sensitive to
the relative phase between the pump and signal. In the nondegenerate case, gain
is independent of this phase.
We develop our models first for degenerate parametric pumping in the
three-wave and four-wave cases, where the pump frequency is either twice or
equal to the signal frequency, respectively. We then derive expressions for the
nondegenerate case where the pump frequency is not a multiple of the signal
frequency, where it becomes necessary to consider idler tones which develop.
For the nondegenerate three-wave case, we present an intuitive picture for a
parametric amplifier containing a flux-pumped SQUID where current at the signal
frequency depends upon the load impedance at an idler frequency. This
understanding provides insight and readily testable predictions of circuits
containing flux-pumped SQUIDs.Comment: 27 pages, 6 figures, 1 tabl
Mg I emission lines at 12 and 18 micrometer in K giants
The solar Mg I emission lines at 12 micrometer have already been observed and
analyzed well. Previous modeling attempts for other stars have, however, been
made only for Procyon and two cool evolved stars, with unsatisfactory results
for the latter. We present high-resolution observational spectra for the K
giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines
at 12 micrometer as compared to the Sun. We also present the first observed
stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and
presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ
standard non-LTE modeling for trace elements in cool stellar photospheres. We
compute model atmospheres with the MARCS code, apply a comprehensive magnesium
model atom, and use the radiative transfer code MULTI to solve for the
magnesium occupation numbers in statistical equilibrium. We successfully
reproduce the observed Mg I emission lines simultaneously in the giants and in
the Sun, but show how the computed line profiles depend critically on atomic
input data and how the inclusion of energy levels with n > 9 and collisions
with neutral hydrogen are necessary to obtain reasonable fits.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
The pumpistor: a linearized model of a flux-pumped SQUID for use as a negative-resistance parametric amplifier
We describe a circuit model for a flux-driven SQUID. This is useful for
developing insight into how these devices perform as active elements in
parametric amplifiers. The key concept is that frequency mixing in a
flux-pumped SQUID allows for the appearance of an effective negative
resistance. In the three-wave, degenerate case treated here, a negative
resistance appears only over a certain range of allowed input signal phase.
This model readily lends itself to testable predictions of more complicated
circuits.Comment: 4 pages, 3 figure
Constraints On Porosity And Mass Loss In O-Star Winds From The Modeling Of X-Ray Emission Line Profile Shapes
We fit X-ray emission line profiles in high resolution XMM-Newton and Chandra grating spectra of the early O supergiant zeta Pup with models that include the effects of porosity in the stellar wind. We explore the effects of porosity due to both spherical and flattened clumps. We find that porosity models with flattened clumps oriented parallel to the photosphere provide poor fits to observed line shapes. However, porosity models with isotropic clumps can provide acceptable fits to observed line shapes, but only if the porosity effect is moderate. We quantify the degeneracy between porosity effects from isotropic clumps and the mass-loss rate inferred from the X-ray line shapes, and we show that only modest increases in the mass-loss rate (less than or similar to 40%) are allowed if moderate porosity effects (h(infinity) less than or similar to R-*) are assumed to be important. Large porosity lengths, and thus strong porosity effects, are ruled out regardless of assumptions about clump shape. Thus, X-ray mass-loss rate estimates are relatively insensitive to both optically thin and optically thick clumping. This supports the use of X-ray spectroscopy as a mass-loss rate calibration for bright, nearby O stars
A Generalized Porosity Formalism For Isotropic And Anisotropic Effective Opacity And Its Effects On X-Ray Line Attenuation In Clumped O Star Winds
We present a generalized formalism for treating the porosity-associated reduction in continuum opacity that occurs when individual clumps in a stochastic medium become optically thick. As in previous work, we concentrate on developing bridging laws between the limits of optically thin and thick clumps. We consider geometries resulting in either isotropic or anisotropic effective opacity, and, in addition to an idealized model in which all clumps have the same local overdensity and scale, we also treat an ensemble of clumps with optical depths set by Markovian statistics. This formalism is then applied to the specific case of boundfree absorption of X-rays in hot star winds, a process not directly affected by clumping in the optically thin limit. We find that the Markov model gives surprisingly similar results to those found previously for the single-clump model, suggesting that porous opacity is not very sensitive to details of the assumed clump distribution function. Further, an anisotropic effective opacity favours escape of X-rays emitted in the tangential direction (the venetian blind effect), resulting in a bump of higher flux close to line centre as compared to profiles computed from isotropic porosity models. We demonstrate how this characteristic line shape may be used to diagnose the clump geometry, and we confirm previous results that for optically thick clumping to significantly influence X-ray line profiles, very large porosity lengths, defined as the mean free path between clumps, are required. Moreover, we present the first X-ray line profiles computed directly from line-driven instability simulations using a 3D patch method, and find that porosity effects from such models also are very small. This further supports the view that porosity has, at most, a marginal effect on X-ray line diagnostics in O stars, and therefore that these diagnostics do indeed provide a good clumping insensitive method for deriving O star mass-loss rates
On the semi-classical analysis of the groundstate energy of the Dirichlet Pauli operator in non-simply connected domains
We consider the Dirichlet Pauli operator in bounded connected domains in the
plane, with a semi-classical parameter. We show, in particular, that the ground
state energy of this Pauli operator will be exponentially small as the
semi-classical parameter tends to zero and estimate this decay rate. This
extends our results, discussing the results of a recent paper by
Ekholm--Kova\v{r}\'ik--Portmann, to include also non-simply connected domains.Comment: 15 pages, 4 figure
Clusters of eigenvalues for the magnetic Laplacian with Robin condition
We study the Schrodinger operator with a constant magnetic field in the exterior of a compact domain in Euclidean space. Functions in the domain of the operator are subject to a boundary condition of the third type (a magnetic Robin condition). In addition to the Landau levels, we obtain that the spectrum of this operator consists of clusters of eigenvalues around the Landau levels and that they do accumulate to the Landau levels from below. We give a precise asymptotic formula for the rate of accumulation of eigenvalues in these clusters, which is independent of the boundary condition. Published by AIP Publishing
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