16,522 research outputs found
A magnetospheric simulation at the space station
It is proposed that a strong magnet (terrella) be flown at or near the Space Station to create an artificial magnetosphere in a laboratory setting. The relative flow of the ionosphere past the terrella will constitute a plasma wind that will interact with the magnetic field of the terrella to produce a localized magnetosphere. This object could then be extensively studied using diagnostic probes attached to the Space Station, or with free flyers. The space and storage requirements would be minimal, since the experiment would be conducted outside the space station. The total equipment would consist of several terrella (with varying surface conductivities), approximately 3 small magnetometer/plasma diagnostic packages, and several gas canisters for upstream seeding. Power requirements would be approximately 60 watts. Several track mounted tethers, each approximately or 200 m long in length, with track parallel to the orbital motion and 100 m long, are also needed. Astronaut time needed would be minimal in the tethered configuration (approximately 4 man hours/week). A free flying configuration, while not needing the tether track, would require much more human interaction
Depolarization volume and correlation length in the homogenization of anisotropic dielectric composites
In conventional approaches to the homogenization of random particulate
composites, both the distribution and size of the component phase particles are
often inadequately taken into account. Commonly, the spatial distributions are
characterized by volume fraction alone, while the electromagnetic response of
each component particle is represented as a vanishingly small depolarization
volume. The strong-permittivity-fluctuation theory (SPFT) provides an
alternative approach to homogenization wherein a comprehensive description of
distributional statistics of the component phases is accommodated. The
bilocally-approximated SPFT is presented here for the anisotropic homogenized
composite which arises from component phases comprising ellipsoidal particles.
The distribution of the component phases is characterized by a two-point
correlation function and its associated correlation length. Each component
phase particle is represented as an ellipsoidal depolarization region of
nonzero volume. The effects of depolarization volume and correlation length are
investigated through considering representative numerical examples. It is
demonstrated that both the spatial extent of the component phase particles and
their spatial distributions are important factors in estimating coherent
scattering losses of the macroscopic field.Comment: Typographical error in eqn. 16 in WRM version is corrected in arxiv
versio
Non-degenerate, three-wave mixing with the Josephson ring modulator
The Josephson ring modulator (JRM) is a device, based on Josephson tunnel
junctions, capable of performing non-degenerate mixing in the microwave regime
without losses. The generic scattering matrix of the device is calculated by
solving coupled quantum Langevin equations. Its form shows that the device can
achieve quantum-limited noise performance both as an amplifier and a mixer.
Fundamental limitations on simultaneous optimization of performance metrics
like gain, bandwidth and dynamic range (including the effect of pump depletion)
are discussed. We also present three possible integrations of the JRM as the
active medium in a different electromagnetic environment. The resulting
circuits, named Josephson parametric converters (JPC), are discussed in detail,
and experimental data on their dynamic range are found to be in good agreement
with theoretical predictions. We also discuss future prospects and requisite
optimization of JPC as a preamplifier for qubit readout applications.Comment: 21 pages, 16 figures, 4 table
Why the braking indices of young pulsars are less than 3?
In this letter we discuss two possible reasons which cause the observed
braking indices n of young radio pulsars to be smaller than 3: (a) the evolving
spin-down model of the magnetic field component increases with
time; (b) the extrinsic braking torque model in which the tidal torques exerted
on the pulsar by the fallback disk, and carries away the spin angular momentum
from the pulsar. Based on some simple assumptions, we derive the expression of
the braking indices, and calculate the spin-down evolutionary tracks of pulsars
for different input parameters.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter
Speeding up simulations of relativistic systems using an optimal boosted frame
It can be computationally advantageous to perform computer simulations in a
Lorentz boosted frame for a certain class of systems. However, even if the
computer model relies on a covariant set of equations, it has been pointed out
that algorithmic difficulties related to discretization errors may have to be
overcome in order to take full advantage of the potential speedup. We summarize
the findings, the difficulties and their solutions, and show that the technique
enables simulations important to several areas of accelerator physics that are
otherwise problematic, including self-consistent modeling in three-dimensions
of laser wakefield accelerator stages at energies of 10 GeV and above.Comment: To be published in the proceedings of DPF-2009, Detroit, MI, July
2009, eConf C09072
Depolarization regions of nonzero volume in bianisotropic homogenized composites
In conventional approaches to the homogenization of random particulate
composites, the component phase particles are often treated mathematically as
vanishingly small, point-like entities. The electromagnetic responses of these
component phase particles are provided by depolarization dyadics which derive
from the singularity of the corresponding dyadic Green functions. Through
neglecting the spatial extent of the depolarization region, important
information may be lost, particularly relating to coherent scattering losses.
We present an extension to the strong-property-fluctuation theory in which
depolarization regions of nonzero volume and ellipsoidal geometry are
accommodated. Therein, both the size and spatial distribution of the component
phase particles are taken into account. The analysis is developed within the
most general linear setting of bianisotropic homogenized composite mediums
(HCMs). Numerical studies of the constitutive parameters are presented for
representative examples of HCM; both Lorentz-reciprocal and
Lorentz-nonreciprocal HCMs are considered. These studies reveal that estimates
of the HCM constitutive parameters in relation to volume fraction, particle
eccentricity, particle orientation and correlation length are all significantly
influenced by the size of the component phase particles
Scattering of cosmic strings by black holes: loop formation
We study the deformation of a long cosmic string by a nearby rotating black
hole. We examine whether the deformation of a cosmic string, induced by the
gravitational field of a Kerr black hole, may lead to the formation of a loop
of cosmic string. The segment of the string which enters the ergosphere of a
rotating black hole gets deformed and, if it is sufficiently twisted, it can
self-intersect chopping off a loop of cosmic string. We find that the formation
of a loop, via this mechanism, is a rare event. It will only arise in a small
region of the collision phase space, which depends on the string velocity, the
impact parameter and the black hole angular momentum. We conclude that
generically, the cosmic string is simply scattered or captured by the rotating
black hole.Comment: 11 pages, 2 figures, RevTe
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