129,172 research outputs found
Low frequency acoustic and electromagnetic scattering
This paper deals with two classes of problems arising from acoustics and electromagnetics scattering in the low frequency stations. The first class of problem is solving Helmholtz equation with Dirichlet boundary conditions on an arbitrary two dimensional body while the second one is an interior-exterior interface problem with Helmholtz equation in the exterior. Low frequency analysis show that there are two intermediate problems which solve the above problems accurate to 0(k(2) log k) where k is the frequency. These solutions greatly differ from the zero frequency approximations. For the Dirichlet problem numerical examples are shown to verify the theoretical estimates
Modeling 3-D objects with planar surfaces for prediction of electromagnetic scattering
Electromagnetic scattering analysis of objects at resonance is difficult because low frequency techniques are slow and computer intensive, and high frequency techniques may not be reliable. A new technique for predicting the electromagnetic backscatter from electrically conducting objects at resonance is studied. This technique is based on modeling three dimensional objects as a combination of flat plates where some of the plates are blocking the scattering from others. A cube is analyzed as a simple example. The preliminary results compare well with the Geometrical Theory of Diffraction and with measured data
Nonreciprocal Electromagnetic Scattering from a Periodically Space-Time Modulated Slab and Application to a Quasisonic Isolator
Scattering of obliquely incident electromagnetic waves from periodically
space-time modulated slabs is investigated. It is shown that such structures
operate as nonreciprocal harmonic generators and spatial-frequency filters. For
oblique incidences, low-frequency harmonics are filtered out in the form of
surface waves, while high-frequency harmonics are transmitted as space waves.
In the quasisonic regime, where the velocity of the space-time modulation is
close to the velocity of the electromagnetic waves in the background medium,
the incident wave is strongly coupled to space-time harmonics in the forward
direction, while in the backward direction it exhibits low coupling to other
harmonics. This nonreciprocity is leveraged for the realization of an
electromagnetic isolator in the quasisonic regime and is experimentally
demonstrated at microwave frequencies
Scattering of Low-Frequency Radiation by a Gyrating Electron
The scattering of electromagnetic radiation by the particle gyrating in an
external magnetic field is considered. Particular attention is paid to the
low-frequency case, when the frequencies of incident radiation are much less
than the electron gyrofrequency. The spectral and polarization features of the
scattering cross-section are analyzed in detail. It is found that the
scattering transfers the low-frequency photons to high harmonics of the
gyrofrequency, into the range of the synchrotron emission of the electron. The
total scattering cross-section appears much larger than that for the particle
at rest. The problem studied is directly applicable to the radio wave
scattering in the magnetosphere of a pulsar. The particles acquire relativistic
rotational energies as a result of resonant absorption of the high-frequency
radio waves and concurrently scatter the low-frequency radio waves, which are
still below the resonance. It is shown that the scattering can affect the radio
intensity and polarization at the lowest frequencies and can compete with the
resonant absorption in contributing to the low-frequency turnover in the pulsar
spectrum. Moreover, the scattering can be an efficient mechanism of the pulsar
high-energy emission, in addition to the synchrotron re-emission of the
particles. Other astrophysical applications of the scattering by gyrating
particles are pointed out as well.Comment: 13 pages, no figures. Accepted for publication in MNRA
Realizing broadband electromagnetic transparency with a graded-permittivity sphere
Broadband electromagnetic transparency phenomenon is realized with a
well-designed graded-permittivity sphere, which has an extremely low scattering
cross section over a wide frequency range, based on the generalized Mie
scattering theory and numerical simulation in full-wave condition. The dynamic
polarization cancellation is revealed by studying the variation of the
polarization with respect to the frequency. Furthermore, a properly-designed
multi-shell sphere is also proposed and examined in order to reduce the
rigorous conditions for realizing the broadband transparency in experiments.Comment: 15 pages, 4 figure
Scattering loss in electro-optic particulate composite materials
The effective permittivity dyadic of a composite material containing
particulate constituent materials with one constituent having the ability to
display the Pockels effect is computed, using an extended version of the
strong-permittivity-fluctuation theory which takes account of both the
distributional statistics of the constituent particles and their sizes.
Scattering loss, thereby incorporated in the effective electromagnetic response
of the homogenized composite material, is significantly affected by the
application of a low-frequency (dc) electric field
Superradiant scattering of electromagnetic waves emitted from disk around Kerr black holes
We study electromagnetic perturbations around a Kerr black hole surrounded by
a thin disk on the equatorial plane. Our main purpose is to reveal the black
hole superradiance of electromagnetic waves emitted from the disk surface. The
outgoing Kerr-Schild field is used to describe the disk emission, and the
superradiant scattering is represented by a vacuum wave field which is added to
satisfy the ingoing condition on the horizon. The formula to calculate the
energy flux on the disk surface is presented, and the energy transport in the
disk-black hole system is investigated. Within the low-frequency approximation
we find that the energy extracted from the rotating black hole is mainly
transported back to the disk, and the energy spectrum of electromagnetic waves
observed at infinity is also discussed.Comment: 15 pages, 2 figures, accepted for publication in Physical Review
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