23,370 research outputs found
Multipole polarizability of a graded spherical particle
We have studied the multipole polarizability of a graded spherical particle
in a nonuniform electric field, in which the conductivity can vary radially
inside the particle. The main objective of this work is to access the effects
of multipole interactions at small interparticle separations, which can be
important in non-dilute suspensions of functionally graded materials. The
nonuniform electric field arises either from that applied on the particle or
from the local field of all other particles. We developed a differential
effective multipole moment approximation (DEMMA) to compute the multipole
moment of a graded spherical particle in a nonuniform external field. Moreover,
we compare the DEMMA results with the exact results of the power-law graded
profile and the agreement is excellent. The extension to anisotropic DEMMA will
be studied in an Appendix.Comment: LaTeX format, 2 eps figures, submitted for publication
Neutrino masses, leptogenesis and dark matter in hybrid seesaw
We suggest a hybrid seesaw model where relatively ``light''right-handed
neutrinos give no contribution to the neutrino mass matrix due to a special
symmetry. This allows their Yukawa couplings to the standard model particles to
be relatively strong, so that the standard model Higgs boson can decay
dominantly to a left and a right-handed neutrino, leaving another stable
right-handed neutrino as cold dark matter. In our model neutrino masses arise
via the type-II seesaw mechanism, the Higgs triplet scalars being also
responsible for the generation of the matter-antimatter asymmetry via the
leptogenesis mechanism.Comment: 4 page
A GPU-based finite-size pencil beam algorithm with 3D-density correction for radiotherapy dose calculation
Targeting at the development of an accurate and efficient dose calculation
engine for online adaptive radiotherapy, we have implemented a finite size
pencil beam (FSPB) algorithm with a 3D-density correction method on GPU. This
new GPU-based dose engine is built on our previously published ultrafast FSPB
computational framework [Gu et al. Phys. Med. Biol. 54 6287-97, 2009].
Dosimetric evaluations against Monte Carlo dose calculations are conducted on
10 IMRT treatment plans (5 head-and-neck cases and 5 lung cases). For all
cases, there is improvement with the 3D-density correction over the
conventional FSPB algorithm and for most cases the improvement is significant.
Regarding the efficiency, because of the appropriate arrangement of memory
access and the usage of GPU intrinsic functions, the dose calculation for an
IMRT plan can be accomplished well within 1 second (except for one case) with
this new GPU-based FSPB algorithm. Compared to the previous GPU-based FSPB
algorithm without 3D-density correction, this new algorithm, though slightly
sacrificing the computational efficiency (~5-15% lower), has significantly
improved the dose calculation accuracy, making it more suitable for online IMRT
replanning
Eigenstructure Assignment Based Controllers Applied to Flexible Spacecraft
The objective of this paper is to evaluate the behaviour of a controller designed using a parametric Eigenstructure Assignment method and to evaluate its suitability for use in flexible spacecraft. The challenge of this objective lies in obtaining a suitable controller that is specifically designated to alleviate the deflections and vibrations suffered by external appendages in flexible spacecraft while performing attitude manoeuvres. One of the main problems in these vehicles is the mechanical cross-coupling that exists between the rigid and flexible parts of the spacecraft. Spacecraft with fine attitude pointing requirements need precise control of the mechanical coupling to avoid undesired attitude misalignment. In designing an attitude controller, it is necessary to consider the possible vibration of the solar panels and how it may influence the performance of the rest of the vehicle. The nonlinear mathematical model of a flexible spacecraft is considered a close approximation to the real system. During the process of controller evaluation, the design process has also been taken into account as a factor in assessing the robustness of the system
Nonlinear alternating current responses of graded materials
When a composite of nonlinear particles suspended in a host medium is
subjected to a sinusoidal electric field, the electrical response in the
composite will generally consist of alternating current (AC) fields at
frequencies of higher-order harmonics. The situation becomes more interesting
when the suspended particles are graded, with a spatial variation in the
dielectric properties. The local electric field inside the graded particles can
be calculated by the differential effective dipole approximation, which agrees
very well with a first-principles approach. In this work, a nonlinear
differential effective dipole approximation and a perturbation expansion method
have been employed to investigate the effect of gradation on the nonlinear AC
responses of these composites. The results showed that the fundamental and
third-harmonic AC responses are sensitive to the dielectric-constant and/or
nonlinear-susceptibility gradation profiles within the particles. Thus, by
measuring the AC responses of the graded composites, it is possible to perform
a real-time monitoring of the fabrication process of the gradation profiles
within the graded particles.Comment: 18 pages, 4 figure
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