14,236 research outputs found
Universal Electromagnetic Waves in Dielectric
The dielectric susceptibility of a wide class of dielectric materials
follows, over extended frequency ranges, a fractional power-law frequency
dependence that is called the "universal" response. The electromagnetic fields
in such dielectric media are described by fractional differential equations
with time derivatives of non-integer order. An exact solution of the fractional
equations for a magnetic field is derived. The electromagnetic fields in the
dielectric materials demonstrate fractional damping. The typical features of
"universal" electromagnetic waves in dielectric are common to a wide class of
materials, regardless of the type of physical structure, chemical composition,
or of the nature of the polarizing species, whether dipoles, electrons or ions.Comment: 19 pages, LaTe
Cold atom Clocks and Applications
This paper describes advances in microwave frequency standards using
laser-cooled atoms at BNM-SYRTE. First, recent improvements of the Cs
and Rb atomic fountains are described. Thanks to the routine use of a
cryogenic sapphire oscillator as an ultra-stable local frequency reference, a
fountain frequency instability of where
is the measurement time in seconds is measured. The second advance is a
powerful method to control the frequency shift due to cold collisions. These
two advances lead to a frequency stability of at 7\times 10^{-16}^{87}^{133}$Cs fountains.
Finally we give an update on the cold atom space clock PHARAO developed in
collaboration with CNES. This clock is one of the main instruments of the
ACES/ESA mission which is scheduled to fly on board the International Space
Station in 2008, enabling a new generation of relativity tests.Comment: 30 pages, 11 figure
E-pile model of self-organized criticality
The concept of percolation is combined with a self-consistent treatment of
the interaction between the dynamics on a lattice and the external drive. Such
a treatment can provide a mechanism by which the system evolves to criticality
without fine tuning, thus offering a route to self-organized criticality (SOC)
which in many cases is more natural than the weak random drive combined with
boundary loss/dissipation as used in standard sand-pile formulations. We
introduce a new metaphor, the e-pile model, and a formalism for electric
conduction in random media to compute critical exponents for such a system.
Variations of the model apply to a number of other physical problems, such as
electric plasma discharges, dielectric relaxation, and the dynamics of the
Earth's magnetotail.Comment: 4 pages, 2 figure
Stored Electromagnetic Energy and Antenna Q
Decomposition of the electromagnetic energy into its stored and radiated
parts is instrumental in the evaluation of antenna Q and the corresponding
fundamental limitations on antennas. This decomposition is not unique and there
are several proposals in the literature. Here, it is shown that stored energy
defined from the difference between the energy density and the far field energy
equals the new energy expressions proposed by Vandenbosch for many cases. This
also explains the observed cases with negative stored energy and suggests a
possible remedy to them. The results are compared with the classical explicit
expressions for spherical regions where the results only differ by ka that is
interpreted as the far-field energy in the interior of the sphere. Numerical
results of the Q-factors for dipole, loop, and inverted L-antennas are also
compared with estimates from circuit models and differentiation of the
impedance. The results indicate that the stored energy in the field agrees with
the stored energy in the Brune synthesized circuit models whereas the
differentiated impedance gives a lower value for some cases. The corresponding
results for the bandwidth suggest that the inverse proportionality between
bandwidth and Q depends on the relative bandwidth or equivalent the threshold
of the reflection coefficient. The Q from the differentiated impedance and
stored energy are most useful for relative narrow and wide bandwidths,
respectively
Extraction of reliable information from time-domain pressure and flow signals measured by means of forced oscillation techniques
This paper aims to give a proof-of-concept for the possible application of the forced oscillation lung function test to assess the viscoelastic properties of the airways and tissue. In particular, a novel signal processing algorithm is employed on non-stationary, noisy, (relatively) short time series of respiratory pressure and flow signals. This novel technique is employed to filter the useful information from the signals acquired under two measurement conditions: pseudo-functional residual capacity (PFRC) and pseudo-total lung capacity (PTLC). The PFRC is the measurement performed at lowest lung volume with maximum deflation, and the PTLC is measurement performed at the maximum lung volume under maximum inflation. The results suggest that the proposed technique is able to extract information on the viscoelastic properties of the lung tissue at a macroscopic level. The conclusion of this preliminary study is that the proposed combination of signal processing method and lung function test is suited to be employed on a large database in order to deliver reference values and perform further statistical analysis
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