2,027 research outputs found
HOM-free accelerating dielectric cavities with metallic inclusions
Previous investigations demonstrated that point-defected photonic bandgap cavities based on periodic and aperiodic dielectric arrangements can be successfully employed as single cell in particle accelerators. In this paper, we present a study aimed at highlighting the possible advantages of using hybrid structures based on the above dielectric configurations, but featuring metallic rods in the outermost regions, for the design of accelerating resonators of extremely high-quality factor. In this framework, we consider diverse configurations, with different (periodic and aperiodic) lattice geometries, sizes, and dielectric/metal fractions. The use of superconducting plates to further increase the cavity performances is taken into account. Results from our comparative studies, based on numerical full-wave simulations backed by experimental validations (at room and cryogenic temperatures) in the microwave region, identify the candidate parametric configurations capable of yielding the highest quality factor
Isotropic properties of the photonic band gap in quasicrystals with low-index contrast
We report on the formation and development of the photonic band gap in
two-dimensional 8-, 10- and 12-fold symmetry quasicrystalline lattices of low
index contrast. Finite size structures made of dielectric cylindrical rods were
studied and measured in the microwave region, and their properties compared
with a conventional hexagonal crystal. Band gap characteristics were
investigated by changing the direction of propagation of the incident beam
inside the crystal. Various angles of incidence from 0 \degree to 30\degree
were used in order to investigate the isotropic nature of the band gap. The
arbitrarily high rotational symmetry of aperiodically ordered structures could
be practically exploited to manufacture isotropic band gap materials, which are
perfectly suitable for hosting waveguides or cavities.Comment: 16 pages, 7 figures, submitted to PR
Critical-state effects on microwave losses in type-II superconductors
We discuss the microwave energy losses in superconductors in the critical
state. The field-induced variations of the surface resistance are determined,
in the framework of the Coffey and Clem model, by taking into account the
distribution of the vortex magnetic field inside the sample. It is shown that
the effects of the critical state cannot generally be disregarded to account
for the experimental data. Results obtained in bulk niobium at low temperatures
are quantitatively justified.Comment: 4 pages, 4 embedded figures, to be published on Eur. Phys. J.
Nonlocal field theory driven by a deformed product: Generalization of Kalb-Ramond duality
A modification of the standard product used in local field theory by means of an associative
deformed product is proposed. We present a class of deformed products, one for
every spin S = 0, 1/2, 1, that induces a nonlocal theory, displaying different form for
different fields. This type of deformed product is naturally supersymmetric and it has
an intriguing duality
The Scalar wave equation in a non-commutative spherically symmetric space-time
Recent work in the literature has studied a version of non-commutative Schwarzschild
black holes where the effects of non-commutativity are described by a mass function
depending on both the radial variable r and a non-commutativity parameter θ. The
present paper studies the asymptotic behavior of solutions of the zero-rest-mass scalar
wave equation in such a modified Schwarzschild space-time in a neighborhood of spatial
infinity. The analysis is eventually reduced to finding solutions of an inhomogeneous
Euler–Poisson–Darboux equation, where the parameter θ affects explicitly the functional
form of the source term. Interestingly, for finite values of θ, there is full qualitative
agreement with general relativity: the conformal singularity at spacelike infinity reduces
in a considerable way the differentiability class of scalar fields at future null infinity.
In the physical space-time, this means that the scalar field has an asymptotic behavior
with a fall-off going on rather more slowly than in flat space-time
Microwave Harmonic Emission in MgB2 Superconductor: Comparison with YBaCuO
We report results of microwave second-harmonic generation in ceramic samples
of MgB2, prepared by different methods. The SH signal has been investigated as
a function of the temperature and the static magnetic field. The results are
discussed in the framework of models reported in the literature. We show that
the peculiarities of the SH signal are related to the specific properties of
the sample. A comparison with the results obtained in ceramic and crystalline
YBa(2)Cu(3)O(7) shows that the second-harmonic emission in MgB2 is weaker than
that observed in ceramic YBa(2)Cu(3)O(7).Comment: 13 pages, 6 figures; Proceedings of Third Workshop on Metamaterials
and Special Materials for Electromagnetic Applications and TLC (Rome, 30-31
March, 2006
Clearance Operation of Teulada Site (Italy): A Novel Approach for Short Term MCM Missions in Seafloor Hard Conditions
In May 2007, following a request by Italian Navy (ITN), NURC agreed to provide the
technology, developed during the port protection studies and 2006 MCM ops. in Baltic sea for Estonian Navy, to survey part of the Capo Teulada firing range (Sardinia Island, Italy), used by NATO Armed Forces.
The goal of the service, fully funded by ITN, was to survey part of the firing range with
acoustic, magnetic and optical instruments in order to detect and classify the ordnance
laying on the seafloor. The team operated from CRV Leonardo to produce a detailed acoustic and magnetic map of the area. The positions of the targets of interest have been provided to Italian Navy for further identification and disposal. The activities have demonstrated that the NURC Autonomous Underwater Vehicles (AUVs) Remus type equipped with high frequency side scan sonar can detect and classify targets of the dimensions of unexploded ordnance laying proud on the sea floor. A multiple sonar images technique has also been used to
improve the classification performances. However, in some areas, targets that were
completely buried or concealed in Posidonia fields wouldn’t have been detectable with AUV sonar. To overcome this limitation, INGV supported the NURC team by using a new high definition magnetometer technique to detect and classify buried metal targets. A proton Overhauser sensor was towed from Leonardo vessel and a magnetometer reference station was set ashore in an appropriate site. A subset of interesting contacts (acoustic and magnetic) have been identified optically by
means of the video camera installed on the ROV (Pluto Plus type by Gaymarine) operated from Leonardo. The positioning accuracy achieved was fully compatible with the reacquisition by ROV and then by divers or mine hunters for future disposal ops. The
survey has also produced an accurate map of the Posidonia fields and a detailed bathymetry of the area. More than 300 contacts have been acquired and more than 200 were classified in less than 15 days.
The success of Teulada operation has convinced ITN to ask NURC for a common
development programme with the aim to validate at sea a fully integrated multi-sensor approach (sonar, magnetic and optical) in MCM short term ops. using USV/AUV
platforms
Microwave intermodulation distortion of MgB2 thin films
The two tone intermodulation arising in MgB2 thin films deposited in-situ by
planar magnetron sputtering on sapphire substrates is studied. Samples are
characterised using an open-ended dielectric puck resonator operating at 8.8
GHz. The experimental results show that the third order products increase with
the two-tone input power with a slope ranging between 1.5 and 2.3. The
behaviour can be understood introducing a mechanism of vortex penetration in
grain boundaries as the most plausible source of non linearities in these
films. This assumption is confirmed by the analysis of the field dependence of
the surface resistance, that show a linear behaviour at all temperatures under
test.Comment: 13 pages, 3 figures; to be published in Appl. Phys. Let
Experimental evidence of s-wave superconductivity in bulk CaC
The temperature dependence of the in-plane magnetic penetration depth,
, has been measured in a c-axis oriented polycrystalline
CaC bulk sample using a high-resolution mutual inductance technique. A
clear exponential behavior of has been observed at low
temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the
standard BCS theory yields Angstroem and
meV. The ratio
gives indication for a conventional weakly coupled superconductor.Comment: To appear in Phys. Rev. Let
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