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Variational approach in weighted Sobolev spaces to scattering by unbounded rough surfaces
We consider the problem of scattering of time harmonic acoustic waves by an unbounded sound soft surface which is assumed to lie within a finite distance of some plane. The paper is concerned with the study of an equivalent variational formulation of this problem set in a scale of weighted Sobolev spaces. We prove well-posedness of this variational formulation in an energy space with weights which extends previous results in the unweighted setting [S. Chandler-Wilde and P. Monk, SIAM J. Math. Anal., 37 (2005), pp. 598–618] to more general inhomogeneous terms in the Helmholtz equation. In particular, in the two-dimensional case, our approach covers the problem of plane wave incidence, whereas in the three-dimensional case, incident spherical and cylindrical waves can be treated. As a further application of our results, we analyze a finite section type approximation, whereby the variational problem posed on an infinite layer is approximated by a variational problem on a bounded region
Shielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magnetic fields
We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets
can also act as efficient magnetic shields. The magnetic screening properties
under an axial DC magnetic field are characterized at several temperatures
below the liquid nitrogen temperature (77 K). Two main shielding properties are
studied and compared with those of Bi-2223, a material that has been considered
in the past for bulk magnetic shields. The first property is related to the
maximum magnetic flux density that can be screened, Blim; it is defined as the
applied magnetic flux density below which the field attenuation measured at the
centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall
thickness of 5 mm and a large ratio of length over radius, Blim is evaluated to
1 T at T = 10 K. This value largely exceeds the Blim value measured at the same
temperature on similar tubes of Bi-2223. The second shielding property that is
characterized is the dependence of Blim with respect to variations of the sweep
rate of the applied field, dBapp/dt. This dependence is interpreted in terms of
the power law E = Ec(J/Jc)^n and allows us to determine the exponent n of this
E(J) characteristics for Bi-2212. The characterization of the magnetic field
relaxation involves very small values of the electric field. This gives us the
opportunity to experimentally determine the E(J) law in an unexplored region of
small electric fields. Combining these results with transport and AC shielding
measurements, we construct a piecewise E(J) law that spans over 8 orders of
magnitude of the electric field.Comment: 16 pages, 7 figure
Magnetic fluctuations and superconductivity in Fe pnictides probed by electron spin resonance
The electron spin resonance absorption spectrum of Eu^{2+} ions serves as a
probe of the normal and superconducting state in Eu_{0.5}K_{0.5}Fe_2As_2. The
spin-lattice relaxation rate 1/T_1^{\rm ESR} obtained from the ESR linewidth
exhibits a Korringa-like linear increase with temperature above T_C evidencing
a normal Fermi-liquid behavior. Below 45 K deviations from the Korringa-law
occur which are ascribed to enhanced magnetic fluctuations within the FeAs
layers upon approaching the superconducting transition. Below T_C the
spin-lattice relaxation rate 1/T_1^{\rm ESR} follows a T^{1.5}-behavior without
the appearance of a coherence peak.Comment: 5 pages, 5 figure
Test of a DC-HTS Busbar Demonstrator for Power Distribution in Hybrid-Electric Propulsion Systems for Aircraft
In the framework of the German project TELOS (Thermo-Electrically Optimised
Aircraft Propulsion Systems) a high-temperature superconducting 40 MVA DC demonstrator
busbar for hybrid-electric propulsion systems for aircraft has been developed. The design current
for a temperature below 25 K is 13.3 kA and the rated voltage is 3 kV. The 2-pole busbar contains
2 stacks of REBCO coated conductors which are supported by a 3D-printed structure allowing
compensation of thermal length changes of the superconductor. It fits in a cryostat tube with an
inner diameter of 25 mm. A special focus has been put on low-resistive joints that are necessary
to connect single elements of the busbar system. The special layout of the joints allows an
effective current redistribution between the different tapes in a stack. We present results for the
test of the DC busbar demonstrator in liquid nitrogen at 77 K. The design current for this
temperature is 3.3 kA which corresponds to a rated power of 10 MW. We applied currents up to
3.5 kA and measured the I-V characteristics and contact resistances of 90° and 180° joints in a
virgin and in a strained state thus simulating thermal length changes. We also present results of
Lorentz-Force tests with short AC current pulses up to 20 kA to demonstrate the viability of the
design for application with currents up to 13.3 k
Inverse Scattering for Gratings and Wave Guides
We consider the problem of unique identification of dielectric coefficients
for gratings and sound speeds for wave guides from scattering data. We prove
that the "propagating modes" given for all frequencies uniquely determine these
coefficients. The gratings may contain conductors as well as dielectrics and
the boundaries of the conductors are also determined by the propagating modes.Comment: 12 page
Site-selective spectroscopy and level ordering in C-phycocyanin
We present a combined fluorescence and hole-burning study of the biliprotein C-phycocyanin. Sharp zero-phonon holes compare with a broad structureless fluorescence. This finding is rationalized in terms of the special level structure in this pigment, the fast energy-transfer processes and a lack of correlation of the energies of the emissive states
Intrinsic electron paramagnetic resonance in La2-ξSrξCuO4: Manifestation of three spin polarons
Electron-Paramagnetic-Resonance (EPR) measurements on La2-xSrxCuO4 single crystals provide experimental evidence of a three-spin polaron of two Cu2+ ions and one p hole. The symmetry properties and the peculiar temperature dependence of the g-value of the EPR line indicate the presence of a dynamical Jahn-Teller distortion (Q2-rnode) and formation of a collective mode of polarons and surrounding strongly correlated Cu ions (bottleneck regime). © 1999 Plenum Publishing Corporation
EPR study of the dynamic spin susceptibility in heavily doped YBa2Cu3O6+δ
We report on measurements of the dynamic spin susceptibility of the CuO2-planes on single crystals of YBa2Cu3O6+δ using EPR techniques. Intrinsic signals due to the existence of paramagentic chain fragments (pcf) show a behaviour revealing the opening of a spin gap above the superconducting phase-transition temperature. This can be directly compared with the results as observed from neutron scattering and NMR experiments. © 1995
EPR study of the dynamic spin susceptibility in heavily doped YBa2Cu3O6+δ
Electron paramagnetic resonance (EPR) studies on single crystals of YBa2Cu3O6+δ were performed in a wide range of oxygen concentration δ. Intrinsic EPR signals due to the existence of paramagnetic chain fragments could be detected only in a limited δ range (0. 7≤δ≤0.9). The linewidth of the signal passes through a minimum near 105 K and broadens exponentially for further decreasing temperatures. This behavior manifests the opening of a pseudogap in the dynamic spin susceptibility of the CuO2 planes. At higher temperatures the linewidth follows a Korringa behavior. The g values reveal axial symmetry with respect to the c axis [g=2.28(1); g=2.03(1)] and are almost temperature independent. We compare our results with inelastic neutron scattering and nuclear-magnetic-resonance data. © 1995 The American Physical Society
An improved design of an inductive fault current limiter based on a superconducting cylinder
The paper deals with basic designs of a fault current limiter of the
transformer type which differ each other by the mutual location of a primary
winding and a superconducting short-circuited cylinder. Theoretical study of
the main parameters of the different designs is performed in the framework of
the critical state model and shows that the most effective is a design in which
the primary winding is divided to two sections with equal turn numbers. The
sections are placed inside and outside of the cylinder and connected in series.
Such arrangement of the windings leads to a substantial reduction of AC losses
in the superconducting cylinder, an increase of the activation current and a
decrease of the inductive reactance in the normal regime of a protected
circuit. The experimental results obtained on the laboratory model with a BSSCO
cylinder confirm the theoretical predictions.Comment: 17 pages, 9 figure
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