239 research outputs found
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
Introduction of artificial pinning centres in Bi2Sr2CaCu2O8 ceramics
Considering the phase equilibrium diagram of the system Bi203-SrO-CaO-CuO, single phase 'Bi2Sr2CaCu208' ceramics have been transformed by a simple annealing procedure into multiphase samples. The transformation results in the formation of second phases and in an increase of the intra-grain critical current density at 1 T of five times. This increase is believed to express improved pinning properties of the superconducting crystals. The prepared pinning centers are believed to be e.g. coherent precipitates (Guinier-Preston-zones) within the superconducting crystals
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
Simulation of the Thermal Performance of HTS Coated Conductors for HVDC SFCL
Within the EU-funded project FastGrid a resistive superconducting fault current limiter for DC-application is actually under development. To reduce substantially the amount of deployed superconducting tape an increase of voltage per length in the limitation case is needed. Moreover, the resistive transition (quench) should be fast and uniform. In the present work, the thermal behavior of three different architectures of coated conductors are investigated by means of transient simulations. The first architecture is composed by an additional Hastelloy shunt layer, which is supposed to avoid overheating of the conductor during fault limitation. The other architectures include layers of silver and copper as stabilizers. Our parameter study including the geometries of the different layers and the inhomogeneity of the critical current density of the superconducting tape shall lead to an optimized tape layout. First comparisons with experiment indicate, that, with the first architecture, electric field strength up to 100 V/m should be within reach
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
Identification, Genotyping and Antimicrobial Susceptibility Testing of Brucella spp. Isolated from Livestock in Egypt
Brucellosis is a highly contagious zoonosis worldwide with economic and public health impacts. The aim of the present study was to identify Brucella (B.) spp. isolated from animal populations located in different districts of Egypt and to determine their antimicrobial resistance. In total, 34-suspected Brucella isolates were recovered from lymph nodes, milk, and fetal abomasal contents of infected cattle, buffaloes, sheep, and goats from nine districts in Egypt. The isolates were identified by microbiological methods and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Differentiation and genotyping were confirmed using multiplex PCR for B. abortus, Brucella melitensis, Brucella ovis, and Brucella suis (AMOS) and Bruce-ladder PCR. Antimicrobial susceptibility testing against clinically used antimicrobial agents (chloramphenicol, ciprofloxacin, erythromycin, gentamicin, imipenem, rifampicin, streptomycin, and tetracycline) was performed using E-Test. The antimicrobial resistance-associated genes and mutations in Brucella isolates were confirmed using molecular tools. In total, 29 Brucella isolates (eight B. abortus biovar 1 and 21 B. melitensis biovar 3) were identified and typed. The resistance of B. melitensis to ciprofloxacin, erythromycin, imipenem, rifampicin, and streptomycin were 76.2%, 19.0%, 76.2%, 66.7%, and 4.8%, respectively. Whereas, 25.0%, 87.5%, 25.0%, and 37.5% of B. abortus were resistant to ciprofloxacin, erythromycin, imipenem, and rifampicin, respectively. Mutations in the rpoB gene associated with rifampicin resistance were identified in all phenotypically resistant isolates. Mutations in gyrA and gyrB genes associated with ciprofloxacin resistance were identified in four phenotypically resistant isolates of B. melitensis. This is the first study highlighting the antimicrobial resistance in Brucella isolated from different animal species in Egypt. Mutations detected in genes associated with antimicrobial resistance unravel the molecular mechanisms of resistance in Brucella isolates from Egypt. The mutations in the rpoB gene in phenotypically resistant B. abortus isolates in this study were reported for the first time in Egypt
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
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
Quasi-degenerate self-trapping in one-dimensional charge transfer exciton
The self-trapping by the nondiagonal particle-phonon interaction between two
quasi-degenerate energy levels of excitonic system, is studied. We propose this
is realized in charge transfer exciton, where the directions of the
polarization give the quasi-degeneracy. It is shown that this mechanism, unlike
the conventional diagonal one, allows a coexistence and resonance of the free
and self-trapped states even in one-dimensional systems and a quantitative
theory for the optical properties (light absorption and time-resolved
luminescence) of the resonating states is presented. This theory gives a
consistent resolution for the long-standing puzzles in quasi-one-dimensional
compound A-PMDA.Comment: accepted to Phys. Rev. Letter
Temperature dependence of the EPR linewidth of Yb3+ - ions in Y0.99Yb0.01Ba2Cu3OX compounds: Evidence for an anomaly near TC
Electron paramagnetic resonance experiments on doped Yb3+ ions in YBaCuO
compounds with different oxygen contents have been made. We have observed the
strong temperature dependence of the EPR linewidth in the all investigated
samples caused by the Raman processes of spin-lattice relaxation. The
spin-lattice relaxation rate anomaly revealed near TC in the superconducting
species can be assigned to the phonon density spectrum changesComment: 10 pages, 4 figures Renewed versio
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