824 research outputs found
Influence of Resonances on the Noise Performance of SQUID Susceptometers
Scanning Superconducting Quantum Interference Device (SQUID) Susceptometry simultaneously images the local magnetic fields and susceptibilities above a sample with sub-micron spatial resolution. Further development of this technique requires a thorough understanding of the current, voltage, and flux ( IVĪ¦ ) characteristics of scanning SQUID susceptometers. These sensors often have striking anomalies in their currentāvoltage characteristics, which we believe to be due to electromagnetic resonances. The effect of these resonances on the performance of these SQUIDs is unknown. To explore the origin and impact of the resonances, we develop a model that qualitatively reproduces the experimentally-determined IVĪ¦ characteristics of our scanning SQUID susceptometers. We use this model to calculate the noise characteristics of SQUIDs of different designs. We find that the calculated ultimate flux noise is better in susceptometers with damping resistors that diminish the resonances than in susceptometers without damping resistors. Such calculations will enable the optimization of the signal-to-noise characteristics of scanning SQUID susceptometers
Three-dimensional analysis of the Pratt and Whitney alternate design SSME fuel turbine
The three dimensional viscous time-mean flow in the Pratt and Whitney alternate design space shuttle main engine fuel turbine is simulated using the average passage Navier-Stokes equations. The migration of secondary flows generated by upstream blade rows and their effect on the performance of downstream blade rows is studied. The present simulation confirms that the flow in this two stage turbine is highly three dimensional and dominated by the tip leakage flow. The tip leakage vortex generated by the first blade persists through the second blade and adversely affects its performance. The greatest mixing of the inlet total temperature distortion occurs in the second vane and is due to the large leakage vortex generated by the upstream rotor. It is assumed that the predominant spanwise mixing mechanism in this low aspect ratio turbine is the radial transport due to the deterministically unsteady vortical flow generated by upstream blade rows. A by-product of the analysis is accurate pressure and heat loads for all blade rows under the influence of neighboring blade rows. These aero loads are useful for advanced structural analysis of the vanes and blades
Meissner response of a bulk superconductor with an embedded sheet of reduced penetration depth
We calculate the change in susceptibility resulting from a thin sheet with
reduced penetration depth embedded perpendicular to the surface of an isotropic
superconductor, in a geometry applicable to scanning Superconducting QUantum
Interference Device (SQUID) microscopy, by numerically solving Maxwell's and
London's equations using the finite element method. The predicted stripes in
susceptibility agree well in shape with the observations of Kalisky et al. of
enhanced susceptibility above twin planes in the underdoped pnictide
superconductor Ba(Fe1-xCox)2As2 (Ba-122). By comparing the predicted stripe
amplitudes with experiment and using the London relation between penetration
depth and superfluid density, we estimate the enhanced Cooper pair density on
the twin planes, and the barrier force for a vortex to cross a twin plane. Fits
to the observed temperature dependence of the stripe amplitude suggest that the
twin planes have a higher critical temperature than the bulk, although stripes
are not observed above the bulk critical temperature.Comment: 16 pages, 9 figure
'STANDING WATCH IN KASSA' CITY WALLS AND WATCHTOWERS AND THEIR PHASES OF CONSTRUCTION
The Research Snippets are extracts from a monthly service called the Journal Article Summary Service. It is a service that summarises new articles in obstetrics and gynaecology published over the preceding month
Meissner response of anisotropic superconductors
The response field of a half-space anisotropic superconductor is evaluated
for an arbitrary weak external field source. Example sources of a point
magnetic moment and a circular current are considered in detail. For the
penetration depth with being any other relevant distance
(the source size, or the distance between the source and the superconductor),
the major contribution to the response is the independent field of
the source image. It is shown that the absolute value of cannot be
extracted from the response field with a better accuracy than that for the
source position. Similar problems are considered for thin films.Comment: 8 pages, 0 figures. 7 pages: section removed, refs. adde
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