Gonzalo Ferro

Inductive Superconducting Fault Current Limiters With Y123 Thin-Film Washers Versus Bi2223 Bulk Rings as Secondaries

We present the comparison between the performance of an inductive superconducting fault current limiter operating with bulk Bi_1.8Pb_0.26Sr₂Ca₂Cu₃O _10+x rings or Y₁Ba₂Cu₃O_7-delta thin-film washers as secondaries. We have measured the impedance offered by the limiter under current faults and the recovery time once the faults are cleared. Our results show that the use of thin films can improve the impedance response of these devices and especially their recovery time, which can be about two orders of magnitude shorter than that of bulk samples

Analytical approach to the thermal instability of superconducting films under high current densities

Using the Greens function of the 3D heat equation, we develop an analytical account of the thermal behaviour of superconducting films subjected to electrical currents larger than their critical current in the absence of an applied magnetic field. Our model assumes homogeneity of films and current density, and besides thermal coefficients employs parameters obtained by fitting to experimental electrical field - current density characteristics at constant bath temperature. We derive both a tractable dynamic equation for the real temperature of the film up to the supercritical current density J^\ast (the lowest current density inducing transition to the normal state), and a thermal stability criterion that allows prediction of J^\ast . For two typical YBCO films, J^\ast predictions agree with observations to within 5%. These findings strongly support the hypothesis that a current-induced thermal instability is generally the origin of the breakdown of superconductivity under high electrical current densities, at least at temperatures not too far from Tc.

Quench and thermal recovery in a superconducting fault current limiter with artificial weak zones

We report results on the behaviour of an inductive fault current limiter prototype with weak zones artificially created. The superconducting elements are Bi-2223 rings and the tests have been realized with the device on a Helium gas environment at 77.3 K. We found that the artificial weak zones can play the main role during the limitation under a current fault and on the thermal recovery process after the fault, which could be used to improve the performance of these devices. Our results show also that the artificial weak zones could balance, in addition, the influence of other natural defects which could lead to non-uniform normal-superconducting transitions, the formation of hot spots, etc.

Measurements of the Doping Effects on the In-Plane Paraconductivity in Cuprate Superconductors

We will summarize here some of our measurements of the superconducting fluctuations effects on the in-plane electrical resistivity (the so-called in-plane paraconductivity) in La2−xSrxCuO4 thin films with different Sr content. Our results suggest that these superconducting fluctuations effects are not related to the opening of a pseudogap in the normal-state of underdoped compounds.

Integrating CAD modules in a PACS environment using a wide computing infrastructure

PurposeThe aim of this paper is to describe a project designed to achieve a total integration of different CAD algorithms into the PACS environment by using a wide computing infrastructure.MethodsThe aim is to build a system for the entire region of Galicia, Spain, to make CAD accessible to multiple hospitals by employing different PACSs and clinical workstations. The new CAD model seeks to connect different devices (CAD systems, acquisition modalities, workstations and PACS) by means of networking based on a platform that will offer different CAD services. This paper describes some aspects related to the health services of the region where the project was developed, CAD algorithms that were either employed or selected for inclusion in the project, and several technical aspects and results.ResultsWe have built a standard-based platform with which users can request a CAD service and receive the results in their local PACS. The process runs through a web interface that allows sending data to the different CAD services. A DICOM SR object is received with the results of the algorithms stored inside the original study in the proper folder with the original images.ConclusionsAs a result, a homogeneous service to the different hospitals of the region will be offered. End users will benefit from a homogeneous workflow and a standardised integration model to request and obtain results from CAD systems in any modality, not dependant on commercial integration models. This new solution will foster the deployment of these technologies in the entire region of Galicia.

Superconducting microlimiters based on YBCO thin-films grown on SrTiO3 substrates

It has been shown recently that superconducting microbridges implemented with YBa2Cu3O7−δ (YBCO) thin-films grown on sapphire substrates could be used as very efficient fault current limiters for microelectronic devices with some elements working at temperatures below TcI the superconducting critical temperature and, simultaneously, under very low power conditions (below 1 W). These so-called microlimiters are then well adapted to important applications of the superconducting microelectronics, as infrared detectors or SQUID based electronics. Here we will present new results obtained by using YBCO microbridges grown on SrTiO3 substrates, which have a thermal conductivity of the order of 50 times lower than the sapphire substrates used in most of our previous work. These new measurements confirm the important role played in the behaviour of microlimiters by the thermal exchanges between the microbridges with their substrates and between these last with their environment. Our present results also show that the temperature for optimal operation of the microlimiter is substrate-dependent.

The impedance of inductive superconducting fault current limiters operating with stacks of thin film Y123/Au washers or bulk Bi2223 rings as secondaries

Inductive fault current limiters operating with stacks of various small superconducting elements acting as secondaries were studied. The stacks consist of Y1Ba2Cu3O7?? thin film washers or Bi1.8Pb0.26Sr2Ca2Cu3O10+x bulk rings. A central result of our work is an experimental demonstration that the limiting capability of the device is strongly reduced when several bulk rings are stacked, whereas it remains almost unchanged for thin film washers. The use of thin films should therefore allow us to build more efficient high power inductive limiters based on stacks of small washers.

Influence of the microbridge width on the transition to the normal state induced by high current densities ramps in YBa2Cu3O7-δ thin films

We report current voltage curves (CVC) on c-axis oriented YBa2Cu3O7-δ superconducting thin films patterned in several microbridges of different width and the same width/length ratio (1/10). The microbridge widths range from 5 µm to 100 µm. Our present measurements extend previous experiments performed in our research group to microbridges with different geometries. Measurements of CVC spanned the whole range: from the critical current Jc (when the dissipation starts) to the supercritical current J* (when an abrupt jump to a highly dissipative state occurs). We have found that Jc values do not change for all the microbridges of the same film whereas J* decreases as the microbridge width increases. These findings indicate that near Jc the current is almost uniformly distributed over all the microbridge volume. Moreover, although our results do not exclude an intrinsic origin for the transition to the normal state induced by high current densities, they suggest that J* could be deeply affected by the thermal interchange between the films and their substrates.

A CAD SCHEME FOR EARLY LUNG CANCER DETECTION IN CHEST RADIOGRAPHY

The purpose of this work is to describe a chest radiography computer-aided diagnostic (CAD) scheme designed to analyze the chest radiographs performed in the framework of the Galician (Spain) Healt...

Influence of thermal, morphological and measuring variables on the thermal runaway of superconducting films under high current densities

We study the abrupt jump in voltage seen in the current-voltage characteristics of high-Tc films at current densities, J*, around 2–3 times the critical one, and for perturbations with characteristic times in the millisecond range. Customarily, even for these long characteristic times, the voltage jump has been ascribed to an electrodynamic vortex instability. Recent results based on finite-element analyses show quantitatively that smooth, i.e., jumpfree isothermal curves may show up themselves as the jumping, i.e., discontinuous curves experimentalists observe, once the nonlinearity, thermal feedback and measurement rate are properly accounted for. The thermal nature of the transition to a highly dissipative state in high-Tc superconductors at zero magnetic field makes the control of the instability transition easier in that there are more parameters available to experimentalists that may be tuned to tailor the phenomenon. This is specially relevant for devices working on their superconducting borderline as current-limiters or even magnets. Here we explore through finite-element analysis the role of film geometry, measurement rate and substrate thermal conductivity in the triggering of the thermal runaway.