Bertrand Dutoit

Comparison of mechanically drawn and protection layer chemically etched optical fiber tips

Abstract Scanning near-field optical microscope tips are produced by mechanically drawing and by chemical etching of standard single mode fibers. The geometrical shapes and taper angles are compared. By mechanical drawing with a commercial micropipette puller, smooth tapers with point diameters below 50 nm are obtained. The final taper cone angle does not exceed 14° for small point sizes. The total length of the taper with decreasing core diameter ranges from 500 to 2000 μm. Strong variations of the cone angle can be observed depending on the chosen pulling conditions. Therefore the variations of the fiber diameter at different distances from the tip are measured for characterizing the shapes as a function of the pulling force. Chemical etching in 40% aqueous hydrofluoric acid covered with an organic solvent protection layer results in tips of conical shape in an autostopping process. The cone angle can be varied from 8° to 41° by the adequate choice of the organic solvent under static etching conditions. Tips with smooth surfaces and point diameters below 40 nm are realized.

Comparison of numerical methods for modeling of superconductors

Different finite-element method (FEM) formulations have been developed in order to model the electromagnetic behavior of type-II superconductors. This paper presents a comparison between simulations with A-V formulation models implemented in two FEM software packages (FLUX2D and FLUX3D) and a numerical method based on analytical model for superconductors in applied magnetic field. These models can be used for superconductors with complex geometry and power-law current-voltage characteristics. Simulated is a 37-filamentary tape with applied transport current in self-field and alternating current (ac) magnetic field parallel to the wide side of the tape. A good agreement is found between the ac-loss and current distributions obtained with the different models.

High performance micromachined Sm2Co17 polymer bonded magnets

Abstract A new technique has been developed to fabricate thick permanent magnets in large batches with a remanence of up to 0.34 T and an energy product of over 22 kJ/m 3 . To our knowledge, these are the best values reached in micromachined magnets. To illustrate the potential of this new technology, a new type of rotational speed microsensor has been fabricated. A 1 mVpp signal is obtained at 0.25 mm from a gearwheel.

Enhancement of power system transient stability using superconducting fault current limiters

Transient stability investigations consist of studying the rotor oscillations of generators (electro-mechanic oscillations, 0.1-2 Hz) after the occurrence of a fault of large amplitude, e.g. a short circuit. The goal is to indicate if the generators are capable to stay synchronous after a fault has occurred. The fault duration is one of the most important factors to be determined. In fact, the shorter the fault, the more the maintaining of synchronisation can be guaranteed. Now in case of a fault, a fault current limiter has an extremely fast current transition in comparison to electromechanical time constants. This implies a quasi-instantaneous elimination of the fault through a limitation of the current and consequently a better ability to maintain the synchronisation of the system. The authors recall that in a classic system, the elimination of a fault, by opening a circuit breaker, is carried out in two or three cycles in the best case. They have studied a simple, radial electric network configuration with a machine and an infinite network. The study covers simulations of a fault that can occur in a network and the consequences of the recovery time of the fault current limiter.

Finite element method simulation of AC loss in HTS tapes with B-dependent E-J power law

The nonlinear behavior of high temperature superconductors (HTS) is often modeled by an E-J power law in order to describe their electromagnetic properties. This paper presents AC loss calculations in HTS tapes, performed by means of FEM commercial software using the A-V method. The implemented nonlinear model of the HTS tapes takes into account the B-dependence of the critical current density J/sub c/ and the power index n. The expressions for J/sub c/(B) and n(B) are obtained from electrical measurements of a Bi-2223 tape under applied DC magnetic field. Numerical simulations of HTS tapes under different experimental conditions have been performed, i.e. the application of a transport current and/or AC external perpendicular magnetic field at 59 Hz. A comparative analysis of AC loss is then presented where J/sub c/ and n are maintained either constant or B-dependent. The combined J/sub c/(B) and n(B) formulation leads to a better understanding of HTS electromagnetic behavior, especially when a perpendicular magnetic field is applied.

Magneto-Thermal Modeling of Second-Generation HTS for Resistive Fault Current Limiter Design Purposes

Coated conductors (CCs) are very promising for the design of novel and efficient resistive fault current limiters (FCLs). However, a detailed knowledge about their thermal and electromagnetic behaviors in the presence of over-critical currents is crucial for their improvement. In this context, we performed finite-element magneto-thermal modeling of CCs under over-critical current on several geometries. Accordingly, we have investigated the influence of the physical properties of stabilizer and substrate on the thermal stability to improve the high-temperature superconductor (HTS)-FCL design. All simulations were performed using COMSOL Multiphysics, a commercial finite-element package, which has a built-in coupling between the thermal and electrical equations, allowing us to compute both quantities simultaneously during the solving process. Our results allow us to determine the current threshold to achieve thermal stability of HTS FCLs made with CCs.

Comparison of the AC losses of BSCCO and YBCO conductors by means of numerical analysis

This paper presents a quantitative comparison of the AC loss performance of two BSCCO and two YBCO conductors, characterized by the same self-field critical current of 150 A. In particular, compared are a 37-filamentary BSCCO tape, a 16-filamentary BSCCO square wire, a standard YBCO tape, and a stack of four narrower YBCO tapes. The comparison is made using a numerical technique, based on the finite-element method, which employs a non-linear E-J relation with the dependence of the critical current density Jc on the local magnetic field. For the simulations of YBCO, a new shell-region model is utilised, which allows overcoming the geometry and mesh problems, typical for superconductors with very high aspect ratio. Different AC working conditions are simulated: self-field, applied external field, and combined transport current and external field of varying orientation. Outlined are the advantages of using BSCCO or YBCO conductors for the different applications. Various magnetic field and current density profiles are investigated in order to illustrate the reasons for the loss difference in the four conductors. Particular attention is drawn to the YBCO tape and the YBCO stack, whose AC loss characteristics are less studied than those of BSCCO conductors.

Conceptual Design of a 24 kV, 1 kA Resistive Superconducting Fault Current Limiter

In recent years many large scale demonstrators and prototypes of superconducting fault current limiters have been successfully developed and tested. Within the European Project ECCOFLOW (www.eccoflow.org), it is the first time that a resistive-type superconducting fault current limiter is developed for two different locations and that a permanent installation is foreseen. The limiter has a rating of 20 kV and 1 kA and will be tested in a busbar and transformer feeder application. The paper summarizes the conceptual design of this innovative limiter and reports in detail about the development of the super- conducting limiting elements, their integration into a cryostat and the design of the whole limiter including cooling and grid integration. As a main result it can be summarized that the ECCOFLOW limiter fulfills all requirements according to the two different specifications. Approximately 3 km of 12 mm wide YBCO tape will be used to realize a three phase system.

Design and Production of the ECCOFLOW Resistive Fault Current Limiter

The European project ECCOFLOW aims at a versatile resistive-type superconducting fault current limiter. For the first time, such a device will be tested at two different sites. The project partners have developed a superconducting fault current limiter design based on REBCO tapes with respect to the specifications provided by two hosting utilities. The limiter will operate at 1 kA at a rated voltage of 24 kV, and will be tested in both a busbar and a transformer feeder application. The design started with extensive investigations on the superconductor tape properties, especially with respect to the limitation behavior in all possible scenarios, and subsequent wide-range modeling of its in-grid behavior. The general integrated layout provides a limitation time of up to 1 s. The limiter is cooled using Gifford-McMahon cryocoolers to recondense the evaporated nitrogen. The present status of production and testing will be presented as well.

Bi(2223) Ag sheathed tape Ic and exponent n characterization and modelling under DC applied magnetic field

We use a dual channel digital lock-in to perform electrical measurement of AC losses at power frequencies. A DC magnetic field between 2 and 400 mT is applied with a varying angle from parallel to perpendicular to the tape surface, thus having a complete view of the loss behavior under DC applied field. Furthermore, the same experimental layout is used to acquire time series of current and voltage across the sample. Using a triangular input current, we measure and average the voltage, which then is fitted to a power law (I/I/sub c/)/sup 11/. The measurements are repeated for the mentioned magnetic field and angle domain to give the dependencies of I/sub c/ and n with magnetic field and angle. For device modeling purposes, we can then express a phenomenological law giving I/sub c/ and n as a function of the applied magnetic fields intensity and direction.