A.J.M. Roovers

AC losses and critical current density of superconducting GdBa2Cu3O7−x

Energy losses occurring in a cylindrical sample of Gd-Ba-Cu-O, subjected to an external AC magnetic field were examined. The loss dependence on the amplitude of the magnetic induction exhibits two stages of flux penetration into the superconductor. Critical current densitities for both stages of penetration were examined and an explanation for such behaviour is proposed. Support for this point of view is obtained by measurements on pulverized sample material. All measurements were performed at a temperature of 4.2 K and in absence of a background field. Analysis of the data provides two critical current densities: an inter-granular critical current density at weak alternating magnetic fields and an intra-granular critical current density at higher magnetic fields. The intra-granular critical current density is at least two orders of magnitude larger than the inter-granular one.

Current distribution and AC losses in twisted multifilamentary AC superconductors

The development of AC superconductors has led to multifilamentary wires having a highly resistive matrix material and a small twist pitch and filament diameter. A numerical one-dimensional model is presented which enables the authors to calculate the current distribution as well as the magnetization and transport current loss in a current-carrying AC superconductor subjected to a transverse alternating magnetic field. The impact on wire design is also discussed. >

A loss measurement system in a test facility for high-current superconducting cables and wires

The AC loss measurement system in the Twente test facility is operational. Preliminary results of the first loss measurements on a vacuum-welded conductor are presented. The Twente test facility is a high-current system in which superconductors can be tested up to 7 T and 25 kA. The loss measurements were done on a monolith conductor designed for TORE SUPRA, manufactured by Vacuumschmelze. >

Critical current and stability effects between 0 and 6 tesla in mono and multifilamentary NbTi conductors having a CuNi matrix

We investigated the current carrying capacities of ten NbTi superconductors having a CuNi matrix with diameters between 50 and 300 μm as function of an applied magnetic field of 0 to 6 tesla. The effects of the method of wire fixation and the electrical connections were studied. The results are compared to those obtained with two common conductors with Cu matrix. We observed large differences of a factor 2 between the critical current densities of the various CuNi matrix conductors. Furthermore, it is remarkable that the best CuNi matrix conductors have critical current densities which are much higher than those in Cu matrix conductors.

AC losses in a prototype NbTi cable for LHC dipole magnets

In this paper loss measurements on a prototype NbTi cable for the Large Hadron Collider dipole magnets are presented. The cable is meant to be used for the outer layer of the magnets. It is a trapezium-shaped (key-stoned) Rutherford cable. The experimental results were obtained in a Twente test facility. Magnetization loss measurements as well as transport current loss measurements were performed. The results are compared with classical loss models. From the magnetization experiments the coupling time constant of the cable at different magnetic fields can be obtained. It is also possible to estimate the critical current density from these measurements. The critical current and the effective transverse resistivity are important for calculations concerning the loss during ramping the magnetic field and the field disturbance due to shielding currents in the filaments and to the coupling currents.

AC Loss Measurements on Superconductors Using a (Low-Inductive) Coil Geometry

In this paper loss measurements on various superconductors are presented. The measurements were performed on samples in a coil configuration. Most of the results will be used in the VAMAS Round Robin Test, in the course of which the experimental data of several laboratories throughout the world are compared. The characteristic loss parameters of the conductors are extracted from the experimental results, when possible, using classical loss theories.

AN ANALYSIS OF LOSS MEASUREMENT SYSTEMS FOR HIGH-CURRENT SUPERCONDUCTORS

A large number of conductors and cables for large-scale applications have been developed over the last few years. This requires test facilities equiped for high magnetic fields and large transport currents. The status of a medium-sized test facility developed at the University of Twente [1,2,3] is reported. An analysis of the loss measurement system in this test facility will be given. Some experimental results of AC loss measurements are also presented. In some cases a simplification of the test set-up is used to estimate the losses under operating conditions. A selection of the various approaches in determining the AC loss in superconducting cables and monoliths will be given.