Influence of production parameters on the superconducting properties of NbTi and Nb3Sn wires

The commercial specification of superconducting NbTi and Nb3 Sn composite wires is normally given by parameters such as the critical current (which is dependent on conductivity, temperature, and magnetic field), an empirically deduced parameter for which the effective resistivity is about 10-14 Ωm, wire and filament diameter, and twist length. To some extent their values cannot be chosen independently, nor do they guarantee proper operation of the system in all aspects. It is argued that for Nb3Sn composite, depending on the production process, an extra set of parameters describing the stress sensitivity seems to be needed to specify the quality of the composite

New developments on calculating and measuring ac losses in composite superconductors

A method for the complete numerical solution of the Maxwell equation for a multifilamentary wire carrying a transport current and subject to applied fields of arbitrary direction, where both current and applied fields have arbitrary time dependence, has been developed. First results show unexpected differences from analytical models used in the past. From both approaches current degradation effects can be estimated and loss contributions resulting from the combined action of applied field and current can be calculate

A.c. stability and a.c. loss in composite superconductors

Two methods of loss calculations are reviewed. The first method, for loss calculations in wires, uses a numerical solution of the Maxwell equations. The second method makes use of Kirchhoff's equations and is much better suited to cables, including braids. Both methods require a knowledge of the constitutive equations relating E and j or V and l in the composite conductor. Experimental results regarding the stability of large cables are presented and a way of improving the stability of a single strand is suggested

Electrodynamic sources of non-uniform heat production in composite superconductors causing instabilities

Model calculations of the electrodynamic properties of superconducting composites are usually carried out assuming that the material constants, such as critical current density and resistivity, are uniform over the cross-section of the composite, or at least rotationally symmetric. Also, the applied field is usually considered to be uniform over a distance compared to the composite diameter. In this paper the possibility will be discussed that non-uniformity in field and material properties may cause significant non-uniformity in heat production so to disturb the local temperature pattern and form sources of instabilities and quenches. The influence of the cooling conditions on the transport current-carrying capacity in the a.c. regime is also discussed

Anomalous skin effect in cylindrical samples

A description is given of the anomalous skin effect in a cylinder placed in an a.c. magnetic field parallel to the cylinder axis. The necessary nonlocal relation between current density and electric field inside the sample is established with the aid of Boltzmann's transport equation. Results are presented in terms of the current density in the sample and its magnetic susceptibility

Deviations from the critical state model observed in niobium

The values of the critical current density of niobium slabs, measured with the help of trapezoidal ac magnetic fields are not in agreement with the critical state model. Extensions of this model are suggested to explain the observed deviation

Hysteresis losses in hollow superconducting filaments and in multifilament systems

The magnetization of hollow superconducting filaments in a perpendicular applied field is calculated numerically using a mathematical model. With this model the hysteresis losses over a closed cycle of the applied field, as well as the instantaneous power dissipation in the filament, can be calculated. Theoretical results are compared with data from experiments using wire with hollow Nb3Sn filaments. Demagnetization effects caused by the influence of the field of one filament on another are studied and, finally, bridging between neighouring filaments and its influence on the effective filament diameter and the hysteresis losses are discussed

Conductivity measurements with trapezoidal a.c. magnetic fields

The inductive response of normal metals to trapezoidal a.c. fields is studied theoretically as well as experimentally. A method is presented to determine the conductivity and its magnetic induction dependence from the decay of the induced voltage during periods of constant field. The correctness of the analysis is confirmed by the close agreement between the complete experimental and theoretical voltage waveforms

Fully superconducting rectifiers and fluxpumps Part 1: Realized methods for pumping flux

The magnetic and electrical properties of superconductors were a challenge for many inventors and designers to use superconducting materials in the construction of fully superconducting voltage and current sources commonly called fluxpumps. In the past twenty years a large variety of mechanically or electrically driven devices have been proposed and successfully operated.\ud \ud In this review the basic principle of operation of each class of devices is shown and specific material problems and limitations are reported. The review will be published in two parts.\ud \ud Part 1 deals with mechanical devices such as flux compressors and dynamos. Although those devices must have been of great importance for technical application, their construction and operation offered great experience with regard to the properties of superconducting materials, their joint techniques switching and mechanical and magnetic stability under ac and dc conditions.\ud \ud In this part also a start is made with the more promising class of electrically driven rectifier fluxpumps. With these rectifiers, current levels over 10 kA can be obtained with high efficiency