R.G. Jenkins

Self-field effects on critical current values in Tl-1223 tapes

Abstract We have measured the properties of a large number of Tl-1223 tapes as part of a programme to optimise the fabrication procedures. During this work we have noticed that the highest I c values are not proportionally dependent on the cross-sectional geometry of the tapes. There seems to be a limiting I c value of around 10–13 A in our samples (and those of many other groups). We have performed a series of experiments on self-field effects in Tl-1223 tapes and compared the results with calculated field distributions in tape-shaped conductors in different configurations. We conclude that the I c are self-field limited in these tapes when the currents being carried are around 10 A.

Fabrication and properties of TBCCO films via air-atomising spray deposition of thallium-free precursors

Abstract The high-field properties of polycrystalline superconducting TlBaCaCuO films fabricated by the incorporation of thallium vapour into air-atomised BaCaCuO precursors are described. Thick films with T c values in the range 106–111 K have been prepared on polycrystalline yttria-stabilised zirconia substrates. The surface morphology, crystal structure and composition of the films are related to their high-field transport and magnetisation properties. Typical 10 mm × 9 mm films show J c values > 1×10 4 A/cm 2 at 77 K (0 T). The best film has a J c =1.3×10 4 A/cm 2 ( I c =3.6 A) at 77 K (0 T). Films prepared on 26 mm×9 mm substrates show typical large-area J c values > 0.5×10 4 A/cm 2 (77 K, 0 T). A square planar specimen of dimensions 4.3 mm ×4.3 mm exhibited magnetisation J c values=1.2×10 5 A/cm 2 at 4.2 K (0.1 T), 9.3×10 4 A/cm 2 at 10 K (0.1 T), 3.3×10 4 A/ cm 2 at 4 K (8 T), and 1.6×10 4 A/cm 2 at 10 K (8 T).

Orientation and thermal cycling effects on the critical currents of high Tc superconducting composites

Abstract Transport critical currents of short lengths of high T c superconducting composite tapes have been measured as a function of magnetic field applied parallel to and perpendicular to the tape surface. Anisotropy factors have been calculated from the results on a number of samples, differing only in their final annealing temperatures, and show great similarity for samples whose critical current densities vary widely. We explain these results in terms of the different microstructures of the samples. We discuss n values and present some data on Ag magnetoresistance. Mechanisms leading to a degradation in J c as a result of thermal cycling are described along with possible solutions.

Control aspects of superconducting magnets for magnetic suspensions and bearings

The paper describes some theoretical and experimental considerations aimed at identifying the main characteristics of high Tc/sub /superconducting magnets for use in controlled applications such as magnetic suspensions and bearings. The theoretical model is formulated to represent the requirements necessary for such applications, and some experiments reinforce this by assessing the effects of periodic variations in both excitation and reluctance over a range of frequencies. In this way a clear understanding can be obtained of the factors which affect the losses in the superconductors, and also the drive voltage requirements are identified for different operating conditions.<<ETX>>

Prototype coils wound from high-T/sub c/ superconducting composites

The construction and performance of three prototype pancake coils wound from high-T/sub c/ superconducting composite tapes fabricated using the techniques of Powder-In-Tube (PIT) and Electrophoretic Deposition (EPD) are described. Numerous potential taps were incorporated in the coils to enable determination of conductor homogeneity. Results are presented of measurements of transport critical currents of complete coils and sections of coils. The homogeneity of these sectional critical currents is shown to be much improved when the composite tape is multilayer rather than single layer. In addition data are presented on critical current anisotropy with respect to applied field direction, at 4.2 K and 77 K, for a short sample of the tape used in the construction of one of the coils. >

Designing HTS coils for magnetic circuits

We discuss some of the main considerations involved in the design of HTS coils to operate in liquid nitrogen and provide ampere-turns for magnetic circuits in general, and then in particular for a small-scale electromagnetic (i.e. attractive) maglev demonstrator. The most important factor affecting design is the sensitive and strongly anisotropic dependence of HTS tapes critical current on magnetic field. Any successful design must limit the field in the windings, especially components perpendicular to the tapes surface (radial components in the case of solenoids), to acceptably low levels such that local critical currents nowhere fall below the operating current. This factor is relevant to the construction of HTS coils for all applications. A second important factor is that the presence of an iron magnetic circuit can greatly alter the flux distribution within the coils from that found when they are in free space. FE modelling has been used to calculate accurate field profiles in proposed designs for comparison with short simple I/sub c/(B) data. We present a design for a maglev demonstrator, illustrating how some of the problems, in particular the reduction of radial field components, may be addressed, and describe its predicted performance.

Fabrication of BiSCCO 2212/MgO conductors by melt processing

Abstract We have investigated the possibility of fabricating robust BiSCCO 2212/MgO composite conductor structures which can carry significant currents. These have been prepared by a simple melting process and final long term annealing, and the transport critical currents measured between 77 and 4 K. We have successfully created metre long conductors which carry more than 10 A supercurrent at 4 K, and have been able to integrate four individual spiral conductors together into a coil structure. The durability of this kind of composite conductor has been demonstrated by repeated testing and remounting of individual samples.

Magnet coils made from high-temperature superconductor

Abstract We review the progress we have made in constructing HTS coils and report our latest results. Also we describe the cryogen-free operation of one of our HTS coils cooled to 55 K using a Stirling cycle cryocooler. Lastly, we describe how 4 Oxford coils are being used in a project to investigate the controllability of HTS magnets in applications such as “maglev” suspension systems. We briefly report the initial findings of this work and describe developments in progress.

Prototype BSCCO 2212 coils: Processing and properties

Abstract High-Tc Superconductivity Group in Oxford has produced a number of HTS coils for demonstrator devices operating in liquid nitrogen refrigerated to 64K. A typical coil contained 5m of tape, comprised 55 turns, had an ID of 13mm and OD of 45mm, and exhibited critical currents of 2A and 10A at 77K and 64K respectively. A small magnet was assembled from two of these pancake coils which in zero applied field at 4.2K generated a peak magnetic field of 0.386T when carrying 85A (corresponding to a criterion of 10μV/m) and 0.434T with 117.5A(100μV/m).

A comparison of the E-J characteristics of four NbTi superconducting composites at equivalent electric field levels from 10/sup -4/ to 10/sup -7/ V m/sup -1/

The four-terminal short sample tcst used to measure the critical currents of commercial s uperconducting composites typically employs equivalent electric field levels of and Vm-’ to specify thc onset of the sup e r co nd u c t i n g -r e s i s t i ve transit i o n . W e de scribe refinements to this technique enabling rcsolution of the transition at the Vm-’ level. Data on four samples of NbTi conductor are compared over the full electric field range available using the above method, illustrating the influence of composite geometry and matrix material on the detailed form of the E-J characteristic.