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The large currents in the cable-in-conduit conductors (CICC) destined for the high field magnets in the International Thermonuclear Experimental Reactor (ITER), cause huge transverse forces on the strands compressing the cable against one side of the conduit. This load causes transverse compressive strain in the strands at the crossovers contacts. Moreover, the strands are also subjected to bending and contact surfaces micro-sliding, which results into friction and anomalous contact resistance versus force behavior. Three Nb/sub 3/Sn central solenoid model coil (CSMC) conductors were tested previously in the Twente Cryogenic Cable Press up to 40 cycles with a transverse peak load of 650 kN/m. This press can transmit a variable (cyclic) transverse force directly to a cable section of 400 mm length at a temperature of 4.2 K (or higher). To explore life-time cycling, we tested a CSMC Nb/sub 3/Sn conductor up to 40,000 cycles. The coupling loss and the associated interstrand resistance between various strands and strand bundles are measured at various loads. The force on the cable and the displacement are monitored in order to determine the effective cable Young's modulus and the mechanical heat generation. Some aspects of strand deformation in CICC's are discussed. The test results are discussed in view of previous press results and data extracted from the ITER model coil tests.

A method to calculate the AC loss of superconducting power devices from the measured AC loss of a short sample is developed. In coils and cables the magnetic field varies spatially. The position dependent field vector is calculated assuming a homogeneous current distribution. From this field profile and the transport current, the local AC loss is calculated. Integration over the conductor length yields the AC loss of the device. The total AC loss of the device is split up in different components. Magnetization loss, transport current loss and the loss due to the combined action of field and current all contribute to the AC loss of the device. Because ways to reduce the AC loss depend on the loss mechanism it is important to know the relative contribution of each component. The method is demonstrated on a prototype transformer coil wound from Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x//Ag superconducting tape. Differences between the model assumptions and devices are pointed out. Nevertheless, within the uncertainty margins the calculated AC loss is in agreement with the measured loss of the coil.

We investigated the local J/sub C/ and microstructures of Bi-2212 multilayer tapes. Multilayer tapes were prepared by wrapping four dip-coated sheets with silver foil. These tapes were fabricated with a melt-solidification method with a Pre-Annealing and Intermediate Rolling (PAIR) process. The local J/sub C/ distribution along the transverse direction of the tapes was determined in a nondestructive manner in which a magnetic field with a steep gradient was applied at 77 K, and the I/sub C/ was measured. The microstructures of the superconducting oxide layers were observed by using an optical micrograph and an SEM micrograph. Bi-2212 superconducting oxide layers existed on silver substrates homogeneously, and the silver/oxide interfaces were almost smooth along the silver substrates. From the J/sub C/-profile, J/sub C/ values at the middle region on the tape were low in comparison with the J/sub C/ values in the edge regions. The depression of the J/sub C/ values around the middle region was due to the overlapping of silver foil and, hence, the inferior microstructure.

A high quality factor, superconducting resonator coil is developed in cooperation with the Dutch companies SMIT Transformers and SMIT DRAAD. The coil system is manufactured in industry, using industrial tooling. It has a design reactive power rating of 1 MVA at a frequency of 50 Hz and the operating temperature is 64 K. The system consists of four concentric but separate coils, made of 2 km of Bi-2223 conductors from two different manufacturers. The coils are optimized by shaping the magnetic field around the coil edges with ferro-magnetic C-cups to reach a design electrical quality factor of 1000. The system is operated in a glass-epoxy cryostat at 64 K and 77 K. The resonator coil design demonstrates all the superconducting elements that are essential for manufacturing superconducting transformers. A numerical model has been developed to predict the DC and AC behavior of the system based on measurements of short samples and test coils. A comparison is made between the experiment and model calculations.

The I-V curves of Bi-Sr-Ca-Cu-O superconductors were measured in DC magnetic fields, oriented both parallel and perpendicular to the tape surface, up to 33 T, 4.2 K, in resistive magnets at the National High Magnetic Field Laboratory. This paper presents the experimental set-up, results of critical current measurements, and an analysis of the obtained I/sub c/(B) curves in terms of strongly and weakly linked current paths. A distinct double-step behavior is observed for field perpendicular to the tape surface. The scaling behavior of the applied field with its component perpendicular to the ab-plane, and the implied average grain-misalignment angle, is investigated.

The quality of the magnetic field in superconducting accelerator magnets is associated with the properties of the superconducting cable. Current imbalances due to coupling currents /spl Delta/I, as large as 100 A, are induced by spatial variations of the field sweep rate and contact resistances. During injection at a constant field all magnetic field components show a decay behavior. The decay is caused by a diffusion of coupling currents into the whole magnet. This results in a redistribution of the transport current among the strands and causes a demagnetization of the superconducting cable. As soon as the field is ramped up again after the end of injection, the magnetization rapidly recovers from the decay and follows the course of the original hysteresis curve. In order to clarify the interactions between the changes in current and magnetization during injection the authors performed a number of experiments. A magnetic field with a spatially periodic pattern was applied to a superconducting wire in order to simulate the coupling behavior in a magnet. This model system was placed into a stand for magnetization measurements and the influence of different powering conditions was analyzed.

Potentially, coils made with BSCCO-2223/Ag tape and operating in liquid nitrogen at 50-60 Hz are interesting for power related applications. The electrical characteristics of such a coil depend on the radial magnetic field in the windings. One way to suppress the radial field is by placing small iron pieces around coil edges. V-I curves of the 100 kVA coil are calculated and measured for two cases: coil edges without and with iron pieces. The coil is a solenoid with inner and outer diameters and a height of 470, 474 and 614 mm respectively. A numerical model adapted to the coil geometry is used to calculate the conductor losses. The loss voltage is also evaluated by measuring the coil temperature. The total loss of the coil is measured electromagnetically. Calculated and measured losses are in agreement. Finally, the loss V-I curves of the coil are simulated in the frequency range of 0 to 400 Hz. The results are applied to the design of a 1 MVA resonator coil.

In order to improve the understanding of the dominant mechanisms that limit the critical current in high temperature superconductors, the dependence of the critical current on magnetic field and temperature of a Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tape has been investigated in detail. The critical current is measured in magnetic fields up to 8 T, at temperatures ranging from 4.2 K to 70 K. The results are compared with existing models that describe the current path as two parallel systems, one depending on weak links and the other on flux pinning. The critical current at low magnetic fields is reduced drastically by the self-field of the superconductor. At intermediate magnetic fields, the field dependence of the critical current is mainly dominated by weak links, while at higher fields it is dominated by the strong-links current path, and depends on flux pinning. To clarify the models used to describe the measurements, the temperature dependence of the parameters used in the models is studied. The temperature dependence of the parameters used to describe the weak-links current path points out that the weak links are formed by remnant Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub x/ phase at the grain boundaries.

The transport properties of superconductors are commonly characterized by means of a 4-probe measuring technique and the critical current is determined on a certain criterion for the electrical field. An alternative method to investigate the transport properties is to measure the magnetic response of a superconductor in a changing magnetic field. This magnetic technique has the interesting advantage that it can be used to investigate long lengths of (insulated) conductor. A detailed analysis is made to develop a reliable measuring procedure for this new test facility. The magnetic response of a superconductor is modeled in a description for an infinitely long tape with a rectangular cross-section and an arbitrary voltage-current relation. The calculated magnetic profiles, in space and time, are compared with experimental results at 77 K. It is demonstrated that the magnetic signal can be used to monitor the quality of a long length of tape (>500 m) with a high accuracy. Additionally it is shown that the shape of the voltage-current relation can be reconstructed based on the frequency dependence of the magnetic response.

The three dimensional surface of the critical current density versus field and temperature J/sub c/(B,T) of niobium-tin is a function of the strain state of the superconductor. A brief review of literature on this subject is presented. The J/sub c/(B) function is described by the relations for flux pinning. The temperature and strain dependencies are added to this relation, This results in a unifying scaling law for A15 materials, which is verified for different niobium-tin conductors with respect to all the relevant variables, i.e. field, temperature and uni-axial strain. Nb/sub 3/Sn conductors from 9 manufacturers are measured in the frame work of the third ITER benchmark tests on critical current. The investigated ranges are: applied field from 7 to 13 T, temperature from 4.2 to 8 K and applied strain from -0.4 to +0.8%. Special attention is paid to the region of compressive axial strain, which is the most relevant state of strain for superconductors under thermal compression in practical applications.

In AC power-engineering applications, the energy dissipation in the superconductor is dominated by the magnetization due to alternating fields. To reduce this type of loss, conductors are being developed with twisted filaments and an increased matrix resistivity. The magnetic AC loss has been well described for low-T/sub c/ (wire) conductors. In Bi-2223 tapes the picture is different due to strong anisotropy, granularity, flux creep and large aspect ratio of the tape. The magnetic AC loss is investigated at power frequencies in various Bi-2223 tapes (twisted and nontwisted) and with different materials for the matrix (Ag, Ag alloys and ceramic barriers). When the field is parallel to the tape plane, the filaments in twisted tapes can be decoupled and the AC loss is decreased even when the matrix is silver. In tapes with ceramic barriers between the filaments, first indications of filament decoupling are observed also in perpendicular field. Compared to a round wire, there are essential differences between the AC loss mechanisms occurring in a long twisted tape and those in a short piece of nontwisted tape.

Characteristics of a circular superconducting coil made with BSCCO-2223/Ag tape depend on the amplitude and direction of the magnetic field in the windings. The effect can be estimated by studying a short sample of the same tape. However, the loss voltage-current and the frequency characteristics of a coil deviate considerably from those that are measured on a short sample. In order to estimate the deviation, the authors compared measured characteristics of a few small coils employing up to /spl sim/10 m of tape with those calculated from the short samples data. The comparison includes several arrangements of coils and field shaping elements around the coil edges and is performed in the frequency and temperature range typical for power applications (1-100 Hz and 64-78 K respectively). The results are applied to the design of a 100 kVA-50 Hz resonator coil with a high quality factor.

The critical current reduction in silver-sheathed (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ superconducting tapes (Bi2223/Ag) is investigated when loaded with uni-axial strains in combination with a magnetic field perpendicular to the tape surface. The number and quality of the grain-to-grain connections and the alignment of the superconducting cores mainly determine the critical current in Bi2223/Ag tapes. It is assumed that the transport current flows simultaneously through two current carrying paths in the tape: one through the network of Josephson junctions and the other is through the well-connected grains. The model describes well the magnetic field dependence of the critical current at various strains. A detailed analysis has shown that strain deteriorates grain connectivity, induces cracking and hence changes the current carrying path. Furthermore, strain may introduce new defects inside the grains along the strong-link current path and increase intra-granular pinning strength.

The influence of compressive and tensile axial strains on the critical current of various Nb/sub 3/Sn conductors is investigated. The investigated multifilamentary wires are especially developed for the ITER project. The critical current is determined as a function of an axial tension in a standard pull set-up. These results are compared with the results that are obtained on a bending spring-type strain device. In this second device an axial compression larger than the integral thermal shrinkage of the matrix, can be achieved. A study of the field dependence of the critical current yields the strain dependence of the extrapolated upper-critical field versus the axial strain. A good correlation is found between the two different strain devices in the tensile strain regime. There is a difference in the (limited) compressive strain regime where such a comparison can be made.

With reference to the application of NbTi tape in a superconducting thermal switch, the critical current of a 20- mu m-thick NbTi tape was measured in several directions of the magnetic field. The critical current was found to behave strongly anisotropically, due to the deformation of the NbTi. The tape is extrasensitive to the component of the magnetic field perpendicular to the surface. Without an external field this component of the self-field reduces the critical current far below its intrinsic value. A one-dimensional model can describe the reduction of critical current due to the self-field in a thin tape. >

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. >

The design and construction of a superconducting magnet system for a high-resolution neutron spin echo spectrometer. The principal solution for the field shape of optimal precession magnets is B/sub 0/cos/sup 2/( pi z/L). In practical precession magnets, this field shape is approximated by 30 superimposed concentric solenoids with a bore of 80 mm. The required field integral of 1 Tm, providing 10/sup 4/ precession turns, is achieved in a magnet with a length of about 1.5 m. The field in the center is 1.5 T maximum. The relative line integral inhomogeneity of about 10/sup -3/ obtained with such a coil is improved to less than 10/sup -6/ by two in-beam correction coils. The advanced homogeneity level means that after 10000 precession turns the precession angle remains still well defined without using tedious correction procedures. >