T. Hughes

THE INFLUENCE OF GEOMETRY ON SELF-FIELD AC LOSSES OF AG SHEATHED PBBI2223 TAPES

We measured axial fields along an Ag sheltered PbBi2223 superconducting tape 3 mm wide and 0.03 mm thick carrying 47 Hz alternating currents. Tte critical current of the tape at 77 K was 1 5 A corresponding to a critical current density of 16 000 A/cm2. The axial electric field was measured by pairs of voltage leads brought out in a plane perpendicular to the tape axis, turned axially to meet and so led out to a voltmeter. The spatial variation and the current dependence of the electric field, obtained from several pairs of taps closed at different radial positions, were consistent with the prediction for a uniform superconducting thin rectangle with a critical current Ic rather than a superconductor of elliptical cross-section. Our results also showed that the hysteretic losses at currents below Ic can be measured with an accuracy of about 5% using a pair of taps closed at a radial distance about three times the tape half-width.

Experimental study on AC losses in Ag sheathed PbBi2223 tapes with twist filaments

Abstract Experimental measurements of AC losses were carried out on Ag sheathed PbBi2223 tapes with twisted and untwisted filaments. Losses were measured at 77 K as function of frequency and magnetic field parallel and perpendicular to the tape surface, using appropriate pick-up loops. Both the first and third harmonics of the signal were measured, in order to distinguish between the hysteresis loss and other types of loss. The effect of filaments uncoupling by twisting was clearly identified. For a tape with a twist pitch of 10 mm and Ic=40 A (∼20 kA cm−2) operating at 43 Hz, the filaments are uncoupled in fields less than 40 mT, which is greater than the full penetration field for both the filaments and the tape. Hence, a reduction in the hysteretic loss of the superconducting core is realised at power frequency between 10 and 40 mT. Results form the self-field loss measurement implies the uncoupling of twisted filaments at relative low transport current (I

Thermometric measurements of the self-field losses in silver sheathed PbBi2223 multifilamentary tapes

Self-field ac losses in Ag sheathed PbBi-2223 tapes were measured using a thermometric method, which determines the losses by measuring the temperature profile of a vacuum insulated sample, with both ends at a fixed temperature. In practice, the samples were placed in a vacuum capsule immersed in LIN bath. By minimizing the bath superheat, thermal emf and heating at the current contacts, a loss induced temperature increase as low as 2 mK was measured using a Si diode thermometer. With a typical sample length of 600 mm, self-held losses between 7/spl times/10/sup -6/ W/m and 4/spl times/10/sup -3/ W/m were measured at different frequencies. The results are in good agreement with both the electric measurement and theoretical calculation. This provides the first independent confirmation that electric measurement with carefully placed voltage loops can give the true losses of the sample.

Measurement of AC losses in textured polycrystalline Bi-2212 thin rods

AC losses of textured polycrystalline Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/ thin rods carrying AC transport currents have been measured in self-field and in DC magnetic fields at 77 K. Measurements of the first and third harmonic of the voltage are presented. The current amplitude and frequency dependence of losses as well as the ratio between the different harmonics of the voltage is analysed. The results are compared with the losses calculated numerically for a superconductor characterised by power-law current-voltage dependence.

Comparative thermometric and electric measurements of the self-field AC losses in Ag-sheathed PbBi2223 tapes

Abstract A method for the thermometric measurement of the self-field AC losses of Ag-sheathed PbBi2223 tapes is described. This technique determines the loss by measuring the temperature profile associated with the losses, along the length of a vacuum insulated tape with each end maintained at a fixed base temperature. In practice the samples were placed in a vacuum capsule immersed in a liquid nitrogen bath. By ensuring the proper cryogenic conditions and with the use of a sensitive diode thermometer self-field losses between 7 μWm −1 and 4 mWm −1 have been measured. Comparisons with the electrical measurement technique show a good correspondence between the two methods. The effect, on the self field AC losses, of an inhomogenous distribution of critical current along the length of the tape is also discussed. Measurement of the frequency response of the losses show the importance of the eddy currents induced in the silver sheath. It is shown that these eddy currents are mainly due to the effect of the perpendicular component of the self field.

Measurement of Self-field AC Losses in PbBi-2223 Ag-sheathed Tapes

Abstract A technique for measurement of self field AC losses of PbBi-2223 Ag sheathed tapes is presented and preliminary results shown.

Magnetic coupling and self-field AC losses of two neighbouring Ag sheathed PbBi2223 tapes

The magnetic interaction between two neighbouring superconducting tapes has significant influences on their self-field AC losses. While the two tapes are independent of each other when separated far apart, they are expected to be fully coupled and behave as a single tape when placed very near. One of the consequences of such coupling is the increase in the self-field loss per tape, which is double of that for an uncoupled tape carrying the same current. In order to assess the interactions among tapes in systems such as a power cable, a better understanding is necessary on the critical distance where the coupling becomes important. Experimental measurements on the self-field losses in two neighbouring Ag sheathed PbBi2223 tapes were carried out with the two tapes separated by various distances, while placed either side by side (side-configuration) or one on top of the other (top-configuration). The results indicates that the critical coupling distance for the top-configuration is about 5 mm, where the increase in loss per tape is about 10%. The critical length for the side-configuration is found to be of the same order of magnitude.

Characteristics of the a.c. losses in Ag-sheathed PbBi2223 tapes

Abstract A mini review of our latest results on the a.c. losses in Ag-sheathed PbBi2223 tapes is presented with the focus on self-field losses in a variety of situations and external losses with and without a d.c. transport current. Comparative measurements with the thermometric and electric methods have demonstrated the validity of the electric method despite the problem of a distribution of the loss electric field. Using these two methods the effect of a longitudinal distribution of critical current along the tape has been shown. The influence of a transverse distribution of the local J c through the core of the tape is also examined. Studies of the magnetic field dependence of the self-field losses have shown that different regions of the core have a different J c ( B ) dependence, in contrast to the temperature dependence where all regions of the core show a similar dependence. Measurements at higher frequencies show the importance of flux diffusion where an increased dB dt gives an increased induced critical current. At sufficiently higher frequencies (> 200 Hz) the self-field of the core is shown to induce eddy current losses within the silver sheath of the tape. For the external a.c. field case in parallel fields the losses are well described by the infinite slab geometry. For the case of the perpendicular field the losses are considerably less than those expected from either the thin ellipse or rectangle geometry. Possible mechanisms for this discrepancy include granularity of the tape or partial uncoupling of the filaments. For the combined action of an a.c. external field and a d.c. transport current our results show that the total losses cannot be obtained using only using a pick-up loop to measure the flux changes within the tape. An additional loss sustained by the d.c. current source also needs to be measured via a pair of voltage taps.

Self-field AC losses of assemblies of Ag sheathed PbBi2223 tapes

Magnetic interaction between two neighbouring superconducting tapes carrying a transport current can cause them to become coupled so that they react as one tape. One of the consequences of such coupling is the increase in the self-field loss per tape. The critical distance at which the tapes begin to couple is important in the assessment of the interactions among tapes in systems such as a power cable or coil. Experimental measurements of the self-field losses in two neighbouring Ag sheathed PbBi2223 tapes carrying the same transport current were carried out with the two tapes separated by various distances, while placed one on top of the other (stack-configuration). This configuration is similar to that found in adjacent layers in a coil. The results indicate that the critical coupling distance for the top-configuration is about 5 mm, where the increase in loss per tape is about 10%. Measurements are also undertaken for the more realistic situation where each of the tapes has a different critical current.

Modelling of AC Losses in High-Tc Superconductors with Flux Creep E-J Characteristics

Abstract A numerical model for simulating high-T c bulk material superconducting tapes subject to AC magnetic fields or transport currents, is presented. The model is based on an electromagnetic diffusion process and describes the superconducting tape as a nonlinear conductor with conductivity being a functions of both local magnetic and electric fields, as deduced from the flux creep E-J characteristics of the material. AC losses for various applied fields or transport currents are calculated and discussed.