Jin Fang

Frequency dependent AC loss in degraded Bi-2223/Ag tape

Abstract The low-frequency ( f ) AC loss per cycle of a high-temperature superconducting Bi-2223/Ag tape is frequency independent when the measurements are performed at 77 K with current amplitude I m below the critical current I c . Its loss vs. I m / I c is consistent with the results derived from the critical state model with a Kim type field dependent critical current density J c . After I c of the sample is degraded from 32 to 16.5 A by repeated cooling and warming, the loss becomes strongly f dependent and it increases by a factor of 2–3 with decreasing f from 405 to 15 Hz. It is found that by defining an effective critical current I c1 , which increases from 14 to 25 A with increasing f from 15 to 405 Hz, the Norris result for an elliptic bar with a Bean type J c fits the data for all values of f . This behavior indicates that the I c degradation results from the division of as-obtained wide Josephson junctions into fine ones, and the thermally activated creep of Josephson vortices formed in the system of fine junctions leads to a frequency dependent I c1 .

Test results and analyses of conductor short samples for HT-7U

The experiments of Cable-in conduit conductor (CICC) short samples with high proportion of segregated copper strands have been carried out in SULTAN facility last September. These experiments aimed to investigate transient stability and AC losses of CICC conductor coated with different resistive barriers (Pb-30Sn-2Sb or Ni plating on strands) and to check the design of PF and TF CICC for HT-7U magnets. The resistive barriers influences on the stability and AC losses of CICC are evaluated. These experimental results are used for the choice of HT-7U TF and PF CICC design

Strain effect of YBCO coated conductor on transport characteristics

YBCO coated conductor is commercially available now and will be widely used in various practical power applications. In these applications, the conductor will be subjected to different kinds of mechanical stresses and strains. To achieve an optimal design of the devices, the transport characteristics under tensile, bending and torsional strains were investigated at 77 K and self-field. It is observed that the critical current Ic increases initially with applied strain, reaches a peak at emax and then decreases with further increases of strain. The improvement of Ic at emax can be attributed to the residual compressive strain and the shield of plastic deformation produced by the laminate layers. In AC cases, the induced Ic degradation causes an increase of transport loss, which can be described by the Norris model with a non-uniform current distribution. The torsion of the conductor can decrease the AC loss by field decoupling. The fluctuation of the n-value and the almost constant slope of the AC transport loss curves at applied strains in the measured range indicate the possibility of plastic deformation without the influence of the current distribution and the changes of the pinning potentials in the weak-link areas of the conductor.

Transport ac loss of elliptical thin strips with a power-law E(J) relation

The transport ac loss Q of an elliptical thin strip of critical current I c with a power-law relation is accurately computed as a function of current amplitude I m and frequency f. The resulting is normalized to following the Norris critical-state formula, and converted to at a critical frequency f c based on a transport scaling law. Having a set of at several values of n as a base, a general expression of is obtained, which can be used to easily calculate for any practical purposes.

Transport ac losses of a second-generation HTS tape with a ferromagnetic substrate and conducting stabilizer

The current–voltage curve and transport ac loss of a second-generation HTS tape with a ferromagnetic NiW substrate and brass stabilizer are measured. It is found that the ac loss is up to two orders of magnitude larger than what is expected by the power-law E(J) determined by the current–voltage curve and increases with increasing frequency. Modeling results show that the overly large ac loss is contributed by the ac loss in the HTS strip enhanced by the NiW substrate and the magnetic hysteresis loss in the substrate, and the frequency-dependent loss occurs in the brass layer covering the substrate but not in the ferromagnetic substrate itself as assumed previously. The ac loss in the brass layer is associated with transport currents but not eddy currents, and it has some features similar to ordinary eddy-current loss with significant differences.

The Influence of the YBCO Tape Arrangement and Gap Between the Two Tapes on AC Loss

This paper mainly focuses on the influence of different arrangements and gaps on ac loss. First, vertical and horizontal arrangement models of the two tapes are set up based on the Clem model. Second, a two-dimensional numerical calculation model based on H-formulation is established in the Comsol Multiphysics finite-element software. In addition, to obtain the real electromagnetic environment of different models, we use the Comsol Multiphysics to simulate the magnetic field distribution of the two models. Finally, we set up the experimental platform to prove the validity of the model. Results illustrate that the ac loss of the two models decreases with the increase of gap, and the impact of the gap change on the ac loss of the vertical arrangement model is larger than that of the horizontal arrangement model.

Geometry dependence of magnetic and transport AC losses in Bi-2223/Ag tapes with different aspect ratios

On five Bi-2223/Ag tapes with different aspect ratios from 5 to 26, AC losses have been measured at 77 K while a parallel AC magnetic field or a perpendicular AC magnetic field or a longitudinal AC transport current is applied. It has been found that at any frequency the perpendicular magnetic losses per cycle increase, but the parallel magnetic losses per cycle and the transport losses per cycle decrease as the aspect ratio increases. These experimental results are in accord with theoretical results. Meanwhile, we investigated the geometry dependence of the decay time constant of coupling current and that of full penetration field.

Some aspects of fabrication of YBCO thin films by inverted cylindrical magnetron sputtering in large area

The use of inverted cylindrical magnetron sputtering to produce high-quality superconducting thin films is investigated. The critical parameters in the sputtering process have been identified, and 10-mm*10-mm YBCO thin films with critical temperatures (T/sub c/s) of 85-90 K and critical current densities (J/sub c/s) of 1*10/sup 6/ A/cm/sup 2/ (77 K, 0 T) have been deposited reproducibly in optimal conditions. In order to extend the work to a large area, the sputtering gun and the heater have been redesigned. The films thickness and stoichiometric variation are less than +or-5% over a 50-mm*50-mm area. The uniformity of substrate temperature plays an important role in large-area in situ deposition of YBCO films. At optimum conditions, the variation of T/sub c/ is found to be less than +or-1 K.<<ETX>>

A Novel AC Loss Measurement Technology for High Temperature Superconducting Cable With Large Current Capacity Using a Compensation Coil

High temperature superconducting (HTS) cable is a new development of applying superconducting technology in power system. AC loss measurement technology is a key factor in designing HTS cable as it decides the stability and operating cost of the cable. In this paper, a novel AC loss measurement technology for HTS cable is proposed based on LabVIEW. A compensation coil with large current carrying capacity and a stepping motor are used to test the variation of AC losses of HTS cable. A closed loop system is developed to obtain AC losses measured using modules of high-precision Data Acquisition (DAQ) and motion controller of National Instruments (NI). Finally, AC losses of a 0.2 m, 110 kV/1.5 kA prototype HTS cable are measured at 49.9 Hz and 60 Hz frequency considering compensation coil and lock-in amplifier. Obtained results show that AC losses linearly increase with the transport current in logarithmic coordinates. The comparison results applying with compensation coil and lock-in amplifier prove that the proposed measurement system is effective in measuring AC loss of HTS cable with large current capacity in different frequency.

In-plane and out-of-plane magnetoresistivity in a MTG Er-doped YBCO crystal

We have measured the temperature dependence of top and bottom voltage signals of a Y0.8Er0.2Ba2Cu3O7?? crystal in magnetic field up to 7.5 T parallel to the c-axis using the pseudo-flux-transformer method. The results show that in a dissipative regime the voltage signal on the side of the applied current contacts is more than that on the opposite side; meanwhile the bottom voltage signal shows a field-dependent peak effect near the critical temperature Tc. We calculated the true resistivities ?ab and ?c of the crystal within an anisotropic model. The difference between the calculated ?ab and ?c and the experimental values ?abst and ?cst is helpful in understanding flux dynamic behaviour. Based on the Yeshurun?Malozemoff model, we have investigated temperature and field dependences of effective activation energy along the ab-plane and c-axis obtained from the ?ab(T) and ?c(T), respectively. The temperature dependence of effective activation energy along the c-axis displays a good thermally activated flux flow (TAFF) behaviour. In the TAFF regime the field dependence of effective activation energy follows a power behaviour, U(H) = 4.24 ? 104 H?1.1. The TAFF regime and the coupling between the layers have been discussed.