E Seiler

AC Loss in Pancake Coil Made From 12 mm Wide REBCO Tape

The design of a superconducting coil from high-performance REBCO coated conductors is often complicated because of complex anisotropy of the critical current density Jc . It is important to understand how much detail of this feature must be taken into consideration in the prediction of maximum achievable current and the expected ac loss. We present the results of investigation performed with a small (ten turns, 60 mm inner diameter) coil made from SuperPower tape of 12 mm width. The knowledge of Jc(B,θ) determined on short sample allowed prediction of the maximum achievable current of the coil and the ac loss behavior. We have also investigated the effect of the tape nonuniformity. Our results confirm that the lateral nonuniformity when Jc at tape edges is lower than in its center leads to significant increase of ac loss. A longitudinal nonuniformity, in particular a reduction of critical current in some portion along the tape length, is hardly observable in the ac loss result. On the other hand, using a piece of tape with lower Jc in the innermost coil turn would significantly reduce the maximum current. We also present calculations showing the change in current-voltage curve and redistribution of ac dissipation in the case of nonuniform tape quality.

How to determine critical current density in YBCO tapes from voltage–current measurements at low magnetic fields

In this paper, an optimization method to determine the magnetic field dependence of the intrinsic critical current density, Jc(B), is introduced. First of all the critical current is measured in various external magnetic fields. Then the Jc(B)-dependence that fits optimally to the measurements is searched for. In these computations the self-field of the sample is taken into account and thereby measurements performed in low external magnetic fields, below 0.1xa0T, can be exploited. Here the Jc(B)-dependence of YBCO material is described by the Kim model, which is modified to include the anisotropy also. Thus, there are four parameters to search for: zero field critical current density Jc0, reference field B0, Kim model exponent α and anisotropy scaling factor γ. The search for these parameters was computationally challenging but computation times were still satisfactory. As examples, the Jc(B)-dependences of two YBCO samples from different manufacturers were found. For both samples, all parameters except B0 were near each other. For example, Jc0 values for both samples were about 0.9 × 1010xa0Axa0m−2.

Predicting AC loss in practical superconductors

Recent progress in the development of methods used to predict AC loss in superconducting conductors is summarized. It is underlined that the loss is just one of the electromagnetic characteristics controlled by the time evolution of magnetic field and current distribution inside the conductor. Powerful methods for the simulation of magnetic flux penetration, like Brandts method and the method of minimal magnetic energy variation, allow us to model the interaction of the conductor with an external magnetic field or a transport current, or with both of them. The case of a coincident action of AC field and AC transport current is of prime importance for practical applications. Numerical simulation methods allow us to expand the prediction range from simplified shapes like a (infinitely high) slab or (infinitely thin) strip to more realistic forms like strips with finite rectangular or elliptic cross-section. Another substantial feature of these methods is that the real composite structure containing an array of superconducting filaments can be taken into account. Also, the case of a ferromagnetic matrix can be considered, with the simulations showing a dramatic impact on the local field. In all these circumstances, it is possible to indicate how the AC loss can be reduced by a proper architecture of the composite. On the other hand, the multifilamentary arrangement brings about a presence of coupling currents and coupling loss. Simulation of this phenomenon requires 3D formulation with corresponding growth of the problem complexity and computation time.

Modeling of current density distributions in critical state by commercial FE codes

The method of minimum magnetic energy variation has proved its merit in modeling the magnetic field interaction with hard type-II superconductors. Its main advantage is that the front of flux penetration is calculated starting from a physical principle equivalent to the Beans critical state model. We have developed a procedure that adapts the above-mentioned method to the environment of the commercial finite element code ANSYS. As a step forward with respect to existing works, this allows to include magnetic materials into simulations. The case of a superconducting strip covered by a ferromagnetic sheath is reported.

Performance Improvement of Superconducting Tapes Due to Ferromagnetic Cover on Edges

Improvement of critical current and reduction of AC loss is presented for a common multifilamentary Bi-2223/Ag tape with thin nickel layer electroplated at the edges. Numerical calculations using a commercial finite-element code have been carried out to find the distributions of electrical current and magnetic field when the whole section is filled with the critical current density. The dependence of critical current density on local magnetic field and its orientation was taken into consideration, allowing to understand the mechanism of Ic increase. Optimization of the cover thickness and width would be possible due to these calculations. AC loss was investigated experimentally in the condition of AC transport and simultaneous action of AC magnetic field in phase with transport current. Reduction of both the transport loss and the magnetization loss has been observed, as well as the loss at simultaneous action of transport AC and applied AC field.

A study of coupling loss on bi-columnar BSCCO/Ag tapes through ac susceptibility measurements

Coupling losses were studied in composite tapes containing superconducting material in the form of two separate stacks of densely packed filaments embedded in a metallic matrix of Ag or Ag alloy. This kind of sample geometry is quite favourable for studying the coupling currents and, in particular, the role of superconducting bridges between filaments. By using the ac susceptibility technique, the electromagnetic losses as a function of ac magnetic field amplitude and frequency were measured at the temperature T = 77 K for two tapes with different matrix composition. The length of the tapes was varied by subsequent cutting in order to investigate its influence on the dynamics of magnetic flux penetration. The geometrical factor χ0 which takes into account the demagnetizing effects was established from ac susceptibility data at low amplitudes. Losses versus frequency dependencies have been found to agree nicely with the theoretical model developed for round multifilamentary wires. Applying this model, the effective resistivity of the matrix was determined for each tape by using only measured quantities. For the tape with a pure silver matrix its value was found to be larger than that predicted by the theory for given metal resistivity and filamentary architecture. In contrast, in the tape with a Ag/Mg alloy matrix, an effective resistivity much lower than expected was determined. We explain these discrepancies by taking into account the quality of the interface between the superconducting filaments and the normal matrix. In the case of a soft matrix of pure Ag, this is of poor quality, while the properties of the alloy matrix seem to provoke the extensive creation of intergrowths which can be actually observed in these kind of tapes.

AC Losses and Current Sharing in an YBCO Cable

Power cables constitute one of the most promising industrial applications for HTS materials. The Super3C project aims at establishing the feasibility of a low loss 30 m long 1 kA 10 kV YBCO cable. A major goal of the design is to minimize AC losses. Therefore, a circuit analysis model was tailored for predicting the losses in YBCO cables accurately. During this project, also a 0.5 m long one layer cable was constructed to test the behavior of a real YBCO cable. The AC losses measured from this cable were in good agreement with the computed results and thereby the feasibility of the developed design tool was verified. However, the measurements revealed that differences in contact resistances caused uneven current sharing between the tapes but computational analysis predicted the current sharing to be nearly even in the final 30 m cable.

The influence of filament arrangement on current distribution and AC loss in Bi-2223/Ag tapes

State-of-the-art deforming techniques allow the preparation of composite Bi-2223/Ag tapes with well controlled filament arrangement. This would allow tailoring of the tapes electromagnetic properties according to specific applications. We studied the possibility to reduce, through a suitable filament ordering, the hysteresis losses caused by an external magnetic field perpendicular to the tapes wide face. For this purpose, the distribution of shielding electrical currents in multifilamentary superconducting arrays, as a response to a perpendicular AC magnetic field, was investigated both theoretically and experimentally. We show that due to optimizing of filament arrangement, the hysteresis loss can be suppressed by one order of magnitude.

Shielding and losses in multifilamentary tapes exposed to perpendicular AC magnetic fields

In multifilamentary high-T/sub c/ tapes for energy transportation, a component of magnetic field is applied normally to the filament surfaces. Since the thickness of any filament of the tape is much smaller than the width (the factor is usually between 10 and 20), it results in large field amplification on the tape surface and consequently a large loss. This problem is studied in detail here, analysing the behavior of Bi 2223/Ag tapes with different structures of the filamentary zone. Experimental data are compared with calculations based on finite element codes. The importance of filament arrangement for AC losses is illustrated.

Modification of critical current in HTSC tape conductors by a ferromagnetic layer

In some applications of tape conductors from high temperature superconductors (HTSC) the magnetic field is created by the transported current itself. This is e.g. the case of power transmission cables or current leads. Quite complex distribution of local magnetic field determines then the ability of the superconducting element to carry electrical current. We have investigated how much the critical current of a tape conductor can be changed by putting a ferromagnetic layer in the vicinity of the HTSC material. Numerical procedure has been developed to resolve the current and field distribution in such superconductor-ferromagnet composite tape. Theoretical predictions have been confirmed by experiments on sample made from Bi-2223/Ag composite tape. The critical current of such tape can be improved by placing a soft ferromagnetic material at the tapes edges. On the other hand, the calculations show that the ferromagnetic substrate of YBCO coated tape reduces its self-field critical current.