A.M. Campbell

A general treatment of losses in multifilamentary superconductors

Abstract Losses are calculated for a range of shapes of superconducting wire and cable. It is shown that the coupling losses can nearly always be expressed to a first approximation in terms of two parameters. One is the shape of the coil, the other (which contains most of the material parameters) is the time constant. The time constant is calculated for several cable types, including rectangular conductors, in which an exact solution for the field can be found at low frequencies. Maximum values for the loss are calculated and general conclusions for the design of cables are drawn.

Numerical solution of critical state in superconductivity by finite element software

A numerical method is proposed to analyse the electromagnetic behaviour of systems including high-temperature superconductors (HTSCs) in time-varying external fields and superconducting cables carrying AC transport current. The E–J constitutive law together with an H-formulation is used to calculate the current distribution and electromagnetic fields in HTSCs, and the magnetization of HTSCs; then the forces in the interaction between the electromagnet and the superconductor and the AC loss of the superconducting cable can be obtained. This numerical method is based on solving the partial differential equations time dependently and is adapted to the commercial finite element software Comsol Multiphysics 3.2. The advantage of this method is to make the modelling of the superconductivity simple, flexible and extendable.

High intergranular critical currents in metallic MgB2 superconductor

Strong evidence for high intergranular critical current densities and large bulk magnetic flux pinning in superconducting polycrystalline MgB2 has been observed. The presence of strongly-coupled grain boundaries in this material has been confirmed by a dramatic collapse of the magnetic hysteresis loop when a bulk specimen is ground into a fine powder and re-measured under similar conditions. Further evidence for strong intergrain links in polycrystalline MgB2 is provided by the continuous variation of the remanent magnetic moment up to the full penetration field of a bulk sample. The absence of weak-link nature in this material has profound implications for its potential in a wide range of engineering applications.

Artificial flux pinning centers in large, single-grain (RE)-Ba-Cu-O superconductors

Second-phase, nanoscale inclusions of composition Y2Ba4CuMOy (M=U, Nb, Ta, W, Mo, and Re), which form artificial pinning centers, have been introduced into large, single-grain [rare-earth (RE)]-Ba-Cu-O superconductors. A significant improvement in critical current density is observed in these samples, due presumably to various combinations of normal conducting, paramagnetic, and geometrical properties of the Y2Ba4CuMOy particles in the superconducting (RE)Ba2Cu3O7−δ (RE-123) phase matrix. These Y2Ba4CuMOy phase particles are chemically stable in the Ba-Cu-O liquid during peritectic solidification, unlike Y2BaCuO5 (Y-211) phase particles in Y-Ba-Cu-O, which opens a processing window for the fabrication of nanostructured large, single-grain (RE)-Ba-Cu-O superconductors with enhanced flux pinning for high-field engineering applications.

AC losses in high T/sub c/ superconductors

The losses in high T/sub c/ superconductors do not differ in principle from those in low T/sub c/ materials, and the same calculations can be used. However it is necessary to allow for the granularity of the materials and the high aspect ratio of the conductors. This paper derives simple, if approximate, expressions for calculating the losses in the most important cases for practical purposes. These are the use of BSCCO tapes in magnets and cables at power frequencies. The predictions of elliptical and thin strip models are compared and it is found that an elliptical geometry agrees best with the experimental results. It is also found that the voltage measured in a transport measurement depends on the position of the contacts and that if an unambiguous loss measurement is required it is necessary to include the flux outside the sample in the voltage measurement loop.<<ETX>>

A trapped field of >3 T in bulk MgB2 fabricated by uniaxial hot pressing

A trapped field of over 3 T has been measured at 17.5 K in a magnetized stack of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness 5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which is a readily scalable, industrial technique, to 91% of their maximum theoretical density. The macroscopic critical current density derived from the trapped field data using the Biot–Savart law is consistent with the measured local critical current density. From this we conclude that critical current density, and therefore trapped field performance, is limited by the flux pinning available in MgB2, rather than by lack of connectivity. This suggests strongly that both increasing sample size and enhancing pinning through doping will allow further increases in trapped field performance of bulk MgB2.

A model for calculating the AC losses of second-generation high temperature superconductor pancake coils

A model is presented for calculating the AC losses of a stack of second-generation high temperature superconductor tapes. This model takes as a starting point the model of Clem and co-workers for a stack in which each tape carries the same current. It is based on the assumption that the magnetic flux lines lie parallel to the tapes within the part of the stack where the flux has not penetrated. In this paper we allow for the depth of penetration of field to vary across the stack, and use the Kim model to allow for the variation of Jc with B. The model is applied to the cases of a transport current and an applied field. For a transport current the calculated result differs from the Norris expression for a single tape carrying a uniform current and it does not seem possible to define a suitable average Jc which could be used. Our method also gives a more accurate value for the critical current of the stack than other methods. For an applied field the stack behaves as a solid superconductor with the Jc averaged locally over several tapes, but still allowed to vary throughout the stack on a larger scale. For up to about ten tapes the losses rise rapidly with the number of tapes, but in thicker stacks the tapes shield each other and the losses become that of a slab with a field parallel to the faces.

Superconducting magnetic bearings for energy storage flywheels

We are investigating the use of flywheels for energy storage. Flywheel devices need to be of high efficiency and an important source of losses is the bearings. In addition, the requirement is for the devices to have long lifetimes with minimal or no maintenance. Conventional rolling element bearings can and have been used, but a noncontact bearing, such as a superconducting magnetic bearing, is expected to have a longer lifetime and lower losses. We have constructed a flywheel system. Designed to run in vacuum this incorporates a 40 kg flywheel supported on superconducting magnetic bearings. The production device will be a 5 kW device storing 5 kWh of retrievable energy at 50000 rpm. The Cambridge University system is being developed in parallel with a similar device supported on a conventional bearing. This will allow direct performance comparisons. Although superconducting bearings are increasingly well understood, of major importance are the cryogenics and special attention is being paid to methods of packaging and insulating the superconductors to cut down radiation losses. The work reported here is part of a three-year program of work supported by the EPSRC.

AC losses in striated YBCO coated conductors

We measured AC losses in RABiTS nonstriated as well as striated coated conductors provided by the US Air Force Research Laboratory. In addition, samples with different kinds of well defined inter-filament bridging have been measured and their losses compared with those of samples with unbridged filaments and with mono-layer samples. This bridging is designed to aid current sharing between filaments, which is advantageous, but it is necessary to determine if the effect on AC losses is detrimental. It was found that AC losses of striated samples with multiple bridges are higher than those of the samples with no bridges due to significant filament coupling but even so the losses are still substantially lower than those of a monolayer sample.

New chemically stable, nano-size artificial flux pinning centres in (RE)?Ba?Cu?O superconductors

Chemically stable, nanoscale (<50 nm) Y2Ba4CuMOy (M = Nb, Ta, W, Mo, Zr, Hf, etc,) phase inclusions, which potentially form a new type of artificial pinning centre, have been introduced successfully for the first time into large, single grain (RE)–Ba–Cu–O bulk superconductors.