In many high-Tc superconductors the critical current density jc is an ill-defined quantity due to the smooth current—voltage characteristic. Since jc is the basic parameter entering the critical state model, its application to such materials becomes problematic. In this paper, a theory of magnetic properties and AC-losses in superconductors with smooth current—voltage characteristics is proposed. It is applied to superconductors with a power law characteristic, E ≈ jα. The AC-losses are calculated analytically; simple scaling rules are obtained for their dependence on the frequency and the field amplitude. Moreover, it is shown that the normal ohmic conductor and the “perfect” type-II superconductor (critical state) emerge as limiting cases, α = 1 and α = ∞, from the theory.

Magnetic properties and AC-losses of superconductors with power law current-voltage characteristics

A critical state model for a granular superconductor is employed to calculate the temperature and AC and DC magnetic field dependence of the complex susceptibility, χ = χ ′ + iχ ″, of a sintered bulk YBa 2 Cu 3 O 7- δ superconductor. Inter granular Josephson vortices are assumed to sweep in and out of the weak-link network while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis losses. The predictions of the model for χ′ and χ″ are consistent with experimental data and model parameters which characterize a high temperature granular superconductor can be determined. These parameters are the inter- and intragranular pinning force densities, the fraction of the superconducting grains, the grain size distribution and a London penetration depth which neglects grain anisotropy.

AC susceptibility of high temperature superconductors in a critical state model

Superconducting flux pumps enable large currents to be injected into a superconducting circuit, without the requirement for thermally conducting current leads which bridge between the cryogenic environment and room temperature. In this work, we have built and studied a mechanically rotating flux pump which employs a coated conductor high-Tc superconducting (HTS) stator. This flux pump has been used to excite an HTS double pancake coil at 77 K. Operation of the flux pump causes the current within the superconducting circuit to increase over time, before saturating at a limiting value. Interestingly, the superconducting flux pump is found to possess an effective internal resistance, Reff, which varies linearly with frequency, and is two orders of magnitude larger than the measured series resistance of the soldered contacts within the circuit. This internal resistance sets a limit for the maximum achievable output current from the flux pump, which is independent of the operating frequency. We attribute this ...

Dynamic resistance of a high-Tc superconducting flux pump

Magnets fabricated with HTS wire can not be operated in a true persistent mode as superconducting joints of sufficient technological quality have not been achieved to date. In order to maintain a constant magnetic field in a HTS magnet a power supply has to be permanently employed, which then leads to heat losses in the cryo-system through the employment of current leads. By using a flux pump these losses can be minimized. We present a new flux pump based on 2G HTS wire. This device energized at 77 K a 2.7 mH 2G HTS double-pancake coil to its critical current of 49 A within 112 seconds. The operating principle will be described and data of the current ramping is shown. Considering the simplicity of the device and the potential to increase the generated current to 200 A and more, this new flux pump is very promising for many superconducting devices including HTS and LTS magnets and rotating machines.

Flux Pump for HTS Magnets

We have developed a model that describes the transverse load degradation in Nb3Sn CICCs, based on strand and cable properties, and that is capable of predicting how such degradation can be prevented. The Nb3Sn cable in conduit conductors (CICCs) for the International Thermonuclear Experimental Reactor (ITER) show a significant degradation in their performance with increasing electromagnetic load. Not only do the differences in the thermal contraction of the composite materials affect the critical current and temperature margin, but mostly electromagnetic forces cause significant transverse strand contact and bending strain in the Nb3Sn layers. Here, we present the model for transverse electro-magnetic load optimization (TEMLOP) and report the first results of computations for the ITER type of conductors, based on the measured properties of the internal tin strand used for the toroidal field model coil (TFMC). As input, the model uses data describing the behaviour of single strands under periodic bending and contact loads, measured with the TARSIS set-up, enabling a discrimination in performance reduction per specific load and strand type. The most important conclusion of the model computations is that the problem of the severe degradation of large CICCs can be drastically and straightforwardly improved by increasing the pitch length of subsequent cabling stages. It is the first time that an increase of the pitches has been proposed and no experimental data are available yet to confirm this beneficial outcome of the TEMLOP model. Larger pitch lengths will result in a more homogeneous distribution of the stresses and strains in the cable by significantly moderating the local peak stresses associated with the intermediate-length twist pitches. The twist pitch scheme of the present conductor layout turns out to be unfortunately close to a worst-case scenario. The model also makes clear that strand bending is the dominant mechanism causing degradation. The transverse load on strand crossings and line contacts, abbreviated as contact load, can locally reach 90 MPa but this occurs in the low field area of the conductor and does not play a significant role in the observed critical current degradation. The model gives an accurate description for the mechanical response of the strands to a transverse load, from layer to layer in the cable, in agreement with mechanical experiments performed on cables. It is possible to improve the ITER conductor design or the operation margin, mainly by a change in the cabling scheme. We also find that a lower cable void fraction and larger strand stiffness add to a further improvement of the conductor performance.

https://iopscience.iop.org/article/10.1088/0953-2048/19/9/011/pdf

Transverse load optimization in Nb3Sn CICC design; influence of cabling, void fraction and strand stiffness

The selffield losses of commercial available single core and multifilament wires with Cu and CuNi matrix have been studied Good agreements have been obtained between experimental results and numerical calculations Analytic relations for low amplitudes of the sinusoidal transport current allow for the determination of the critical current density at zero field and the conductivity of the matrix material

Self field losses in superconducting circular wires

Plasma sprayed YBa2Cu3O7-x on nickel-rich substrates: XPS analysis and interface interaction

A superconductive magnet system solely cooled by thermal conduction and two Gifford-McMahon cryocoolers has been developed at our institute. One cooler is redundant to obtain reliable and serviceable operation. The magnet operates at a temperature of 12 K. In order to reduce the heat flux into the system when one cooler is out of service, two thermal switches were developed with the following features. In both cases thermal contact is made by pressing two or more pieces of metal against each other. The first switch is a lathe-chuck type and consists of three metal pieces symmetrically arranged around a metal bar. They are simultaneously pushed in a radial direction thus making mechanical and thermal contact. The switch is closed by an external force applied via a Bowden cable. The second is a bench-vise type. A metal bar is clamped between two metal jaws by means of the action of a screw driven by an external torque. In both cases relatively fast switching is possible. Thermal resistances were measured as a function of the contact force and with various surface materials. The thermal resistance obtained in the on-state was better than 0.5 W/K, and in the off-state at least a factor of 1000 less. Thermal and mechanical cycling appeared to have no large influence on the switch performance.

A Mechanically Driven Switch for Decoupling Cryocoolers

Within the scope of our research program for a 25 kA superconducting rectifier, we have built a 25 kA s.c. coil being a single layer solenoid with a bore of 0.45 meter and a volume of 52 litre. The starting point for the design was to avoid any metallic structural material. This unique coil consists of 26 turns of a Rutherford cable in one layer covered at the outside with 6 millimeter glassfibre reinforced epoxy, in order to lower the azimuthal and axial stresses in the conductor to acceptable values. The coil has been vacuum impregnated with a glassfilling factor of 0.529. The paper describes the strength calculations and the construction details. A theoretical analysis of the mechanical behaviour of the glassfibre-epoxy-conductor lamination is given.

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A 25 kA, 2T, 78 kJ, 52 litre superconducting test coil. Strength calculations and construction

The inductive response of metals to applied fields with trapezoidal time dependence is described for samples with cylindrical geometry and for slabs. The influence of coil effects and magnetoresistance is accounted for but mean free path and surface effects are ignored. Experimental data obtained for tin show a close agreement with the theoretical predictions.

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L.J.M. van de Klundert

Papers

Self field losses in superconducting circular wires

Plasma sprayed YBa2Cu3O7-x on nickel-rich substrates: XPS analysis and interface interaction

A Mechanically Driven Switch for Decoupling Cryocoolers

A 25 kA, 2T, 78 kJ, 52 litre superconducting test coil. Strength calculations and construction

The response of metals in trapezoidal magnetic fields