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

Application of the network model 60 superconducting cables

A colliding energy of 16 TeV as requested for the proposed new hadron collider LHC of CERN can only be attained if the techniques to construct 10 T dipole magnets are available since LHC should be installed in the existing LEP tunnel. At the moment a development programme is carried out at CERN in collaboration with European laboratories and industries to construct several 10 tesla model magnets following either the NbTi/2 K or the Nb3Sn/4 K routes. Here we present a new initiative to explore the Nb3Sn route. a 4 years R&D programme was started in 1988, in the framework of a cooperation agreement with CERN, by the Applied Superconductivity Centre at the University of Twente and NIKHEF, the National Institute for Nuclear and High Energy Physics in collaboration with ECN, HOLEC and SMIT WIRE. The paper reports on various design aspects and the present state of development.

Development of a 10 T Nb3Sn “Twin Aperture” Model Dipole Magnet for the CERN LHC

A method for the complete numerical solution of the Maxwell equation for a multifilamentary wire carrying a transport current and subject to applied fields of arbitrary direction, where both current and applied fields have arbitrary time dependence, has been developed. First results show unexpected differences from analytical models used in the past. From both approaches current degradation effects can be estimated and loss contributions resulting from the combined action of applied field and current can be calculated. >

/pdf/new-developments-on-calculating-and-measuring-ac-losses-in-4y0oywwh38.pdf

New developments on calculating and measuring AC losses in composite superconductors

Non-isothermal flux penetration in type II superconductors

A discrete modeling technique is used to analyze the 29-strand NET (Next European Torus) braid in changing magnetic fields. Input for the model is the V-I relation of an individual strand and the contact resistance between touching strands. The model gives three-dimensional current patterns, V-I relations, and corresponding energy losses for the cable. The analysis shows a strong dependence of the cable properties on the direction of the field.

/pdf/a-numerical-model-for-the-29-strand-net-braid-kw7ugx7aw8.pdf

L.J.M. van de Klundert

Papers

Application of the network model 60 superconducting cables

Development of a 10 T Nb3Sn “Twin Aperture” Model Dipole Magnet for the CERN LHC

New developments on calculating and measuring AC losses in composite superconductors

Non-isothermal flux penetration in type II superconductors

A numerical model for the 29-strand NET braid