Transition temperature of strong-coupled superconductors.

The feasibility of high field magnet applications of the twisted stacked-tape cabling method with 2G YBCO tapes has been investigated. An analysis of torsional twist strains of a thin HTS tape has been carried out taking into account the internal shortening compressive strains accompanied with the lengthening tensile strains due to the torsional twist. The model is benchmarked against experimental tests using YBCO tapes. The critical current degradation and current distribution of a four-tape conductor was evaluated by taking account of the twist strain, the self-field and the termination resistances. The critical current degradation for the tested YBCO cables can be explained by the perpendicular self-field effect. It is shown that the critical current of a twisted stacked-tape conductor with a four-tape cable does not degrade with a twist pitch length as short as 120 mm. Current distribution among tapes and hysteresis losses are also investigated. A compact joint termination method for a 2G YBCO tape cable has been developed. The twisted stacked-tape conductor method may be an attractive means for the fabrication of highly compact, high current cables from multiple flat HTS tapes.

https://iopscience.iop.org/article/10.1088/0953-2048/25/1/014011/pdf

HTS twisted stacked-tape cable conductor

This review focuses on the superconducting properties of MgB2 that are relevant for power applications. The reversible mixed state parameters are the most important, since they define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current. They also determine the flux pinning energy, the pinning force and the elastic properties of the flux line lattice and, therefore, strongly influence the critical current densities. The magnetic properties of magnesium diboride are anisotropic and influenced by the two different energy gaps of the σ- and π-bands. Whereas the transition temperature could not be enhanced significantly during the past five years, the upper critical field was considerably increased by impurity scattering or doping. Flux pinning is very weak in MgB2 single crystals and was only improved by irradiation techniques so far. In polycrystalline samples, grain boundary pinning seems to play the dominant role. High critical currents close to the theoretical limit were found in c-axis oriented thin films. The anisotropy of the upper critical field strongly reduces the critical currents in untextured MgB2 at high magnetic fields, where the supercurrents become highly percolative, since not all grains are superconducting anymore. The performance of polycrystalline wires and tapes was significantly improved during the past few years by increasing the upper critical field and by reducing its anisotropy. Pinning seems to be nearly optimized in many forms of this material, but the connectivity between the grains might be further improved.

https://iopscience.iop.org/article/10.1088/0953-2048/20/12/R01/pdf

Magnetic properties and critical currents of MgB2

Superconducting magnets have a variety of industrial, medical and research applications. This review discusses the prospects for MgB2 superconductor for practical magnet applications vis-?-vis the intermetallic low (LTS) and high temperature superconductor (HTS) cuprates. It has high TC (39?K), high JC (105?106?A?cm?2 at 4.2?K and in the self-field) and HC2 (15?20?T at 4.2?K) in wire/tape geometry, with great scope for further improvement in the coming years, making it a promising candidate for practical applications. The superconducting properties of MgB2 differ from those of LTS and HTS in many ways. Besides the unusually high TC, MgB2 has a large coherence length, low anisotropy and transparent grain boundaries. The most important difference between MgB2 and other practical superconductors is that it has two superconducting gaps originating from two different bands. Tuning the scattering rates between the two bands improves the superconducting properties and the practical applicability of MgB2. The different methods of fabrication of MgB2 conductors are described and compared. Fabrication of long length MgB2 conductors is relatively easy and less expensive as compared to HTS and the method allows the use of a variety of sheath materials with suitable barriers or reinforcement. The conductors have much better mechanical properties for practical applications. The critical issues and the challenges to be addressed for realization of MgB2 superconductors as the first choice for high field magnet applications are discussed. At present MgB2 is most suited for 20?25?K operation in fields of 1?2?T.

https://iopscience.iop.org/article/10.1088/0953-2048/20/1/R01/pdf

Prospects for MgB2 superconductors for magnet application

Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (AC) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of AC loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA to up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology (KIT) and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low AC losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Times are therefore mature for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cables.

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

RBCO (R = Y or rare earth element) coated conductors (CC) are the most promising HTS materials for future high field coils operated at moderately high temperature (40-50 K). Coils are planned for the second generation of fusion reactors (DEMO, "DEMOnstrator") and beyond. A ROEBEL bar concept for a high current (kA-class) low AC loss cable is the most suitable assembling technique for conductors in magnet windings due to the flat rectangular cross section. The presented RACC-cable technique (RACC=ROEBEL assembled coated conductors) works with pre-shaping of tapes into strands with the ROEBEL specific meander geometry. The usually very good bending properties of the CC support the assembling procedure of the RACC-cable. We report on a 16 strand RACC-cable with 19 cm transposition length made from CC material from the commercial supplier SuperPower which reached 1020 A transport critical current at 77 K (Jeng = 11.3 kAcm-2). The basic properties of the virgin YBCO tapes and the shaped strands like orientation and field dependent transport currents, current homogeneity and bending effects, were investigated and correlated with the measured properties of the RACC-cable. Calculation of the self field effects by means of a model adapted to the specific RACC-cable geometry and in particular taking into account the current distribution in the cable, explained the 30% current reduction in the cable quantitatively.

ROEBEL Assembled Coated Conductors (RACC): Preparation, Properties and Progress

For technical application of high current MgB2 wires, a multifilamentary composite with small filaments and good thermal stabilization is required to avoid damage from excessive heat generation in the resistive regime above Jc With reduced filament size, however, a degradation of Jc, due to a more and more inhomogeneous microstructure, sausaging effects and a reaction layer between filament and sheath, usually increases dramatically The influence of filament size on Jc was investigated in the light of these aspects, developing very thin steel reinforced wires for a satellite application in the ASTRO-E2 XRS instrument (NASA), applying the 'in situ route', which starts with Mg + B powder mixtures Compared to wires processed at temperatures above 900 °C, a phase formation heat treatment at 640 °C significantly reduces the reaction layer between filament and sheath to about 1–2 µm and favours the nucleation of small MgB2 grains in the nanometre scale This leads to the best transport currents achieved so far in undoped round wires made from commercial powders Low n-values indicate an improved intrafilamentary current sharing during the Ic transition However, at low fields and very high currents the E(I) characteristics indicate a still thermally driven transition with excessive heat generation and irreversible damage of the wire

In situ MgB2 round wires with improved properties

We have measured the ac transport current loss of eight different Bi(2223) tapes with varying filament geometries in the frequency range from 13 to 500 Hz at 77 K. The investigated tapes have an Ag sheath (i.e. 37 or 703 filaments) or a modified sheath using AgMg. Furthermore tapes with novel geometries such as multifilamentary wire-in-tube (WIT) tapes and jelly-roll tapes have been studied. In addition we have investigated twisted tapes with a twist-pitch of 1.4 cm in the 37 filamentary Ag and the 85 filamentary AgMg tapes. To compare these tapes with different I c values we calculated the loss factors of the tapes. We find that all the loss factors lay within the limits given for an elliptical or a strip like conductor. Furthermore we find differences in the loss factors due to the filament geometries.

AC transport current losses of multifilamentary Bi(2223) tapes with varying filament geometries

In mono- and multifilamentary MgB2 wires with small filament diameters a homogeneous and fine-grained MgB2 phase is required for high transport currents and reactions with the sheath have to be avoided. Taking this into account we developed very thin steel reinforced MgB2 wires for application as current leads in a research satellite. In situ phase formation in the wire from Mg+B powders was chosen as the most favourable method to achieve good grain connectivity and thus high transport currents. We tested the current lead wires with transport current measurements and investigated the mechanical performance with applied tensile and bending strains. With an improved processing route applying a low-temperature anneal the superconducting properties of standard undoped in situ MgB2 wires could be further improved and critical current densities of 38 kA cm−2 at 6 T and 10 kA cm−2 at 8 T, 4.2 K could be achieved.

https://iopscience.iop.org/article/10.1088/0953-2048/17/5/055/pdf

Development and performance of thin steel reinforced MgB 2 wires and low-temperature in situ processing for further improvements

In potential ac and dc applications technical MgB 2 wires need a multifilament composite structure with small filament diameters. To stabilize the wire at cryogenic temperatures against thermal runaway, a sheath component with high electrical and thermal conductivity is desirable. At present stabilization in the high current regime (low field, low temperature) is often still insufficient due to reaction layers at the sheath/filament interface, large filaments with irregular geometrical shape and an inhomogeneous microstructure. In this paper, we describe the latest development of extremely thin, stainless steel sheathed MgB 2 wires (wire diameter down to 50 μm, filament diameter down to 26 μm) and of mono- and multifilament wires with sheath composites of Nb, Cu, CuSn, CuNiZn, Fe and stainless steel. We show that the choice of the sheath material or composite strongly influences the current carrying capability of MgB 2 wires through its effect on cold-workability, reactivity of sheath with Mg or B and compatibility of thermal contraction of filament and sheath composite. Taking this into account, we obtained an engineering transport critical current density J e of 55 kA/cm 2 ( J c  = 387 kA/cm 2 ) at 4.2 K and 3 T for an only 164 μm thin MgB 2 wire with Nb barrier, Cu stabilizer and stainless steel reinforcement.

W. Goldacker

Papers

ROEBEL Assembled Coated Conductors (RACC): Preparation, Properties and Progress

In situ MgB2 round wires with improved properties

AC transport current losses of multifilamentary Bi(2223) tapes with varying filament geometries

Development and performance of thin steel reinforced MgB 2 wires and low-temperature in situ processing for further improvements

Suitability of sheath materials for MgB2 powder-in-tube superconductors