Cuprate superconductors have found limited application for high-field magnets because of difficulties related to grain boundaries. Now, this issue is partially overcome and round wires suitable for magnetic coils are fabricated from Bi2Sr2CaCu2O8−x.

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Isotropic round-wire multifilament cuprate superconductor for generation of magnetic fields above 30 T

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

Recent progresses in the second generation REBa2Cu3O7 − x (RE123) coated conductor (CC) have paved a way for the development of superconducting solenoids capable of generating fields well above 23.5 T, i.e. the limit of NbTi−Nb3Sn-based magnets. However, the RE123 magnet still poses several fundamental and engineering challenges. In this work we review the state-of-the-art of conductor and magnet technologies. The goal is to illustrate a close synergetic relationship between evolution of high-field magnets and advancement in superconductor technology. The paper is organized in three parts: (1) the basics of RE123 CC fabrication technique, including latest developments to improve conductor performance and production throughput; (2) critical issues and innovative design concepts for the RE123-based magnet; and (3) an overview of noteworthy ongoing magnet projects.

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Progresses and challenges in the development of high-field solenoidal magnets based on RE123 coated conductors

Since the 1960s, Nb-Ti (superconducting transition temperature T/sub c/=9 K) and Nb/sub 3/Sn (T/sub c/=18 K) have been the materials of choice for virtually all superconducting magnets. However, the prospects for the future changed dramatically in 1987 with the discovery of layered cuprate superconductors with T/sub c/ values that now extend up to about 135 K. Fabrication of useful conductors out of the cuprates has been difficult, but a first generation of silver-sheathed composite conductors based on (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ (T/sub c//spl sim/110 K) has already been commercialized. Recent progress on a second generation of biaxially aligned coated conductors using the less anisotropic YBa/sub 2/Cu/sub 3/O/sub 7/ structure has been rapid, suggesting that it too might enter service in the near future. The discovery of superconductivity in MgB/sub 2/ below 39 K in 2001 has brought yet another candidate material to the large-scale applications mix. Two distinct markets for superconductor wires exist-the more classical low-temperature magnet applications such as particle accelerators, nuclear magnetic resonance and magnetic resonance imaging magnets, and plasma-containment magnets for fusion power, and the newer and potentially much larger market for electric power equipment, such as motors, generators, synchronous condensers, power transmission cables, transformers, and fault-current limiters for the electric utility grid. We review key properties and recent progress in these materials and assess their prospects for further development and application.

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Superconducting materials for large scale applications

Development of high-field magnets using high temperature superconductors (HTS) is a core activity at the NHMFL. Magnet technology based on both YBCO-coated tape conductors and Bi-2212 round wires is being pursued. Two specific projects are underway. The first is a user magnet with a 17 T YBCO coil set which, inside an LTS outsert, will generate a combined field of 32 T. The second is a 7 T Bi2212 demonstration coil set to be operated in a large bore resistive magnet to generate a combined magnetic field of 25 T. Owing to the substantial technological differences of the two conductor types, each project faces different conductor and magnet technology challenges. Two small coils have been tested in a 38-mm cold bore cryostat inserted in a 31 T resistive magnet: a Bi2212 round-wire layer-wound insert coil that generated 1.1 T for a total of 32.1 T and a YBCO double-pancake insert that generated 2.8 T for a total central field of 33.8 T. Four larger layer-wound coils have been manufactured and tested in a 20 T, 186-mm cold bore resistive magnet: a sizeable Bi-2212 coil and three thin large-diameter YBCO coils. The test results are discussed. The current densities and stress levels that these coils tolerate underpin our conviction that >30 T all-superconducting magnets are viable.

High Field Magnets With HTS Conductors

Bi-2212/Ag conductor is one of the most promising materials for expanding superconducting magnet applications to higher fields and/or temperatures than present LTS systems. From the view point of practical application, Bi-2212 round wires have significant advantages over more typical HTS tape conductors, such as no anisotropy, ease of handling and simpler coil winding, allowing considerable flexibility in the magnet design. Development efforts at OST have been aimed at enhancing transport properties in long length round wires for high field magnet applications. Recently, significant improvements in the J/sub E/ and J/sub c/ performance have been achieved by optimizing the starting powders, the filament size and fill factor, the deformation processes, and the melt-solidification parameters. The highest J/sub E/ of 266 A/mm/sup 2/ and J/sub c/ of 950 A/mm/sup 2/ at 4.2 K, 45 T with n-value of 17 was obtained in 0.81 mm wire. In this paper progress on the development of Bi-2212 round wires will be reported.

Development of round multifilament Bi-2212/Ag wires for high field magnet applications

A 25.05 T magnetic field was generated by a 5.11 T superconducting Bi2Sr2CaCu2Ox insert magnet within a 19.94 T resistive magnet. The Bi2Sr2CaCu2Ox magnet is constructed using fully reacted powder-in-tube conductor and insulated stainless steel reinforcement. Three concentric sections are used to minimize the total stress in the Bi2Sr2CaCu2Ox conductor: two double pancake stacks and an outer layer-wound section. The insert coil operates at 4.2 K in a 0.168 m diameter cryostat fitted to the resistive magnet. Here we provide an overview of the design and construction of the insert and the results of self-field and in-field testing. Mechanical and electrical safety issues, related to testing in a large resistive magnet, are discussed.

The generation of 25.05 T using a 5.11 T Bi2Sr2CaCu2Ox superconducting insert magnet

Most studies of Bi2Sr2CaCu2Ox (Bi2212) show that the critical current density Jc is limited by the connectivity of the filaments, but what determines the connectivity is still elusive. Here we report on the role played by filament porosity in limiting Jc. By a microstructural investigation of wires quenched from the melt state, we find that porosity in the unreacted wire agglomerates into bubbles that segment the Bi2212 melt within the filaments into discrete sections. These bubbles do not disappear during subsequent processing because they are only partially filled by Bi2212 grains as the Bi2212 forms on cooling. Correlating the microstructure of quenched wires to their final, fully processed Jc values shows an inverse relation between Jc and bubble density. Bubbles are variable between conductors and perhaps from sample to sample, but they occur frequently and almost completely fill the filament diameter, so they exert a strongly variable but always negative effect on Jc. Bubbles reduce the continuous Bi2212 path within each filament and force supercurrent to flow through Bi2212 grains that span the bubbles or through a thin Bi2212 layer at the interface between the bubble and the Ag matrix. Eliminating bubbles appears to be a promising new path to raise the Jc of Bi2212 round wires.

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Bubble formation within filaments of melt-processed Bi2212 wires and its strongly negative effect on the critical current density

Multifilamentary Bi
 2
Sr
 2
CaCu
 2
O
 x
 (Bi-2212) wire made by the powder-in-tube technique is the only high-temperature superconductor made in the round shape preferred by magnet builders. The critical current density (J
 C
) of Bi-2212 round wire was improved significantly by the development of overpressure heat treatment in the past few years. Bi-2212 wire is commercially available in multiple architectures and kilometer-long pieces and is a very promising conductor for very high-field NMR and accelerator magnets. We studied the effects of precursor powder and heat treatment conditions on the superconducting properties and microstructure of recent Bi-2212 wires. Short samples of recent wire with optimized overpressure processing showed J
 C
(4.2 K, 15 T) = 6640 A/mm
 2
 and J
 C
(4.2 K, 30 T) = 4670 A/mm
 2
, which correspond to engineering critical current densities J
 E
(4.2 K, 15 T) = 1320 A/mm
 2
 and J
 E
(4.2 K, 30 T) = 930 A/mm
 2
.

High Performance Bi-2212 Round Wires Made with Recent Powders.

Three types of BSCCO-2212 conductor are in development at Oxford Superconducting Technology (OST). Prototype size batches of multifilament tape continue to be manufactured for use in a 5 T high field insert magnet demonstration in collaboration with the National High Magnetic Field Lab. Progress will be reported in improving uniformity of microstructure and transport properties in these tapes. Development of multifilament wire has been renewed in response to interest from the high energy physics community. Issues of interest in these wires include the effects on J/sub c/ from filament size and distribution, as well as chemical composition and impurity content in the ceramic. Progress in J/sub c/ and J/sub E/ at 4.2 K in these wires will be discussed. Optimization of dip-coated 2212 tape for MRI magnet applications is also underway, as part of a Superconductivity Partnership Initiative. Various cost and performance tradeoffs will be considered, including the silver/ceramic ratio, the thickness of the ceramic, and the choice of batch or continuous heat treatment. The status of these optimization efforts will be reported.

Hanping Miao

Papers

Development of round multifilament Bi-2212/Ag wires for high field magnet applications

The generation of 25.05 T using a 5.11 T Bi2Sr2CaCu2Ox superconducting insert magnet

Bubble formation within filaments of melt-processed Bi2212 wires and its strongly negative effect on the critical current density

High Performance Bi-2212 Round Wires Made with Recent Powders.

BSCCO-2212 conductor development at Oxford Superconducting Technology