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

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

Taking the relay of the large Hadron collider (LHC) at CERN, ITER has become the largest project in applied superconductivity. In addition to its technical complexity, ITER is also a management challenge as it relies on an unprecedented collaboration of seven partners, representing more than half of the world population, who provide 90% of the components as in-kind contributions. The ITER magnet system is one of the most sophisticated superconducting magnet systems ever designed, with an enormous stored energy of 51?GJ. It involves six of the ITER partners. The coils are wound from cable-in-conduit conductors (CICCs) made up of superconducting and copper strands assembled into a multistage cable, inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the toroidal field (TF) and central solenoid (CS) coils require about 600?t of Nb3Sn strands while the poloidal field (PF) and correction coil (CC) and busbar conductors need around 275?t of Nb?Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and has called for a significant worldwide production scale up. The TF conductors are the first ITER components to be mass produced and are more than 50% complete. During its life time, the CS coil will have to sustain several tens of thousands of electromagnetic (EM) cycles to high current and field conditions, way beyond anything a large Nb3Sn coil has ever experienced. Following a comprehensive R&D program, a technical solution has been found for the CS conductor, which ensures stable performance versus EM and thermal cycling. Productions of PF, CC and busbar conductors are also underway. After an introduction to the ITER project and magnet system, we describe the ITER conductor procurements and the quality assurance/quality control programs that have been implemented to ensure production uniformity across numerous suppliers. Then, we provide examples of technical challenges that have been encountered and we present the status of ITER conductor production worldwide.

https://iopscience.iop.org/article/10.1088/0953-2048/27/4/044001/pdf

Challenges and status of ITER conductor production

High-field superconducting solenoids have proven themselves to be of great value to scientific research in a number of fields, including chemistry, physics and biology. Present-day magnets take advantage of the high-field properties of Nb3Sn, but the high-field limits of this conductor are nearly reached and so a new conductor and magnet technology is necessary for superconducting magnets beyond 25 T. Twenty years after the initial discovery of superconductivity at high temperatures in complex oxides, a number of high temperature superconductor (HTS) based conductors are available in sufficient lengths to develop high-field superconducting magnets. In this paper, present day HTS conductor and magnet technologies are discussed. HTS conductors have demonstrated the ability to carry very large critical current densities at magnetic fields of 45 T, and two insert coil demonstrations have surpassed the 25 T barrier. There are, however, many challenges to the implementation of HTS conductors in high-field magnets, including coil manufacturing, electromechanical behavior and quench protection. These issues are discussed and a view to the future is provided.

High Field Superconducting Solenoids Via High Temperature Superconductors

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

Oxford Instruments, Superconducting Technology (OI-ST) produces Nb/sub 3/Sn wire via several "internal Sn" routes. Recently, 12 T, 4.2 K non-Cu critical current density (J/sub c/) values of /spl sim/2900 A/mm/sup 2/ have been achieved by increasing the Nb and Sn fractions of the filament subelements. Similar conductors for high field use have shown engineering current density (J/sub e/) values of 170 A/mm/sup 2/ at 23.5 T, 1.8 K. OI-ST is also involved with research for the High Energy Physics (HEP) National Conductor Program. Results on composites made entirely by hot extrusion are described. Finally, the present status of Ta-Sn powder-in-tube (PIT) and Nb/sub 3/Al precursor strand development are presented. PIT strands have irreversibility fields over 26 T at 4.2 K, while Nb/sub 3/Al precursor strand has been produced by a route that promotes bonding of the billet components.

High field Nb/sub 3/Sn conductor development at Oxford Superconducting Technology

Nb/sub 3/Sn conductor made by the internal tin route is the material of choice for the highest field superconducting magnets. These include systems ranging from solenoids used in 900MHz NMR and 20 T laboratory magnets, to large-scale applications such as ITER and possible LHC upgrades. We present our latest results on internal tin strands having critical current density (J/sub c/) values of 3000 A/mm/sup 2/ (4.2 K, 12 T), as it relates to such magnet systems. One obstacle to wider use of internal tin strand is the relatively small billet size, typically limited to 50 kg or less. As part of the R&D for the U.S. High Energy Physics National Conductor Program, we have developed a method of scaling up the distributed barrier internal tin process to billet sizes several times larger. In the past year we have successfully produced a high J/sub c/ distributed barrier strand made entirely by hot extrusion. Results are also presented on a new method of supplying Ti dopant for the Nb/sub 3/Sn that does not rely on alloying the Sn cores, matrix Cu, or the Nb filaments directly. Material made with this new doping method reached a J/sub c/ (4.2 K, 12 T) value of 2500 A/mm/sup 2/. Finally, the state of development of composites having lower AC losses is described. Such conductors are being developed for possible future fusion applications including ITER.

Advances in Nb/sub 3/Sn strand for fusion and particle accelerator applications

The critical current density (Jc) of Nb3Sn strand has been significantly improved over the last several years. For most magnet applications, high Jc internal tin has displaced bronze process strand. The highest Jc values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved Jc values of 3000 A/mm2 (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm2 at 15 T. To reduce the effective filament diameter in these high Jc strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu Jc values over 1100 A/mm2 (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm3 over non-Cu volume. The Jc-strain behavior of such composites is also presented.

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=900165

Internal Tin ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Conductors Engineered for Fusion and Particle Accelerator Applications

Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable property of powder-in-tube conductors is that there is about 30% void space in the as-drawn wire. We have recently shown that the gas present in the as-drawn Bi-2212 wire agglomerates into large bubbles and that they are presently the most deleterious current-limiting mechanism. By densifying short 2212 wires before reaction through cold isostatic pressing, the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (JE) of 810 A/mm2 at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased JE (4.2 K, 5 T) from 486 A/mm2 for as-drawn wire to 808 A/mm2 for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major JE improvements in this round-wire magnet conductor.

/pdf/reduction-of-gas-bubbles-and-improved-critical-current-8cz0zxypro.pdf

Seung Hong

Papers

High field Nb/sub 3/Sn conductor development at Oxford Superconducting Technology

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

Advances in Nb/sub 3/Sn strand for fusion and particle accelerator applications

Internal Tin ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Conductors Engineered for Fusion and Particle Accelerator Applications

Reduction of Gas Bubbles and Improved Critical Current Density in Bi-2212 Round Wire by Swaging