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High performance Nb-Ti conductor was produced to meet the stringent requirement of LHC. Conductor with fine filaments (~ 6 μm dia) cladded with diffusion barriers, in a resistive matrix (to reduce filament coupling) and low fraction of Cu were produced. For the upcoming projects on high luminosity LHC (HL-LHC) and the future circular collider (FCC) and the consequent field regime changing from 8 T to 12 T and 16 T regime, focus has shifted to A-15 Nb3Sn superconductor. Magnets, using RRP (restacked rod processed) Nb3Sn cables, have been tested for 12 T and 16.5 T fields successfully. We however seem to have reached the limits of these two materials.
Sustained research yielded huge success in the production of three most popular cuprates, namely, REBCO, Bi-2223 and Bi-2212 with high critical current densities. The best known 2G REBCO ‘coated conductor’ is produced by depositing highly oriented thin film of (GdY)BCO on biaxially textured Hastelloy substrates buffered with a number of oxide layers and cladded with Ag-alloy and then a high conducting hardened copper for stability and strength. Critical current density, Jc = 20 MA/cm2 (30 K, 3T) have been achieved in 0.1 mm thick (GdY)BCO tape deposited on a 30 µm Hastelloy substrate and doped with 25 mole% Zr. REBCO cables capable of carrying currents as high as 90 – 100 kA have been produced in different configurations like Roebel bar cables by KIT, CORC (conductor on round core) by Advance Conductor Technologies and CROCO (Cross Conductor) by KIT for future fusion machine EU-DEMO and future accelerator detector magnets.
In recent times, NHMFL was able to solve the problem related to the presence of bubbles in the filaments of PIT Bi-2212 flexible round wires by employing an over-pressure heat treatment (OPHT). A coil wound using this OPHT Bi-2212 wire produced a magnetic field of 33.6 T in a background field of 31 T. The Bi-2223 tapes marketed by Sumitomo since 1989 are produced through the standard PIT technique involving multiple drawing and rolling followed by sintering under pressure. Bi-2223 tapes are widely used for current leads because of its very low thermal conductivity. Sumitomo’s HT-AC wires have very low AC losses and find application in power transmission and other power devices. Even though the Tc of MgB2 is 39 K only, it has already positioned itself as a practical superconductor to be used at 20 K and thus superior to Nb-Ti and Nb3Sn which need liquid helium for operation. High Jc = 1.0 x 105 A/cm2 (4.2 K, 10 T) and high Hirr = 25 T have been achieved in IMD (internal magnesium diffusion) processed MF wires doped with 5 % carbon. The ASG (Ansaldo Supercondacttri of Genova) Superconductor is already marketing a conduction cooled 0.5 T MRI Scanner with the brand name “Paramed MROpen System” using MgB2 magnets. Intense work is going on to develop 3 T MRI scanners using MgB2 magnet.
The arrival of high performing HT superconductors in recent years enabled technologist to establish new records in magnet applications. To name a few, NHMFL established a world record by producing a field of 45.5 T using an insert of 2G REBCO tape conductor backed by an outsert having a number of conventional as well as HTS coils. Bruker Biospin has set a world record by building and installing a 1.2 GHz NMR spectrometer at the Uni. of Florence, Italy as recently as May 2020. ACT (Advanced Conductor Technologies) has operated a laboratory solenoid first time at a current of 4404 A. The magnet produced a field of 15.86 T in a background field of 14 T. The peak field on the conductor was 16.77 T. ACT is now developing two varieties of CICC, one required for high field fusion magnets and the other for accelerator detector magnets in collaboration with MIT, LBNL, NHFML, CERN, KIT, Uni. of Twente and Paul Sherrer Institute with funding from DOE. Magnet requirements of the HL-LHC, FCC and just launched EIC will be discussed.
G. Keppel