Hitachi Cable, Ltd., has developed two(2) sets of the world's largest class superconducting AC magnets capable of generating stably a central magnetic field of 1.5 Tpeak at 50 Hz in a 100 mm-dia clear bore, and that of 1.0 Tpeak at 50 Hz in a 150 mm-dia clear bore, utilizing an AC use NbTi cable.
Also, another superconducting AC magnet has been built from AC use Nb3Sn cable by the R & W (React and Wind) method, that is, the winding of the coil after applying reaction heat treatment to the cable to form superconducting Nb3Sn layers. Among Nb3Sn AC magnets ever built, it is one of the largest ones ever built in the world capable of generating a central magnetic field of 2.0 Tpeak at 50 Hz in a 50 mm-dia clear bore.
These developments have been carried out as a part of the R & D on conducted technology for electric power apparatuses by the Engineering Research Association for Superconductive Generation Equipment (Super-GM) under the Agency of Industrial Science and Technology, MITI's, New Sunshine Program consigned by NEDO.

By utilizing their outstandingly large clear bores, the above mentioned magnets can be used as experimental tools for making various physical measurements utilizing the outstanding large clear bores, while in the past almost all the superconducting AC magnets have only been built on a laboratory scale. These achievements have been realized through the following two factors :

(1)	The superconducting cables used have been designed and manufactured applying the fruits of this decade's development results on low AC loss NbTi and Nb3Sn strands attained as a member of Super-GM.
(2)	Due to the improvements of the designing and winding techniques of the coils, the above mentioned AC magnets have shown the load factors around 90 % on each load line at the AC excitation, due to the improvements in designing and winding techniques of the coils, while typical load factors for AC superconducting coils attained up to now have been around 50%.

NbTi Magnets

The NbTi coils are wound from 1.6 mm-dia insulated 1st stage cable, in which twelve (12) pieces of mm-dia NbTi/Cu-30wt%Ni-Mn strands, each with 0.1 µm-dia NbTi filaments each are cabled around a high resistivity normal strand. The NbTi strand shows the world's lowest AC loss (hysteresis loss) index, that is, hysteresis loss generated in the strand per 1 A of supplied current, corresponding to 1/8 of the index for the Alsthom T-type strand. This results from the duplicated effect of the suppression of the proximity effect due to the alloying of the ferromagnetic element Mn into the Cu-30Ni matrix, and the appearance of a reversible flux motion in each NbTi filament because of the small uniform filament diameter attained under 0.1 µm. A sufficient amount of thick Enough thick Formvar insulation has been applied to each NbTi strand and the central normal strand in order to avoid the circuiting through pinholes in the Formvar layer between the neighboring strands.
In the designing of the coil, extensive study has been made on the insulation technique and the cable fixing method.
Special care has been taken with the insulation around terminals in order to suppress corona. The terminal voltage reaches ~7,000 Vrms (~10,000 Vpeak) when the magnet generates a central magnetic field of 1 Tpeak at 60 Hz. On the other hand, the coil winding structure has been designed to avoid electrical field concentration at any point. The coil winding bobbin structure and the epoxy impregnation technique have been fully studied in order to fix the cable firmly, since the most important technical point for achieving a high load factor in superconducting the AC magnet seems to be to suppress the movement of the wound cable as small as possible, against the electromagnetic force loaded to it during exciting.
The 150 mm-bore superconducting AC magnet can be operated at any specified frequency within the 20~60 Hz range, adjusting a LC resonance circuit. The magnet has been stably operated , generating a central magnetic field of 1 Tpeak at a current supply of 238 Apeak of 50 Hz.

A Nb3Sn Magnet

Medium to small scale superconducting DC Nb3Sn magnets are built typically through the W & R (Wind and React) method , that is, by applying reaction heat treatment to the coil for forming Nb3Sn layers after winding it. This is because Nb3Sn strands with a few µm-dia Nb3Sn filaments show an irreversible uniaxial tensile strain limit of ~0.6 % due to the inherent mechanical brittleness of the intermetallic compound Nb3Sn. We have found that this irreversible tensile strain limit exceeds 1.5 % when the Nb3Sn filament diameter is reduced to ~0.5 µm . It has been verified that AC Nb3Sn coils can be built through the R & W method utilizing reaction heat treated Nb3Sn cables, due to the fact that the R & W processed 50 mm-dia clear bore magnet has attained a designed high load factor of over 90 %. This means superconducting AC Nb3Sn magnets can be built easily through a similar technique to that for NbTi magnets. Thus, a great step has been made for the realization of high performance superconducting electric power equipments, through the mentioned achievement of the R & W processed Nb3Sn magnet with its inherent high temperature margin and super high-field characteristics.

Among the superconducting AC magnets, the 150 mm-dia clear bore NbTi magnet is used as a bias magnet to generate the AC field of the test apparatus for large current-carrying capacity superconducting AC cables. It was developed for evaluating the characteristics such as critical currents and AC losses. etc., for the NbTi and Nb3Sn under development by Super-GM. This test apparatus is installed at the Agency of Industrial Science and Technology, MITI's Electrotechnical Laboratory .