In order to make a 1 GHz NMR magnet compact, it is operated under high hoop-stress conditions. In our design the hoop-stress of a 1 GHz NMR magnet exceeds 180 MPa. We prepared two types of sample coils. One of them was constructed with niobium-tin superconducting wire, which has a tantalum core. The other one was wound with niobium-titanium superconducting wire. These wires are rectangular in cross-section. The niobium-tin sample coil was energized in a backup magnetic field of 13.5 T and 14 T. It was possible to operate it up to a hoop-stress of 272 MPa. The niobium-titanium sample coil was tested up to 99% of its critical current where the hoop-stress was 226 MPa. These results confirm that our design of the 1 GHz NMR magnet is appropriate.
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Ozaki et al. (1999) studied this question.