Design and Development of a Compact 1 GHz (23.5 T)-Class NMR Magnet With Bi-2223 Inner Coils

The design and development for a compact 1 GHz (23.5 T)-class NMR magnet are presented. The magnet uses inner coils wound with Ni alloy-reinforced Bi-2223 conductors in combination with LTS outer coils. The series-connected coils are operated in a power supply-driven mode. The total magnet size is very compact, and is comparable to that of a currently commercialized 600-MHz (14.1 T) NMR magnet. This owes to a high current density operation of the Bi-2223 inner coils, which contribute as much as 53% of the total magnetic field. This paper investigated the following properties of the Bi-2223 inner coils: First, joint resistance under external magnetic fields; second, tensile stress tolerances of the Bi-2223 conductor under bending condition; third, high compressive stress on Bi-2223 coils; fourth, the effect of screening current-induced magnetic field on the field homogeneity and stability; and fifth, the maximum permissible Joule heating to avoid a thermal runaway. Two Bi-2223 inner coils for the magnet were built and tested in liquid nitrogen, which provided important knowledge for the magnet excitation.