Current decay evaluation of closed HTS coil circuits

Persistent current mode operation of HTS coils is one of the key technologies required for very high-field NMR magnets composed of LTS and HTS coils. We prepared four closed circuits which consisted of an HTS coil made of a single conductor, a persistent current switch and their joints. Three HTS persistent current switches were fabricated for this study and they functioned well. The current decay behavior of the four circuits was measured in liquid helium by monitoring the magnetic field in the HTS coil with a Hall probe. One circuit maintained 80 % of the initial current even after 50 hours. A resistance less than 1.6/spl times/10/sup -11/ /spl Omega/ was obtained in another circuit. The small resistivity of HTS coils, caused by their small index number values, was thought to accelerate the current decay.