Speaker
Description
A standard tube-type Nb3Sn conductor was used for the winding of a sub-size MRI-like coil segment. The conductor was 0.8 mm OD and had 217 filaments, with a 45% non-Cu fraction. The wire was twisted, reacted, and then insulated with s-glass. After insulation, the coil was wound on a 1 m OD copper former. The total length of the conductor used was 1.6 km. The coil was instrumented for low temperature testing and then epoxy impregnated. Ten sets of voltage taps and ten thermocouples were used, along with two Cernox sensors and two hall probes (these latter for field measurements). The coil was installed into a large conduction cooled cryobox for cooldown and testing. The coil was cooled by a series of Cu straps which were bolted to the coil ID and then to a Cu cooling ring which was itself connected to two Sumitomo cryocoolers (1.5 W at 4 K each). The coil was surrounded with superinsulation and then sealed in the cryobox. Cool down hit a base temperature of 4 K, with a cool down time of 2 days. A small (1 A) current was applied to the coil during cool down and the coil was seen to have a Tc of 17 K. The maximum temperature difference across the coil was 1.5 K. After the initial cooldown the coil temperature was increased, and Ic was measured as a function of temperature with decreasing temperature. The coil transitioned by n-transition. The radial field of the coil was measured on the former (near the winding) and used to compare coil Ic to short sample Ic via a load line plot.