Force-cooled Small Compound Superconducting Coil

A four-layer pancake coil, which has an inside diameter of 20mm, an outside diameter of 100mm, a hight of 24.8mm and a maximum field of 6.5T, is wound by a Nb3Sn tape (5mm wide and 0.155mm thick). The coil is force-cooled by a helium flow (liquid phase flow, two phase flow or supercritical flow) which passes radially through the layers from innermost turns to outside turns. The coil is energized and performance is investigated under each cooling condition. The experimental results show critical currents of the coil depend on sweep speeds of energizing current. The dependency for each condition is qualitatively explained by heat transfer q-ΔT curve of helium. It is revealed that, if the heat transfer rate and the cooling capability of supercritical helium is raised by increasing the flow rate, the coil has a possibility to show a better performance than it is immersed in liquid helium.