He I-He II electrical lead transition

Abstract

In large He I1 superconducting applications, reliable transition leads are needed to transfer large currents from4.2 K He to He II(Tc2.17 K). The lead consists of a plug made of alternating cu/ss thin laminates with attached s/c strands. The laminates minimizes thermal conduction from 4.2 K to He I1 and provide large heat capacity for protection compared to solid ss transition. We have constructed and tested a 50 kA transition lead that is fully stable up to about 35 kA. Between 35 and 55 kA, the lead will have a stable temperature profile if it becomes normal. Between 55 and 75 kA, its temperature will increase slowly enough with time (following a disturbance) so the protection circuit can turn off the connected power supply.