Development of a low heat leak current-lead system

This paper describes a low heat leak current-lead system for a high temperature superconducting magnet which is refrigerated by conduction cooling and operated in a persistent-current mode. Under the condition that the current lead is cooled by conduction without gas cooling, we investigated the current-lead structure with which the heat load to the cryocooler is sufficiently low. The current lead consists of two parts. One of them spans the temperature interval between room temperature and around 70 K. The other spans the temperature interval between around 70 K and the lower end temperature. The former is made of copper alloy and the latter is made of a high temperature superconductor. To decrease the heat leak to a thermal anchor in the 70 K region, a detachable joint is installed in the copper alloy part. The driving mechanism for the joint is set in the room temperature region. An ultrasonic rotation motor, which works in the vacuum and the magnetic field, is adopted for the driving mechanism. We designed and constructed a detachable current lead of 600 A class, and obtained a test result that the heat leak to the thermal anchor is sufficiently low.