A 5 T persistent current niobium–titanium magnet with a 4 K pulse tube cryocooler

Abstract

We report on the design and testing of a cryogen-free superconducting magnet that is conductively cooled by means of a two-stage pulse tube cryocooler (PTC). The cooler provides a cooling power of 0.4 W at 4.2 K, when operated on a helium compressor with a nominal input power of 6 kW. The 5 T NbTi magnet is equipped with a persistent current switch with a resistive heater. The thermal coupling between the switch and the 4 K stage is chosen so as to allow operation of the switch without a marked heat input to the 4 K stage during the charging process. The PTC is capable of cooling the complete magnet system to a stationary temperature of 2.7 K, which was found to be stable for a period of more than two months. The magnet was charged to a field of 5.5 T, and persistent mode operation was monitored successfully for a period of three weeks.