Cryogenics of a conduction-cooled Nb3Sn SMES

A cryogen-free Nb₃Sn SMES system operating at 10 K has been constructed in order to compensate a short term loss of power. The system is cooled with two Gifford-McMahon cryocoolers with a cooling power of 8 W at 10 K. SMES is an AC device and the cryogenic design is assigned by the application of the system and its specific AC requirements. Thus the minimization of both the coil AC losses and all the external losses becomes highly relevant. The former demand requires especially a careful design of a copper cryogenic interface between the coil and the cryocooler, the latter pretension implies a heat economical cryostat structure with HTS current leads. The cooling experiences of the system are discussed. In addition the cryogenic interface acts as a protection system of the magnet. The philosophy of this so called quench back effect is also outlined.