Manufacture and Test of a Prototype Nb3Sn-NbTi Joint Sample for the CFETR Central Solenoid Model Coil

The project of the Central Solenoid Model Coil (CSMC) was launched in the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) to develop the manufacturing technology of large-scale superconducting magnet for the China Fusion Engineering Testing Reactor (CFETR). The maximum magnetic field of the CFETR CSMC will be 12 T when the running current is 47.65 kA, and the maximum field change rate will be 1.5 T/s. The CSMC consists of two coaxial Nb3Sn coils in the internal high field and a stack of three NbTi coils in the external low field. All the coils are electrically connected in a single circuit by joints. To verify the electrical performance of the joint, a prototype Nb3Sn-NbTi joint sample has been designed and manufactured in ASIPP. The sample is composed of two different conductor legs, the tested joint of praying hands structure to be placed in the high field region, and two upper terminals to be connected to the transformer of the test facility. The prototype sample has been tested in the SULTAN facility at Villigen (Switzerland). In this paper, the sample design, the techniques used for the manufacturing steps of the different conductor legs, and the joint assembly are described in detail. The test results show dc resistance below 5 nΩ criterion, and ac losses below 30 J/cycle with the amplitude of 0.2 T and frequencies in the range of 0.03-1 Hz.