AC Loss of $\hbox{Nb}_{3}\hbox{Sn}$ Butt Joint for JT-60SA Central Solenoid

The upgrading project of the JT-60 to the JT-60 Super Advanced (JT-60SA) at Japan Atomic Energy Agency has started as a joint effort between Japan and the EU. The coils of the JT60-SA are separately fabricated in Japan and EU. The CS and EF coils are fabricated in Japan. In this paper, we describe the butt joint of the JT-60SA CS and measured the ac loss under a time-varying external magnetic field, and analyzed the ac loss at the butt joint by using the FEM simulation software COMSOL Multiphysics. As a result, the measured ac loss of the butt joint were found to be the smallest of all joints (the CS butt joint, the EF pancake joint, the terminal EF joint and the EF prototype joint), and to strongly depend on the thickness of the copper sleeve in full-size coil joint.