Stability of a NbTi forced-cooled superconducting coil by subcooled supercritical helium

The stability and quench characteristics of a NbTi forced-cooled superconducting coil at the subcooled condition were investigated. The test coil was wound in a one-layer solenoid from a cable-in-conduit-type conductor of 1.5 kA at 7 T, whose total length was 34 m. It was installed in a background coil and tested between the subcooled condition below 4.5 K and the normal condition above 4.5 K. Stability, quench current, normal propagation velocity, and pressure rise during a quench of the subcooled coil were measured. The stability margin increased and the pressure rise during a quench decreased as the inlet coolant temperature was lowered