Reduction of the Hot Spot Temperature in HTS Coils

A potential future muon collider requires high field solenoids ( 30 T) for the final cooling stage; the Magnet Division at Brookhaven National Laboratory is undertaking the task of demonstrating feasibility using high-temperature superconductors (HTS). The aim is to construct an all-HTS dual-coil system capable of delivering more than 20 T. Recently, a new record for an all-HTS solenoid has been established with a field of 15 T on-axis. In coil tests, it was noticed that during a fast energy extraction, the current in the solenoids decays faster in comparison to the expected exponential decay. This paper describes the effect and shows how it can be simulated using commercial finite element code. The faster current decay helps to lower the integral current density squared with time by about 10% and is therefore beneficial for quench protection.