Fast quench detection via coolant-gas-temperature monitoring of a high-temperature superconducting magnet

High-temperature superconductors (HTS) are increasingly being employed to construct electromagnets to generate strong magnetic field for a broad range of applications: from medical devices, through electric motors, to future fusion reactors. These superconductors can experience a quench phenomenon, manifesting as a fast increase of temperature in HTS cables leading to disruption of the magnet operation and even serious damage. In-time, fast quench detection, to prevent operation disruption, is currently one of the most actively developing areas. Currently, the HTS cables have cooling channels to maintain the operating temperature and in this paper a technique based on frequency and time domain reflectometry (FTDR) using electromagnetic microwave signal propagating inside coolant gas channels is suggested and discussed. We demonstrate that the variation of gas permittivity during the heat release can be measured using the reflectometry allowing fast evaluation of the gas temperature. The methods to monitor the heat release and analysis required to recover the information about the gas temperature and heat release location are described. The numerical and experimental studies to demonstrate the application of FTDR to monitor the coolant gas temperature were carried out and the results, as well as the future plans are discussed.