Quench detection in HTS coils using thermocouple chains

High-temperature superconducting (HTS) coils are vulnerable to thermal runaway or quenching, typically triggered by local defects, thermal disturbances, or local overcurrents. Once the conductor becomes resistive, Joule heating leads to a rapid rise in temperature. If the current is not reduced in time, this heating can burn the conductor and cause irreversible damage to the magnet. A crucial step towards reliable quench protection is the fast and sensitive detection of anomalies in temperature, voltage, etc. that occur during the onset of a quench. This research investigates co-wound thermocouple chains (TCCs) as a temperature-based quench detection method for HTS coils. During a quench, a TCC generates a voltage signal that is closely correlated with the maximum temperature in the coil. As a result, this signal responds before the conductor reaches its current sharing temperature and before a measurable resistive voltage develops, enabling preventive quench detection. A modelling framework based on the electromotive forces of the individual materials of the TCC was developed to predict the voltage response of arbitrary sensor layouts and was experimentally validated on two individual TCCs. A major challenge of TCCs is their position-dependent response arising from the discrete nature of the sensor. To address this, an electrothermal model of an insulated HTS pancake coil was developed to simulate quench propagation and evaluate different layouts for the discrete sensing points of the co-wound TCC. Based on these simulations, a novel double constant segment angle (CSA) layout is proposed, which largely eliminates the position dependence within the modelled pancake geometry while preserving a voltage signal that closely tracks the maximum coil temperature. Although the sensor layout is magnet-specific and requires predictive electrothermal modelling, the proposed approach provides a robust, non-inductive quench detection method that is inherently less susceptible to electromagnetic noise than conventional voltage-based detection by integration as a twisted-pair, making it a promising alternative for HTS magnets in fusion applications.