Self-powered heater-driven quench protection of an HTS magnet

High-temperature superconductors (HTS) exhibit high engineering current densities of ~1000 A/mm 2 at 4.2 K in an 18-20 T magnetic field. This makes them attractive for high-current devices such as fusion magnets. Although HTS are favourable for high-current operation, they can still quench, so magnets must be designed with a quench protection system. During a quench, a region of the magnet winding becomes resistive, releasing a large amount of energy and creating a hot spot. Because the normal zone propagation velocity in HTS is low (on the order of cm/s), local temperatures can rise quickly unless the heat is redistributed, potentially damaging the magnet winding. Quench heaters can be used to protect the magnet from damage; these are thin resistive strips (e.g., stainless steel) that provide controlled heating and an alternate dissipation path. In this work, an HTS magnet with quench heaters is modelled in ANSYS. Because the heaters are powered by the magnet's stored energy, the model is used to determine the stored energy required to cause the heaters to quench the magnet. To demonstrate the principle, a double-pancake coil was wound with a quench heater electrically connected to the inner and outer turns of the winding. The magnet was tested, and the experimental results were compared with the modelling results. Two scenarios were compared, charging to 300 A and 350 A with stored energies of 0.74 kJ and 1.01 kJ, respectively. The magnet was then disconnected from the power supply, forcing the current to discharge through the quench heater. The 300 A test showed that the stored energy was insufficient to quench the magnet, resulting in low circuit resistance and a longer discharge time. At 350 A, the stored energy was sufficient to raise the winding temperature to cause a quench, increasing the circuit resistance and fully discharging the magnet within 2-3 s. This protection method also protects the magnet in the event of a power supply failure.