Electro-thermal-mechanical modeling of quench and stress evolution triggered by various factors in high-temperature superconducting coils

In high-temperature superconducting magnets, slow quench propagation of YBa2Cu3O7-delta coils makes the time delay of detection signal and active protection, resulting in the local heat accumulation and further coil degradation accompanied by strong mechanical response. Based on Maxwell's equations, the heat conduction equation, and basic equations of elasticity, we build and theoretically validate a two-dimensional axisymmetric electro-thermal-mechanical model for an insulated pancake coil with real dimensions in this work. Such a model has an important advantage that the simulation domain can be restricted to the coil itself by applying appropriate boundary conditions. The operating current of the coil is ramped up to a maximum and then remains unchanged. By using the model, we perform a systematic study of the quench and stress evolution triggered by various factors in the coil. The results indicate that the quench triggered by a heater is most likely to occur at the inner and outer turns. As the temperature keeps rising, the insulation layer near the heater is most prone to mechanical failure. Whereas if the quench is triggered by a local degradation caused by defects, the electromagnetic stress is dominant in the initial period, and after the initial period, the thermal stress will be dominant. The mechanical failure probably occurs at both ends of the insulation layer close to the degradation layer. In addition, the quench in an axial stack of pancake coils containing a local degradation is also analyzed. It is found that the position where the quench occurs first is mainly determined by the position of the local degradation and the number of the stack's layers.