Simulation Tool Development for Quench Modelling: Re-Thinking Heat Diffusion in High Temperature Superconductors

Loss of the superconducting state, namely quench, is a dangerous event for superconducting magnets. This is especially the case for possible future accelerator magnets including high temperature superconductors (HTS) . These magnets are characterized with high magnetic fields and high current densities, and in case of a quench the current is transferred to the copper matrix, which easily leads to a damaged magnet. To predict the time scale and temperature evolution in case of a possible quench, numerical simulations are crucial. Understanding the quench in HTS magnets, by means of numerical simulations, is required to develop adequate quench detection and protection systems.In this thesis, we first describe the background of the research by presenting the quench event in detail. Then we move on to the numerical modelling of quench, which requires a multiphysical approach where at least magnetostatic and heat diffusion problems have to be solved. To solve such problems, we have developed an in-house software QueST (finite element method based Quench Simulation Tool) with an object-oriented mindset for convenient future development. Then we proceed to study the quench characteristics of HTS magnets, utilizing QueST, while comparing to their low temperature superconductor (LTS) counterparts. We show that the quench evolution is different in HTS magnets, where the quench frontier does not propagate with the temperature front due to the large temperature margin. In addition, due to the large temperature margin of HTS magnets, we propose that the traditional analytical approaches, utilized with LTS magnets, are questionable e.g. for computing hot spot temperatures or minimum quench energies (MQE), and thus, numerical approaches are required. Finally, we scrutinize quench for an HTS research magnet showing that the hot spot temperature increases rapidly and a fast quench detection system is required to protect the magnet. We present an alternative method to ignite the quench in simulations, removing the temperature peak, which is present when utilizing a short heat pulse trigger