This project will explore Quench initiation and propagation in a high temperature superconductor (HTS) coil by creating new coupled electromagnetic and thermal models. In order to achieve magnetic fields in excess of approximately 20 Tesla at an operating temperature of 4.2 Kelvin, HTS must be utilised. Typically, these are used as inserts within low temperature superconducting (LTS) 'outserts'. The ability to model quench behaviour accurately in LTS coils has been explored, however due to the differences in behaviour of HTS materials these techniques are not directly applicable to HTS insert coils. Accurate modelling capability in this area will aid the design of compact, high field magnets quantum materials analysis, high-resolution NMR and energy applications such as fusion. Models must couple Electromagnetic and thermal behaviour of the material, and its interaction with its environment, including with LTS magnets.
Heat transfer within a superconducting coil is governed by a partial differential equation with a first term representing heat diffusion and other terms representing mechanisms of heat generation including resistive heating and AC losses in response to changes in current density and magnetic field. Current in the HTS insert coil is governed by an inductor-resistor (LR) circuit of coils and protection circuit with mutual inductance with between the coils and resistance in the HTS coils arising from EM and thermal effects. The magnetic field is determined by two-dimensional Biot-Savart integration of the coil currents.
Before the instant of quench initiation in the HTS the problem is axisymmetric. At the instant of quench, the axial symmetry is broken and the simulation of quench propagation must then be switched to three dimensions. The role of random variations in the material properties in determining the location of the quench initiation will need to be considered.
In this project new models will be created to explore this behaviour. A two-dimensional axisymmetric model will be created to predict the behaviour of the HTS coil in the lead up to the quench. The concept of Minimum Quench Energy for a superconductor at critical state will be exploited to determine the time at which the quench begins to propagate. The propagation of the quench in the circumferential direction will then be solved in three dimensions by application of a Fourier series, with parallel processing applied to solve for each Fourier coefficient independently.
The project will focus on the case of a HTS test coil operating in a LTS background field of ~10T. This will enable experiments to be carried out to test the predictions of the model. These experiments are in addition to this project, not part of it.