High field superconducting magnets have many present and potential future applications, such as medical magnetic resonance imaging (MRI) machines, particle accelerators like the Large Hadron Collider for research, fusion reactors for energy applications, and high field scientific experiments, which require tens of Teslas of magnetic field strength. These magnets usually consist of several pancake coils with many turns and for these magnets, electro-thermal quench (a sudden rise in temperature) is an issue that magnet designers need to take into account. Thus, there is a need for a fast and accurate software to numerically model the overall electromagnetic and thermal behavior of full-scale magnets. Anang Dadhich and his colleagues have developed a novel software programmed in C++ and Fortran, which performs coupled electro-magnetic and electro-thermal analysis using our original methods, i.e., variational methods based on Minimum Electro-Magnetic Entropy Production (MEMEP) and Finite Difference. The developed software, which takes screening currents into account, is applied to full-scale magnets of more than 32 T field strength under the SuperEMFL project. Several magnet cooling conditions are investigated and the impact of turn-to-turn resistance on quench propagation in the pancake coils is explored.
Figure 1: (a) and (b) shows temperature and current density evolution for 10-7 Ω.m2 contact resistance case, respectively. The magnet quenches at 283 seconds due to a damage turn in pancake 1.
These magnets have radial as well as angular electrical currents. The results show that for cases like quench, radial currents are more responsible than angular currents. The case of magnet under adiabatic conditions and magnet being cooled from sides by liquid helium is investigated, and as expected, without cooling, quench can not be prevented. We also show that the non-insulated coils are more reliable against quench than the metal insulated coils. Several turn-to-turn contact resistance are explored – 10−6 Ω· m2, 10−7 Ω· m2 for metal insulated case and 10−8 Ω· m2 for non-insulated case. A small defect can raise the temperature of the whole magnet, as can be seen in Fig. 1 for the metal-insulated case. In the case of low enough (non-insulated) turn-to-turn resistance we saw that the damaged turns can be tolerated as seen in Fig. 2. The model developed can be used for a quick and complete electro-magnetic and electro-thermal analysis of superconducting high field magnets.
Authors: Anang Dadhich, Philippe Fazilleau, Enric Pardo
Link: https://iopscience.iop.org/article/10.1088/1361-6668/ad68d3