Suppression of Catastrophic Thermal Runaway for a REBCO Innermost Coil of an LTS/REBCO NMR Magnet Operated at 400–600 MHz (9.4–14.1 T)

The use of REBCO coils is a promising approach to achieve super-high field NMR magnets. As the first step in this direction, we have started to develop an LTS/REBCO NMR magnet operated at 400-600 MHz (9.4-14.1 T). In this paper, based on numerical simulations, we describe a study of the thermal runaway of the REBCO innermost coil, which is one of the major problems of this type of magnet. Based on the simulation results, if an LTS coils quench occurs, the current in the REBCO coil continues to increase due to the electromagnetic induction and leads to a natural thermal runaway with a very high current density. The thermal runaway is limited to only the top and bottom ends of the REBCO coil, as the radial magnetic field is highest there and the quench propagation velocity is quite low. The current density for this type of thermal runaway is much higher than that of the operating current, resulting in the catastrophic overheating of the coil. This is the worst case scenario which must be prevented for the LTS/REBCO NMR magnet. An optimal size dump resistor for the REBCO coil can prevent the natural thermal runaway by suppressing the current increase due to electromagnetic induction.