Dry magnets using high temperature superconductors and MgB₂ are attractive options to depart from dependence on liquid helium in MRI scanners. In the dry magnets, however, lack of thermal mass of cryogen makes a controlled quench difficult and extends time for restarting the magnets after the quench. In this study, a novel rapid ramp-down procedure, which can be substituted for the controlled quench in emergency rundown, is proposed, and its feasibility is proven for a 1.5 T whole-body MgB₂ MRI magnet. In this procedure, a power supply receives current from a persistent current switch (PCS), the PCS is turned off by heating, the power supply is interrupted by a breaker, and the stored energy in the magnet is mostly consumed at an external resistor. Owing to the large energy margin of MgB₂ wires, the AC loss during the ramp-down does not bring a quench of the MgB₂ coils. A niobium–titanium sheathed MgB₂ wire 0.60 mm in diameter is made, and a PCS with high off-resistivity is designed using this wire. The shunt current during the ramp-down does not bring the burnout of the PCS when the wire length is sufficiently long, typically hundreds of metres. Because heat generation inside the cryostat during the ramp-down is a few per cent of the stored energy in the magnet, the magnet is not heated excessively. As a result, the proposed ramp-down procedure should shorten the downtime of MRI scanners.

使用高温超导体和 MgB _{2 的}干磁体是一种有吸引力的选择，可以摆脱 MRI 扫描仪中对液氦的依赖。然而，在干磁体中，缺乏致冷剂的热质量使得受控失超变得困难并且在失超后延长了重新启动磁体的时间。在这项研究中，提出了一种新的快速减速程序，可替代紧急停机时的受控淬火，并证明其适用于 1.5 T 全身 MgB ₂磁共振磁体。在此过程中，电源从恒流开关 (PCS) 接收电流，PCS 因加热而关闭，电源被断路器中断，磁体中存储的能量主要由外部电阻消耗。由于 MgB ₂导线的能量裕度大，斜降期间的交流损耗不会导致 MgB ₂线圈失超。铌钛包覆的 MgB ₂制成直径为 0.60 mm 的导线，并使用该导线设计了具有高截止电阻率的 PCS。当导线长度足够长（通常为数百米）时，斜坡下降期间的分流电流不会导致 PCS 烧毁。由于在减速期间低温恒温器内部产生的热量仅占磁体中存储能量的百分之几，因此磁体不会被过度加热。因此，建议的减速程序应缩短 MRI 扫描仪的停机时间。