Feasibility study of novel rapid ramp-down procedure in MgB 2 MRI magnet using persistent current switch with high off-resistivity

Dry magnets using high temperature superconductors and MgB 2 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 MgB 2 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 MgB 2 wires, the AC loss during the ramp-down does not bring a quench of the MgB 2 coils. A niobium–titanium sheathed MgB 2 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.