Thermal and Electrical Behaviour of the Persistent Current Switch for a Whole-Body Superconducting MRI Magnet

A prototype persistent current switch is developed for an actively shielded whole-body 1.5 T MRI magnet having an operating current of ∼500 A. The switch is developed using a six-strand CuNi-NbTi conductor. The total length of the conductor used in the PCS is ∼40 m using a bifilar winding technique having a room temperature resistance of 15 Ω and an inductance of 6.6 μH. Two numbers of thermo-foil heaters having a resistance of 90 Ω each are placed between the layers of the winding pack of the switch. The wet-winding technique is followed for the switch using a cryogenic grade epoxy. The characteristics of the switch are performed using a 4 K test rig for its applicability in the 1.5 T MRI magnet. The normal resistance of the switch is measured to be 12.5 Ω at 15 K which is 15% less than the estimated value. The total energy loss onto the switch is estimated to be 0.24% of the ramping energy of the magnet at 6 V charging voltage. The thermal switching profile of the switch is studied and correlated with the total energy loss.