Protection system for inductively coupled magnets (and experience in MRI magnet systems)

A completely passive protection system for complex assemblies of superconductive magnets is presented. Quench-back heaters are used to initiate normal zones in multiple coils. Silicon diodes operating at 4.5 K are utilized to permit rapid charging and discharging of the magnet system, to protect the persistent current switches, and to prevent the long-lived field disturbances associated with resistively shunted magnets. Protection modules employing a self-limiting heater are used to provide the rapid response necessary to protect inductively coupled shim coils while maintaining protection system integrity during a worst-case quench of the main coils. Operating experience with this protection technique on three Magnetic Resonance Imaging (MRI) magnet systems is reported.