Sorry, but you're wrong. The main magnetic field is always on. It's produced by superconducting rings that are "ramped up" when the magnet is initially instal

Sorry, but you're wrong. The main magnetic field is always on. It's produced by superconducting rings that are "ramped up" when the magnet is initially installed and remains on at all times, even when the operator console is powered off. Basically, the ramp up procedure sets up a current loop in the superconducting coil via induction in a controlled manner. After the initial ramp up, the field can only be shut off by bringing in an engineer and special equipment to perform a ramp down procedure, the operator executing an emergency quench, or an accidental quench occuring due to a failure of the refrigeration system. Basically, a quench happens if the temperature of the superconductive rings exceeds the temperature threshold required to maintain superconductivity, the resistivity of the ring material becomes non-zero and runaway heating occurs because of the high current in the ring (lots of amperes), the runaway heating of the ring causes the surrounding liquid helium to go supercritical and rapid (and potentially explosive) boil off occurs. The electromagnets which are only turned on during operation (used for the gradient fields and for shimming the main field) are insignificant in magnitude compared with the main field. The main field is powerful enough to lift a ferromagnetic chair off the floor if the chair gets close enough to the bore without the gradients being active.