Magnet cooldown thermal switch apparatus

A method for reducing a cooldown time of an MRI magnet, the method comprising:

• providing a thermal switch between a first stage and a second stage of a cryocooler of the MRI magnet, the thermal switch having a first end connected to the first stage and a second end connected to the second stage;
• closing the thermal switch above a threshold temperature to provide a thermal connection between the first stage and the second stage of the cryocooler;
• cooling the MRI magnet above the threshold temperature with the thermal switch closed to gain cooling of a cold mass of the MRI magnet connected to the second stage of the cryocooler from the first stage of the cryocooler;
• opening the thermal switch at the threshold temperature;
• and
• cooling the MRI magnet below the threshold temperature with the thermal switch open to disconnect the thermal connection between the first stage and the second stage,
• wherein the thermal switch comprises:
• a first thermally conductive contact thermally coupled to the first stage and a second thermally conductive contact thermally coupled to the second stage, the first and second thermally conductive contacts having mating faces that are in physical contact when the thermal switch is closed and are separated by a gap when the thermal switch is open;
• a mechanical preload structure that applies a preload force at room temperature urging the mating faces into the physical contact, the mechanical preload structure comprising thermally insulating members including a first thermally insulating member and a second thermally insulating member having different thermal contraction;
• and
• wherein, during cooldown, differential thermal contraction of the first and second thermally insulating members reduces the preload force and causes separation of the mating faces at the threshold temperature to open the thermal switch.