7, opening and closing the thermal switch 9, and changing the voltage/power output by the power supply 11.

More particularly, the controller 12 controls the MPCS 7 to selectively enter a closed state, i.e. the superconducting state, and an open state. i.e. the normal or non-superconducting state. The MPCS 7 includes composite superconducting wire made of superconducting filaments inside a copper matrix, similar to the superconducting wire used in the superconducting coil 6. Like other superconducting wires, this composite superconducting wire acts as a normal conductor at higher temperatures and as superconductor at cryogenic temperatures. When the MPCS 7 is in a closed (superconducting) state, it is able to carry the main magnet current, and the superconducting magnet is able to enter the persistent current mode. The MPCS 7 may be switched to the closed state by cooling, e.g., using the cryocooler 3. When the MPCS 7 is in an open (non-superconducting or normal) state, it cannot carry the main magnet current. The MPCS 7 may be switched to the closed state by heating, e.g., using a MPCS heater (not shown). However, the MPCS 7 has a small (normal) resistance in the open state, which is high enough that, when the magnet is connected to the power supply 11, only a small amount of current flows through the MPCS 7 and the rest of the current flows in the superconducting coil 6. Accordingly, when the MPCS 7 is open, the magnet may be in a ramping state, during which the MPCS 7 dissipates power because the ramping voltage across the MPCS 7 generates current flowing through its normal resistivity. When the MPCS 7 is closing, there is no ramping voltage present and no power dissipation. The MPCS 7 transitions from the open state to the closed state by means of cooling, via the convective cooling loop 8 discussed below, and the power supply 11 maintains magnet operating current during the transition. When the MPCS 7 is in the fully closed state (as opposed to transitioning between open and closed), the power supply 11 ramps down the current. The high self-inductance of the superconducting coil 6 ensures that the coil current does not change, so as a result, current through the MPCS 7 ramps up as the power supply 11 current ramps down.

In addition, the controller 12 controls the thermal switch 9 in the convective cooling loop 8 to open and close depending on the action required by the operator. For example, when the MPCS 7 is in the open state while the magnet needs to be put into persistent current mode after a ramping activity, the thermal switch 9 is opened to enable the flow of coolant through the convective cooling loop 8, thereby thermally connecting the MPCS 7 to heat exchanger 10 via the loop tube 13 to provide additional cooling for closing the MPCS 7. When the MPCS 7 is in the closed state, but needs to be opened, for instance to ramp the magnet up or down. the thermal switch 9 is closed to stop the flow of coolant through the convective cooling loop 8, e.g., by blocking the loop tube 10 as discussed below, thereby thermally disconnecting the MPCS 7 from heat exchanger 10, and allowing it to heat up to open without overloading the second stage 5 of the cryocooler 3. When the magnet is in the ramping state, and the MPCS 7 is in the open state, the thermal switch 9 is in closed state to ensure that power generated by the MPCS 7 does not overload the second stage 5 of the cryocooler 3 (which keeps the superconducting coil 6 cold). When the magnet enters the persistent current state, the thermal switch 9 is opened to keep the MPCS 7 thermally connected to the heat exchanger 10, ensuring that the MPCS 7 stays in superconducting state.

The second stage 5 of the cryocooler 3 is able to bring the superconducting coil 6 of the magnet system to the desired cryogenic temperature of about 4 Kelvin, although it has a limited capacity for absorbing power. Thus, heat coming from the MPCS 7 in the open state would otherwise overload the cryocooler 3. As discussed above, when the power supply 11 is connected across the MPCS 7 in its open state, most of the current will flow into the superconducting coil 6, and only a small current will flow through the normal, resistive wires of the MPCS 7. Once current flowing through the superconducting coil 6 has reached its target value (target current), the controller 12 controls the MPCS 7 to enter the closed state, enabling the superconducting coil 6

to operate in a persistent current mode with effectively zero resistance, after ramping down the power supply 11. This may be referred to as a closed superconducting circuit. The target current is the current needed to flow in the wire to make the target field at the center of the superconducting magnet.

In general, the magnet MPCS 7 generates heat, i.e. thermal energy, when in the open state due to current flow through the normal resistance, and also may continue to generate heat when controlled to transition from the closed state to the open state or vice versa. When the current in the superconducting coil 6 has reached the target current, the controller 12 turns off the voltage of the power supply 11, but the high inductance of the superconducting coil 6 causes the current to continue to flow through the power supply 11. In this condition, there is no power dissipation in the MPCS 7 anymore, and the MPCS 7 is ready to be cooled down to switch from the open state to the closed state. Cooling the MPCS 7 is done, in part, by controlling the thermal switch 9 to allow the coolant to flow in the loop tube 13 of the convective cooling loop 8, and thus to thermally connect the MPCS 7 to the second stage 5 of the cryocooler 3.

The loop tube 13 may be formed of a non-magnetic metal, such as copper, aluminum, titanium, zinc, tin or lead, for example, or other non-magnetic material. The loop tube 13 is hermetically scaled, and the coolant contained in the loop tube 13 may be helium gas or helium liquid, for example, for enabling the convective transfer of thermal energy between the MPCS 7 and the heat exchanger 10. Other types of gas and/or liquid coolant may be incorporated.

The thermal switch 9 is configured to open and close the loop tube 13 in order to selectively enable and block flow of the coolant, respectively. When the thermal switch 9 is open, the coolant can flow through the loop tube 13 between the MPCS 7 and the heat exchanger 10 in order to dissipate the heat being generated. When the thermal switch 9 is closed, the flow of the coolant through the loop tube 13 is blocked.

According to preferred embodiments of the invention further described below with reference to FIGS. 2 to 7, the thermal switch 9 comprises a valve 14 integrated with the loop tube 13 between the magnetic persistent current switch 7 and the heat exchanger 10. The valves 14 of the different preferred embodiments of the invention each comprise a valve body 15 with and inlet 16 and an outlet 17 with which the valve body 15 is connected to the loop tube 13, a movable shaft 18 which is arranged inside the valve body 15 and which comprises a permanent rod magnet