Variable thermal conductance link for superconducting persistent current switch for fast magnet ramping

1 . A system for rapid ramping of an MRI magnet, the system comprising:

a cooling source for the MRI magnet;

a persistent current switch in a circuit with coils of the MRI magnet; and

a thermal link having a first end thermally coupled to the persistent current switch and a second end thermally coupled to the cooling source;

wherein the thermal link is configured to provide a low thermal conductance connection between the cooling source and the persistent current switch while the MRI magnet is ramping; and

wherein the thermal link is configured to provide a high thermal conductance connection between the cooling source and the persistent current switch when the MRI magnet reaches a target current corresponding to a target magnetic field to rapidly cool the persistent current switch;

wherein the thermal link comprises a variable thermal conductance interface including at least two thermally conductive members having mating surfaces; and

wherein the thermal link further comprises an actuator configured to change a contact force between the mating surfaces, and to selectively configure the variable thermal conductance interface between:

(i) a first state in which the mating surfaces are pressed together under a first contact force to provide the high thermal conductance connection; and

(ii) a second state in which the mating surfaces are separated or in contact under a second, lower contact force to provide the low thermal conductance connection.

2 . The system of claim 1 , wherein the thermal link comprises:

a fixed thermal connection having a non-zero thermal conductance; and

a variable thermal connection comprising the variable thermal conductance interface.

3 . The system of claim 1 , wherein the actuator comprises a thermal actuator having a temperature-dependent dimensional change, the thermal actuator being configured such that:

as a temperature of the thermal actuator decreases below a threshold temperature, the thermal actuator changes dimension to increase the contact force between the mating surfaces and thereby provide the high thermal conductance connection; and

as the temperature of the thermal actuator increases above the threshold temperature, the thermal actuator changes dimension to decrease the contact force between the mating surfaces and thereby provide the low thermal conductance connection.

4 . The system of claim 3 , wherein the thermal actuator is mechanically connected to a split thermal conductor, and wherein the split thermal conductor comprises the thermally conductive members having the mating surfaces of the variable thermal conductance interface.

5 . The system of claim 4 , wherein a thermal insulator is disposed between the thermal actuator and the split thermal conductor.

6 . The system of claim 3 , wherein the temperature of the thermal actuator is regulated by at least one of:

a heater circuit powered by an independent power supply;

a heater circuit powered by a voltage difference created by an MRI magnet power supply during magnet ramping; and

an increased heat dissipation of the persistent current switch during magnet ramping.

7 . The system of claim 1 , wherein the variable thermal conductance interface further comprises an interposer material between the mating surfaces, whereby the interposer material increases the thermal conductance between the mating surfaces when the variable thermal conductance interface is in the first state in which the mating surfaces are pressed together.

8 . The system of claim 1 , wherein the actuator comprises a mechanically controlled actuator configured to mechanically move at least one of the thermally conductive members so as to change the contact force between the mating surfaces and thereby selectively place the variable thermal conductance interface in the first state or the second state.

9 . The system of claim 8 , wherein the mechanically controlled actuator comprises at least one of:

a motor or actuator configured to transmit mechanical force through a vacuum interface such as a bellows;

an actuator driven by pressurizing a gas or hydraulic fluid;

an actuator internal to a vacuum vessel of the magnet, such as a piezoelectric device;

wherein electrical power to the mechanically controlled actuator is supplied from at least one of:

an external power supply activated by an MRI system controller; and

a magnet ramping voltage.

10 . A system for rapid ramping of an MRI magnet, the system comprising:

a cooling source for the MRI magnet;

a persistent current switch in a circuit with coils of the MRI magnet; and

a thermal link having a first end thermally coupled to the persistent current switch and a second end thermally coupled to the cooling source;

wherein the thermal link is configured to provide a low thermal conductance connection between the cooling source and the persistent current switch while the MRI magnet is ramping; and

wherein the thermal link is configured to provide a high thermal conductance connection between the cooling source and the persistent current switch when the MRI magnet reaches a target current corresponding to a target magnetic field to rapidly cool the persistent current switch;

wherein the thermal link comprises at least one variable thermal conductivity material component configured to change thermal conductivity thereof between:

(i) a low thermal conductivity state providing the low thermal conductance connection during magnet ramping; and

(ii) a high thermal conductivity state providing the high thermal conductance connection after magnet ramping.

11 . The system of claim 10 , wherein the variable thermal conductivity material component changes thermal conductivity in response to a change in temperature thereof.

12 . The system of claim 11 , wherein the variable thermal conductivity material component comprises high purity copper.

13 . The system of claim 11 , further comprising a heater configured to change state of the variable thermal conductivity material component, powered by at least one of: an independent power supply; a voltage difference created by an MRI magnet power supply during magnet ramping; and heat dissipation from the persistent current switch during magnet ramping.

14 . The system of claim 10 , wherein the variable thermal conductivity material component changes thermal conductivity in response to application of electrical current or voltage thereto.

15 . The system of claim 10 , wherein the variable thermal conductivity material component changes thermal conductivity in response to application of magnetic field thereto.

16 . The system of claim 10 , wherein the variable thermal conductivity material component comprises a superconducting material wherein thermal conductance thereof changes between a superconducting state and a non-superconducting state.

17 . The system of claim 16 , wherein the superconducting material comprises at least one of: a thin layer disposed between thermal conducting surfaces; and a strand, tape, or wire on a low thermal conductance backing.

18 . The system of claim 16 , wherein the superconducting material is positioned proximate a coil of the magnet such that a magnetic field therefrom affects the superconducting state thereof.

19 . A method for rapid ramping of an MRI magnet, the method comprising:

providing a cooling source for the MRI magnet and a persistent current switch in a circuit with coils of the MRI magnet;

while the MRI magnet is ramping, configuring a thermal link to provide a low thermal conductance connection between the cooling source and the persistent current switch; when the MRI magnet reaches a target current corresponding to a target magnetic field, configuring the thermal link to provide a high thermal conductance connection between the cooling source and the persistent current switch to rapidly cool the persistent current switch; wherein configuring the thermal link between the low thermal conductance connection and the high thermal conductance connection comprises at least one of:

(a) changing a contact force between mating surfaces of a variable thermal conductance interface using an actuator to selectively configure the interface between a first pressed state providing the high thermal conductance connection and a second low-force state providing the low thermal conductance connection; and

(b) changing thermal conductivity of at least one material component of the thermal link between a low thermal conductivity state providing the low thermal conductance connection and a high thermal conductivity state providing the high thermal conductance connection.