Quench protection circuit for superconducting magnet coils

A superconducting magnet includes at least one superconducting coil and a quench protection circuit electrically coupled to said at least one coil in parallel. The circuit includes at least one quench heater assembly thermally coupled to the at least one coil, and at least one superconducting current limiter electrically connected in series with the at least one quench heater assembly. The superconducting current limiter has a superconducting state with zero resistance, and a normal state with a normal resistance to decrease an electric current flowing through the quench heater assembly.

BACKGROUND

Embodiments of the disclosure relate generally to superconducting magnets and more particularly relates to a quench protection circuit for superconducting coils of the superconducting magnets.

In theory, superconducting magnets conduct electricity without resistance as long as the magnets are maintained at a suitably low temperature, which is referred to as “critical temperature” of the superconductor herein after. Accordingly, when a power source is initially connected to the superconducting magnet coils for a period to introduce a current flow through the magnet coils, the current will continue to flow through the coils after the superconducting switch is closed and power supply is disconnected because of the absence of electrical resistance in the coils, thereby maintaining a strong magnetic field in, for example, magnet resonance imaging (MRI) systems, and generators.

Cooling systems are used for maintaining the superconducting magnets below the critical temperature by, for example, immersing the superconducting coils in liquid helium, or by arranging other cooling apparatus such as cooling tubes thermally coupled to the superconducting coils to remove heat from the coils. A vacuum vessel and a thermal shield are provided for receiving the superconducting coils and minimize the convection and radiation heat load from ambient to the superconducting coils that should be maintained below the critical temperature. However, the magnets or part of the superconducting coils still may become normal (no longer superconducting) and develop a resistance that causes current flowing through the coils to decay rapidly converting the stored magnetic energy into Joule I2R heat that raises the temperature of the region. This is an irreversible action known as “quenching” or “quench”, which causes undesirable heat that can lead to increased temperature. The entire magnet can then become normal and no longer be superconducting. In addition, quench can lead to overheating within the superconducting coil or voltage spikes and arcing damage to components as well as the magnet. It is therefore desirable that quench protection devices can quickly spread the normal zone to the other portions of the coils and dump the magnetic energy into joule heat more evenly across the entire coils or magnet. This quench protection can limit the maximum temperature and voltage in the superconducting coil to be within the safe range, prevent any coil damage caused by over-heating, over-voltage, or over-stress.

One conventional quench protection apparatus includes a set of electrical quench heaters. When a quench occurs, temperature of the heaters arises quickly, and the heaters transmit heat to a larger area of the superconducting coils. The quench protection apparatus also includes a current limiter connected in series to the heaters for limiting the current flowing through the heaters to protect the heaters from overheating. Conventional current limiters, such as positive temperature coefficient resistors, are usually arranged outside the vacuum vessel. Accordingly, there are electrical wires extending through the vacuum vessel, which requires additional cost and might adversely decrease vacuum reliability.

It is desirable to have a different superconducting magnet with a simpler quench protection circuit.

BRIEF DESCRIPTION

In accordance with an embodiment disclosed herein, a superconducting magnet is provided. The superconducting magnet includes at least one superconducting coil and a quench protection circuit electrically coupled to the at least one coil in parallel. The circuit includes at least one quench heater assembly thermally coupled to the at least one coil, and at least one superconducting current limiter electrically connected in series with the at least one quench heater assembly. The superconducting current limiter has a superconducting state with zero resistance, and a normal state with a normal resistance to decrease an electric current flowing through the quench heater assembly.

In accordance with another embodiment, a superconducting magnet system includes a thermal shield, at least one superconducting coil in the thermal shield, a vacuum vessel enclosing the thermal shield, and a quench protection circuit electrically coupled to said at least one superconducting coil in parallel. The circuit includes at least one quench heater thermally coupled to the at least one superconducting coil, and at least one superconducting current limiter connected in series with the at least one quench heater assembly. The at least current limiter includes a superconducting state with zero resistance and normal state with a finite resistance.

In according to still another embodiment, a method for quenching superconducting coils includes coupling a quench protection circuit in parallel to at least one superconducting coil. The quench protection circuit comprises at least one quench heater assembly and at least one superconducting current limiter. A quench event is triggered by an increasing quench voltage across the at least one superconducting coil. The method further includes powering the quench heater upon said quench event and normalizing the at least one superconducting coil at heater regions, and turning the superconducting current limiter from a superconducting state to a normal state to increase the resistance of the quench protection circuit heater branch and limit the current of quench heater assembly to be less than its maximum allowable current.

DETAILED DESCRIPTION

Embodiments of the disclosure relate to a superconducting magnet having a quench protection circuit. The quench protection circuit in one example comprises an electric quench heater assembly in thermal connection with a superconducting coil and a superconducting current limiter electrically connected in series with the quench heater assembly for limiting the electric current flowing through the quench heater assembly during a quench. The quench heater assembly and the superconducting current limiter are then in parallel connection with at least one superconducting coil. Once a quench event to the superconducting coil occurs, the increasing quench voltage across the superconducting coil powers the quench heater assembly, which then heats up and normalizes the superconducting coil at the quench heater regions, thereby spreading the energy and preventing damage to the superconducting magnet. As used herein “normalize” refer to that the superconductor of the coil is turned from a superconducting state to a resistive or normal state because of coil temperature exceeding the superconductor's critical temperature.

In certain embodiments, the superconducting current limiter comprises superconducting materials, and is maintained at its superconducting state during normal operation of the superconducting magnet. After the quench heater assembly has normalized the superconducting coils at the quench heater regions, and before the heater current increases to its critical current, the superconducting current limiter is turned to its normal state, and the resistance of the superconducting current limiter increases quickly to decrease the heater current. The “critical current” as used herein after refers to a maximum heater current at which the quench heater might be damaged by overcurrent or overheat. In one embodiment, the superconducting current limiter is arranged within a vacuum vessel and a thermal shield surrounding the superconducting coil. In certain embodiments, the superconducting magnets can be used in, for example but not limited to, Magnetic Resonance Imaging (MRI) systems, a power generator, and an electric motor.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items, and terms such as “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation.

Referring toFIG. 1, a superconducting magnet10(“magnet10”) according to one embodiment of the invention comprises at least one superconducting coil12(“coil12”), and a quench protection circuit14electrically coupled in parallel to the coil12. In the illustrated embodiment, the at least one superconducting coil12comprises a group of superconducting coils12electrically connected in series. In the illustrated embodiment ofFIG. 1, the quench protection circuit14comprises a main heating branch25which comprises an electric quench heater assembly16(“quench heater16”) which comprise a few quench heaters electrically connected in-series or in-parellel, and a superconducting current limiter18in series connection with the quench heater assembly16. In certain embodiments, the quench heater assembly16is thermally coupled to one or more of the coils12. In one embodiment, the quench heater assembly16can be glued to the coils12by adhesive material, such as epoxy resin, thermal grease, to form very low thermal resistances between the quench heater assembly16and the coils12. In other embodiments, the quench heater assembly16can be mechanically attached to the coils12by clamping for example. Thermally coupled as used herein refers to a proximate placement allowing thermal conduction to occur.

In certain embodiments, to reduce or eliminate unwanted currents in the quench protection circuit14during magnet ramps and magnet normal operations, the quench protection circuit14further comprises a voltage blocker20for blocking current flowing through the quench heater16before the voltage of the coils12reaches a threshold voltage (V0). In certain embodiments, the threshold voltage (V0) is larger than a ramp voltage and the maximum voltages across the coils12during magnet normal operations and the voltage blocker20reduces or eliminates the unwanted current flowing in the quench protection circuit14or other voltage noise. In the illustrated embodiment, the voltage blocker20is in series connection with the quench heater16and the superconducting current limiter18. In certain embodiments, the voltage blocker20can include a pair of back-to-back series connected components such as Zener diodes or a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). The backward breakdown voltage of the Zener diodes is selected or controlled to be higher than the maximum voltages during magnet normal operations and yet low enough for the quench heater assembly16to act during a magnet quench.

In the normal operation of the magnet10, the coils12work at a superconducting state and transmit electric current with zero resistance. The voltage across the coils12is zero, and no current flows through the quench heater assembly16during the normal operation of the magnet10. The superconducting current limiter18is also at its superconducting state without any electrical current flowing therethrough. When a quench occurs to one or more of the coils12, a quench voltage (Vq) across the coils12increases. At this state, the quench voltage (Vq) exceeds the threshold voltage (V0) of the voltage blocker20, and a heater current (ih) flows through the quench heater assembly16and the superconducting current limiter18. The quench heater assembly16is heated and transmits heat to one or more heater regions on the coils12to protect the original quench spot of the coils12. After the coils12have been normalized, and before the heater current (ih) reaches a critical current of the quench heater assembly16, the superconducting current limiter18is turned to its normal state by the joule heating of heater22and the resistance of the superconducting current limiter18increases quickly to decrease the heater current. The quench heater16is thus protected from overcurrent.

FIG. 2illustrates several exemplary curves related to the time periods associated with a quench that occurs to one or more of the coils12. Curve C1illustrates the quench voltage (Vq) across the coils12; curve C2illustrates resistance of the superconducting current limiter18after the quench occurs; and curves C3and C4respectively illustrate the heater current with and without the superconducting current limiter. Referring again toFIG. 2, once a quench occurs to one or more of the coils12, the increasing quench voltage across the coils12increases as shown by curve C1. The increasing quench voltage powers the quench heater assembly16. Without the superconducting current limiter18, the heater current increases along with quench voltage, shown as curve C4inFIG. 2. If the heater current increases over the current limit (ilimit) of the quench heater assembly16at a critical time point (t0), there is a risk that the quench heater assembly16will be harmed by the overcurrent and over temperature or otherwise degraded in operation or life expectancy.

With the superconducting current limiter18electrically coupled to the quench heater assembly16in series, the heater current curve is shown as curve C3. The superconducting current limiter18has a substantially zero resistance at the beginning of the quench event, and is turned to its normal state at a safe threshold time point (t1) before the heater current reaches the current limit (ilimit) at the critical time point (t0). The corresponding heater current at the safe threshold time point (t1) is a safe threshold current (i1). Once the superconducting current limiter18is turned to the normal state at the time point t1, the resistance thereof increases quickly, and the heater current decreases quickly. Accordingly, the quench heater16is protected from overcurrent.

Referring back toFIG. 1, in the illustrated embodiment, the quench protection circuit14comprises a limiter heater22thermally coupled to the superconducting current limiter18. The limiter heater22that heats the superconducting current limiter18to turn the superconducting current limiter18from the superconducting state to the normal state when the quench heater16has normalized the superconducting coils12and before the heater current ihreaches the current limit.

In the illustrated embodiment ofFIG. 1, the quench protection circuit14comprises a limiter-heating branch24electrically coupled in parallel to the main heating branch25, wherein the main heating branch25includes the quench heater assembly16, the superconducting limiter18, and the voltage blocker20. In another embodiment, the limiter-heating branch24can be electrically coupled in parallel to the quench heater assembly16. The limiter-heating branch24comprises the limiter heater22, limiter diode blocker26, and a resistor28electrically connected in series. In certain embodiments, the limiter diode blocker26can be back-to-back in series connected Zener diodes or a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), and has a limiter threshold voltage (V1). In the illustrated embodiment, the limiter-heating branch24comprises a resistor28in series connection with the limiter heater22and the limiter voltage blocker26for adjusting a limiter current (i1) flowing through the limiter heater22.

In one embodiment, the limiter threshold voltage (V1) of the limiter voltage blocker26is larger than the quench threshold voltage (V0) of the quench voltage blocker20. During a quench event of the coils12, the quench voltage (Vq) across the coils12reaches the quench threshold voltage (V0) of the voltage blocker20, and the heater current (ih) flows through the quench heater assembly16. The quench heater assembly16heats up and starts to transmit heat to the heater regions on the coils12. When the quench voltage (Vq) across the coils12exceeds the limiter threshold voltage (V1) of the limiter voltage blocker26, a limiter current (i1) flows through the limiter heater22which heats the superconducting current limiter18. Accordingly, the superconducting current limiter18turns from the superconducting state to the normal state, after the quench heater assembly16has normalized the coils12and before the heater current (ih) reaches the critical current (i0).

In certain embodiments, a thermal analysis of the quench heater assembly16is conducted to ensure that the superconducting current limiter18is turned to the normal state after the quench heater18has normalized the coils12. In one example of a magnet10, a minimum heat (Q) generated by the quench heater assembly16for normalizing the coils12during a quench event can be determined by simulation or experimental results. Referring toFIG. 3, the minimum heat (Qh) generated by the quench heater system16for normalizing the coils12can be calculated according to equations:
Qh=∫0tmih2Rdt>Qe+Qout;
and
|Qe=∫T0TccmdT
wherein “Qe” represents the heat enthalpy increase of the superconductor of the coil12at the heater region when the superconductor temperature increases from the low working temperature (T0) (for example 4.2K at liquid helium) to its critical temperature Tc(for example 9.8K for NbTi superconductor); “Qout” is the heat flows from the superconductor at the heater region to the farthest superconductors, shown as Qout1, Qout2, and Qout3inFIG. 3; “ih” is the heater current flowing through the quench heater assembly16; “R” is the resistance of the quench heater assembly16; “tm” is the minimum time for normalizing the coils12after a quench occurs to one or more of the coils12; “c” is the specific heat of the superconductor; and “m” the mass of the superconductor. Accordingly, a minimum time (tm) and heater current (ihm) are established for normalizing the superconducting coils12, for example, by observing the current curve C3inFIG. 2. In one embodiment, the superconducting current limiter18is turned to the normal state at a time point that lies between the minimum time (tm) when the coils12have been normalized and the critical time (t0) when the heater current reaches the critical current. In another embodiment, the superconducting current limiter18is turned to the normal state when the heater current (ih) is between the minimum heater current (ihm) and the current limit (ilimit).

In another embodiment, the limiter threshold voltage (V1) of the limiter voltage blocker26is substantially the same as the quench threshold voltage (V0) of the quench voltage blocker20. Once a quench occurs to one or more of the coils12, and the quench voltage exceeds the (V1) and (V0), the quench voltage will gradually drive electric currents flowing through both the quench heater assembly16and the limiter heater22. The limiter heater22turns the superconducting current limiter18into the normal state when the quench heater assembly16has normalized the coils12and before the heater current reaches to the current limit. In this embodiment, the limiter current increases much more slowly than the heater current.

Referring toFIG. 4, a quench protection circuit30for providing quench protection of coils12according to another embodiment is illustrated. In this embodiment, a limiter heater32is thermally coupled to the superconducting current limiter18and is electrically connected in series with the voltage blocker20and the quench heater assembly16. During a quench event, the quench voltage exceeds the threshold voltage V0of the diode blocker20, and a heater current (ih2) flows through both the quench heater assembly16and the limiter heater32. In certain embodiments, heat generated by the limiter heater32turns the superconducting current limiter18into the normal state when the quench heater assembly16has normalized the coils12, and before the heater current reaches to the limiting current of the quench heater assembly16. In one embodiment, this is obtained by selecting proper resistance values of the quench heater assembly16and the limiter heater32.

Referring toFIG. 5, the quench protection circuit14,30illustrated inFIG. 1or4is used in a superconducting magnet system34(“system34”) includes at least one superconducting coil12, a thermal shield38and a vacuum vessel36, and a quench protection circuit14, or30for providing quench protection for the coils12. The quench protection circuit14, or30comprises at least series connected quench heater assembly16, superconducting current limiter18and voltage blocker20as illustrated inFIGS. 1 and 4. In certain embodiments, the quench protection circuit14, or30comprises a plurality of quench heater assembly16thermally coupled to the coils12. In the illustrated embodiment, the quench heaters in the assembly16are each a thermally conductive film attached on an outer or inner surface of the corresponding coil12or even embedded between two layers of the winding of superconducting coil12. Once a quench occurs to one or more of the coils12, the quench heater assembly16heats up and spreads heat to the heater regions on the coils12to normalize the coils12at that region. The normalized portions of the coils12have resistances and generate joule heating that further propagate within the coils12and gradually transit the magnetic energy into thermal energy, reduce the maximum temperature and voltage within the quenched coil. The superconducting current limiter18protects the quench heater assembly16from overcurrent. In the illustrated embodiment, the superconducting current limiter18is arranged within the vacuum vessel36and the thermal shield38. Accordingly, no additional wiring is required that penetrates the vacuum vessel36for electrically coupling the superconducting current limiter18with the quench heater assembly16. In the illustrated embodiment ofFIG. 5, the voltage blocker20is also arranged within the vacuum vessel36and the thermal shield38. In other embodiments, the voltage blocker20may be arranged outside of the vacuum vessel36.

In the illustrate embodiment, the system34comprises a thermal shield38disposed within the vacuum vessel36. The thermal shield38prevents radiation heat of the room temperature from affecting the coils12. In the illustrated embodiment, the system further comprises a cryogen tank40within the thermal shield38. The cryogen tank40receives the coils12and stores cryogen such as liquid helium. In the illustrated embodiment, the coils12are immersed in the liquid helium, and are maintained at the low temperature of the liquid helium during normal operation of the system34. In the illustrated embodiment, the system34comprises a plurality of supporting members42for supporting the coils12. In the illustrated embodiment, the superconducting current limiter18is immersed in the liquid helium. In the illustrated embodiment, the superconducting current limiter18is mounted to the supporting member42. In other embodiments, the superconducting current limiter18can be mounted to the coils12, or mounted to an inner surface of the cryogen tank40. In the illustrated embodiment, the voltage blocker20is also arranged within the cryogen tank40. In the illustrated embodiment, the voltage blocker20is mounted on the supporting member42.

Referring toFIG. 6, in one embodiment, the superconducting current limiter18comprises a bobbin44and at least one superconducting wire46wound on the bobbin44. In one embodiment, the at least one superconducting wire46is bifilarly wound on the bobbin44, and current (i) following through adjacent winding turns are in opposite directions that the magnetic field generated by adjacent turns cancels each other therefore the net inductance of the superconducting current limiter is substantially zero. In the illustrated embodiment, the superconducting wire46is folded and then the folded wire is spirally wound on the bobbin44as a solenoid. In certain embodiments, the bobbin44comprises Fiberglass Reinforced Plastics (FRP), or metals such as copper and aluminum), and is adapted for mounting the superconducting current limiter18in the system30(FIG. 5). In certain embodiments, the superconducting wire46comprises superconducting material, preferably to be Niobium-Titanium (NbTi) wire with Copper-Nickel (Cu—Ni) matrix.

It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. The various features described, as well as other known equivalents for each feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure.