Persistent magnet current switch for superconducting magnets

Abstract

A persistent current switch 10 for a superconducting magnet comprises a single superconducting wire having an electrical connection port 16 at its end, and a heater 13 for supplying heat to the superconducting wire. The single superconducting wire includes an LTS segment 11 of low-temperature superconducting material and an HTS segment 12 of high-temperature superconducting material. Preferably, two LTS sub-segments 11 and one HTS segment 12 are electrically connected alternately in series.

Description

[Technical Field]

[0001] The present invention relates to a magnet persistent current switch for a superconducting magnet. [Background technology]

[0002] A superconducting magnet is formed as an electromagnet with magnet windings of superconducting material. When a current is passed through the magnet windings and the magnet windings are cooled below the critical temperature for superconductivity of the magnet winding material, the current flows permanently through the windings, generating a permanent magnet field.

[0003] The persistent mode operation of superconducting magnets is described by M. Wilson in his book "Superconducting Magnets" (Oxford University Press, 1983, ISBN 0-19-854805-2), section 11.2.

[0004] Persistent mode operation of a superconducting magnet is achieved by connecting a superconducting switch in series with the superconducting field-generating coil. The switch, in the form of a switchable electrical connector, includes an electrical switch in the form of a length of superconducting wire wound around a support structure so that it does not contribute to the magnet's magnetic field. In persistent operation, it carries the full magnet current. Heating the switch above its superconducting transition temperature makes it resistive. A power source connected in parallel with the switch can then be used to vary the current in the field-generating coil.

[0005] Such a magnetic persistent current switch is known from International Patent Publication WO2019 / 096567.

[0006] The magnet persistent current switch known from International Patent Publication WO 2015 / 096567 has one superconducting wire that is part of the magnet winding, i.e. one winding of a superconducting field-generating coil is circuited to act as one superconducting wire of the magnet persistent current switch.

[0007] From European patent application EP 40 24 415 a magnetic persistent current switch made of high-temperature superconductors is known. Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION An object of the present invention is to provide a magnet persistent current switch that makes it possible to reduce the time required for ramping up a superconducting magnet. [Means for solving the problem]

[0009] The purpose of this is to a single superconducting wire, the single superconducting wire having an electrical connection port at an end of the single superconducting wire; a heater that supplies heat to one superconducting wire; wherein one superconducting wire includes an LTS segment of low-temperature superconducting material and an HTS segment of high-temperature superconducting material.

[0010] The insight of the present invention is that the heat dissipation in a magnet persistent current switch (MPCS) during ramp-up or ramp-down, i.e., when current flows through the single wire, is inversely proportional to the electrical (ohmic) resistance of the single wire material. The single wire HTS segment is formed of a high-temperature superconductor material that has a very high electrical resistance in its standard state. Therefore, above the critical temperature for superconductivity of the HTS segment, the heat dissipation of the HTS segment, and therefore the entire single wire, becomes very low during ramping of the magnet. Because only a very small amount of heat dissipation by the magnet persistent current switch needs to be carried away from the superconducting magnet, the heat dissipation of the magnet persistent current switch is carried away quickly, allowing for a high ramp speed. This high ramp speed is made possible primarily by the higher ramp voltage that can be tolerated, with little (thermal) penalty for dissipation to the magnet's cryogen system. Conventionally, a ramp voltage of 5 V is used in most cases. The present invention makes it possible to increase the ramp voltage, for example, to 20 V or more. Because the resistivity of the high-temperature superconducting ceramic material is one or several orders of magnitude higher than the resistivity of the low-temperature superconducting material of the LTS segment, the length of the HTS segment may be one or several orders of magnitude smaller than the length of the wire in a conventional MPCS. In practice, the LTS segment is composed of a metal (Cu) matrix or cladding that holds the low-temperature superconducting material, and the resistance of the LTS segment may be dominated by the resistivity of the metal matrix or cladding.

[0011] In summary, the MPCS of the present invention is used in superconducting magnets to energize them to switch them to persistent mode and vice versa to discharge these magnets. In state of the art superconducting magnets for MRI magnets, the magnet coils are made of standard superconducting wire (LTS, i.e. also called low temperature superconductor) such as NbTi, and the MPCS is made of the same material. The switch is opened (turned normal) by heating the wire above the critical temperature of the wire. During magnet energization, the same voltage (= inductance voltage across the magnet coils) used to energize the magnet is also applied to the MPCS in the "open" state. This reduces the dissipation (V 2 / R). This dissipation leads to helium boil-off and temperature rise in so-called sealed magnets (magnets not immersed in a liquid helium bath). Furthermore, it achieves a reduction in the heat load on the low-temperature (4K) structure of the magnet. When applied to magnets whose windings are cooled by immersion in liquid helium, it reduces the amount of helium loss.

[0012] According to the present invention, an MPCS comprises a wire of at least two different materials in series: one part (the LTS segment) is made of LTS wire, and another part (the HTS segment) is made of HTS wire. HTS is a ceramic material whose resistance is very high under normal conditions. Therefore, when the MPCS is heated above the critical temperature of both the LTS and HTS wires, the resistance of the MPCS becomes very high and the dissipation during current application (or discharge) becomes very low.

[0013] The HTS and LTS segments may be shaped as straight or twisted wires or may be wound, for example, like a solenoid.

[0014] The MPCS of the present invention generally accomplishes the following: Reduced dissipation within the magnet (excluding heaters) due to MPCS during energization or discharge. Less heat load means the magnet heats up less during energization (or discharge), which reduces the time required for cooling. A magnet power supply with a higher ramp voltage can be used to allow for fast ramp up and / or ramp down. Gradient amplifiers can be used instead of dedicated magnet power supplies. This results in shorter energization and discharge times for ramp up / down. This is important for a 3T sealed magnet, i.e. in the temperature range (T AutoDischarge -T StartEnergize ) and automatic discharge termination before quenching or proper T AutoDischarge Using high-speed current application results in a higher T StartEnergize is possible. Fast (controlled) ramp-down is possible instead of using a quench button (if a high ramp-down voltage is used): a 1.5T magnet was ramped down in, for example, 5 minutes.

[0015] These and other aspects of the invention will be explained in more detail with reference to the embodiments defined in the dependent claims.

[0016] In an embodiment of the MPCS of the present invention, the HTS segment is made of a ceramic material (MgB2 or YBaCuO, or other HTS copper oxide), preferably without a metallic conductive matrix or cladding, so that the entire HTS segment has a high ohmic resistance in its standard state above its critical temperature for superconductivity. Yttrium barium copper oxide (YBCO) is a family of crystalline compounds that exhibit high temperature superconductivity, including the first material known to become superconducting above the boiling point of liquid nitrogen (77 K), near 93 K. Many YBCO compounds are YBa2Cu3O 7-x ), but YBa2Cu4O y (Y124) and Y2Ba4Cu7O y Materials with other Y:Ba:Cu ratios exist, such as (), which is part of the more general family of rare earth barium copper oxides (ReBCO), where other rare earths replace yttrium. These materials are suitable for HTS segments. Alternatively, MgB2, with a critical temperature of approximately 39 K, can be used for HTS segments.

[0017] In an MPCS embodiment, the invention is embodied as a tandem arrangement of two LTS segments with an HTS segment between the LTS segments.

[0018] Because the LTS and HTS segments are connected in series, their currents may fluctuate due to drift or voltage noise in the magnet power supply. In particular, when the HTS segments cool to recover their superconducting state during the final stage of ramping of a superconducting magnet near the critical temperature of the HTS segments, various localized paths to superconductivity may occur, and localized dissipation due to these current changes may weaken the superconducting state. While the HTS segments transition from their normal state to their superconducting state, the LTS segments remain in their normal (resistive) conductive state. The resistance of the LTS segments reduces the voltage noise across the HTS segments of the MPCS, thus reducing the dissipation of localized paths within the HTS segments. In practice, the ohmic resistance of the LTS segments is governed by the resistivity of the conductive matrix (e.g., Cu) (cladding or matrix) in which the low-temperature superconducting material (e.g., NbTi) is disposed. The HTS segments preferably do not contain conductive materials as cladding or matrix. During the final cooling stage of the ramp-up process, i.e., when cooling the MPCS, there are moments when the HTS segment is partially superconducting and partially in its normal state. Ceramic superconductor materials are composed of numerous grains or crystalline domains that may have slightly different ratios of composite elements, slightly different crystal structures, or slightly different temperatures and (stray) magnetic fields. Therefore, the effective superconducting critical temperature has a distribution of various values ​​throughout the HTS segment. During the recovery of the superconducting state, near the final stage, only some grains or domains are still in the normal resistive state. That is, there exists a "near-superconducting pathway" where a series of grains are in the superconducting state and one or several grains are still in the normal state. The presence of a series LTS segment that is still resistive attenuates the overall voltage noise across these one or several grains. However, in this situation, the large heat dissipation is accommodated by the LTS segment in series with the HTS segment, and therefore the final grain or domain is also efficiently cooled below its individual value of the superconducting critical temperature.The voltage applied by the magnet power supply across the length of the MPCS wire is divided so that a larger contribution is across the LTS segment, and at most a fraction of the voltage is across the HTS segment. Therefore, there is at most low heat dissipation across the HTS segment. Therefore, near the superconducting critical temperature of the HTS material, the HTS segment can be efficiently cooled by the cryogenic system, ensuring that the LTS segment regains its superconducting state while still in its normal resistive state. Some form of conductive metallic material or metal oxide can be applied as a matrix, substrate, or cladding to increase the uniformity of dissipation across the HTS segment during cooling. This is because the current is directed away from the ceramic HTS material, which is still in its normal conductive state, and toward the metal or metal oxide. Because this metal or metal oxide in the HTS segment has a much lower electrical resistance than the LTS segment, power dissipation across the HTS segment remains low, and therefore the ramping time required is reduced due to the higher ramping voltage. The HTS segment, in its normal state, has a much higher resistance than the LTS segment. Thus, the addition of the matrix / substrate / cladding may still make this resistance much higher than the resistance of the LTS segment, but may be a compromise that facilitates the transition to the standard state.

[0019] In another embodiment of the MPCS of the present invention, a capacitance is circuited in parallel with the connection port. Thus, when ramping the electrical voltage source (often called the magnet power supply), a capacitance is circuited in parallel with the magnet power supply. This acts as a low-pass filter, i.e., a shortcut to high-frequency noise that may occur during ramping. The capacitor shortcuts high-frequency noise caused by variations in the conductivity of the HTS segments. The low-pass filter avoids fluctuations in the current through the MPCS caused by EM field fluctuations due to fluctuations caused by external noise sources, such as fluctuations in the mains power supply, to which the magnet windings are sensitive, and propagating voltage noise to the MPCS.

[0020] In a further embodiment of the MPCS of the present invention, a shunt conductor is circuited in parallel with the HTS segment and in series with a controllable switch that is also circuited, optionally, through the LTS segment. The controllable switch is set to an open state and a voltage is applied by the magnet power supply while the HTS segment is in a normal state and has a high ohmic resistance, i.e., during ramp-up / ramp-down. The controllable switch is closed while the superconducting magnet is in persistent mode during cool-down. The shunt conductor protects the HTS segment if its superconductivity is quenched, because in this case, current passes through the shunt and the resistive LTS segment, where energy from the HTS segment is dissipated.

[0021] In yet another embodiment of the MPCS of the present invention, a thermal switch is provided between the HTS segment and a thermal connection port. The thermal connection port establishes an entrance to a thermal path to the low-temperature (e.g., 4 K) structure of the magnet, which is cooled by the magnet's cryogenic system. When implemented in a superconducting magnet, the HTS segment is thermally coupled to the thermal buffer of the superconducting magnet's cryogenic system via the thermal switch. During cooling of the HTS segment, the thermal switch is closed, thus more efficiently transporting heat from the HTS segment to the thermal buffer, thereby reducing the time it takes for the HTS segment to cool and become superconducting. In the persistent mode, the thermal switch is open so that the HTS leads are thermally isolated.

[0022] In another embodiment, the LTS segment includes multiple LTS subsegments, and the HTS segment includes one or more HTS subsegments, with the LTS subsegments alternately connected in series with one of the multiple HTS subsegments. Thus, the HTS subsegments are connected in series with one another. In particular, the LTS subsegments are wound as solenoids connected in series with a relatively short piece of wire forming the HTS segment.

[0023] The present invention also relates to a superconducting magnet comprising a superconducting magnet winding assembly and an MPCS with a single superconducting wire including an LTS segment of low temperature superconducting material and an HTS segment of high temperature superconducting material. The superconducting magnet of the present invention allows for ramping at high voltages, thereby reducing ramp-up / down times.

[0024] In an embodiment of the superconducting magnet of the present invention, a cryogenic system is coupled to the magnet winding assembly, and the magnet persistent current switch has a thermal connection port that is thermally coupled to the thermal buffer of the cryogenic system via a thermal switch. When the thermal switch is closed, the thermal connection port is thermally associated with the HTS segment of the MPCS. This provides a thermal path for heat to be transferred from the HTS segment to the thermal buffer while thermally isolating the HTS segment during persistent mode operation.

[0025] Furthermore, the present invention relates to a method for ramping up / down of the superconducting magnet of the present invention. A thermal switch is closed during cooling of the MPCS, particularly toward the end of the ramp-up process. The thermal switch is also closed at the end of the ramp-down process to allow heat generated during ramp-down to be transported away in a controlled manner. In one implementation, the method for ramp-up / ramp-down includes controlling a controllable switch in a shunt conductor. In another implementation, the method of the present invention includes controlling a thermal switch between an HTS segment and a thermal connection port thermally linked to a thermal buffer in the cryogenic system of the superconducting magnet. Control of the controllable switch and the thermal switch can be performed independently or in combination.

[0026] The present invention allows the possibility of using gradient amplifiers integrated into a magnet as a magnet power supply when used in a magnetic resonance examination system to be fully utilized for ramping the magnet, as well as the use of gradient coils to ramp down the magnet, both with the potential for high ramp rates. See previous invention disclosures (filed for patent). Of course, dedicated MPSs providing high voltages are also possible.

[0027] These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawings. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 shows a diagrammatic representation of an MPCS incorporating the present invention with an LTS segment of a specific length of LTS wire and an HTS segment as a one-piece HTS, with a heater in the HTS segment. [Figure 2] FIG. 2 shows a diagrammatic representation of an MPCS incorporating the present invention with an LTS segment of a specific length of LTS wire and an HTS segment as a one-piece HTS, also having a heater for the LTS segment. [Figure 3] Figure 3 shows a diagrammatic representation of an MPCS incorporating the present invention with an electrical switch in parallel that closes during cooling (recovery to the superconducting state) of the HTS segment. In Figure 3, there is an electrical switch across only the HTS segment. [Figure 4] Figure 4 shows a diagrammatic representation of an MPCS incorporating the present invention with an electrical switch in parallel that closes during cooling (recovery to the superconducting state) of the HTS segment. In Figure 4, there is a switch across (part of) the HTS segment and the LTS segment. [Figure 5] FIG. 5 shows a diagrammatic representation of an MPCS incorporating the present invention, particularly an MPCS with a thermal switch to a thermal buffer to quickly cool the HTS segment. DETAILED DESCRIPTION OF THE INVENTION

[0029] 1 and 2 show diagrammatic representations of an MPCS incorporating the present invention, including an LTS segment of a specific length of LTS wire and an HTS segment as a single HTS component, with heaters in the HTS segment or for the LTS segment. In the embodiment of FIGS. 1 and 2, the LTS segment 11 is formed from two LTS subsegments, and the HTS segment 12 is serially connected between the LTS segment 11. An electrical connection port 16 is provided, which is switchably connected to a magnet winding (not shown) and a magnet power supply for injecting current into the magnet winding. A heater 13 is positioned near the HTS segment so that, during operation, it spreads heat to the LTS segment as well as the HTS segment. The heater 13 is controlled by a control unit 14, which activates the heater when the magnet is ramped so that the HTS segment 12 and / or the LTS subsegment 11 are at their normal resistance state. The control unit 14 turns off the heaters once the LTS subsegment 11 and the HTS segment 12 have regained superconductivity. This is because the magnet cryogenic system cools the LTS subsegment 13 and the HTS segment 12 when the heaters 13, 21 are turned off. In the embodiment of Figure 2, an additional heater 21 is installed near the LTS subsegment 21 to more efficiently spread heat over the length of the superconducting wires 11, 12.

[0030] Figures 3 and 4 show diagrammatic representations of an MPCS incorporating the present invention with an electrical switch in parallel that closes during cooling (recovery to the superconducting state) of the HTS segment. In Figure 3, there is a switch across only the HTS segment, while in Figure 4, there is a switch across (part of) the HTS and LTS segments. In the embodiment of Figure 3, a shunt conductor with a switch is placed in parallel with the HTS segment 12. In the embodiment of Figure 4, a shunt conductor 41 with a switch 42 is circuited in parallel across both the LTS sub-segment and the HTS segment.

[0031] 5 shows a diagrammatic representation of an MPCS incorporating the present invention with a thermal switch to a thermal buffer for rapid cooling of the MPCS, particularly the HTS segment. The superconducting wires 11, 12 of one MPCS 10 are thermally coupled to a thermal conductor 54 made of a highly thermally conductive material. This thermal conductor is in thermal contact with one end of a thermal switch 51. The other end of the thermal switch 51 is in thermal contact (52) with a thermal buffer 53. A control unit 14 activates the thermal switch.

Claims

1. A persistent current switch for a superconducting magnet, comprising: a single superconducting wire having an electrical connection port at an end of the single superconducting wire; a heater that supplies heat to at least a portion of the one superconducting wire; In a persistent current switch comprising: A persistent current switch, wherein the single superconducting wire includes an LTS segment made of a low-temperature superconducting material and an HTS segment made of a high-temperature superconducting material.

2. 10. The persistent current switch according to claim 1, wherein the HTS segment is formed from a ceramic high temperature superconducting material.

3. 2. The persistent current switch according to claim 1, wherein a capacitance is circuited in parallel with said electrical connection port.

4. The persistent current switch according to claim 1 , wherein a shunt conductor in series with a controllable switch is circuited in parallel with the LTS segment and / or the HTS segment.

5. The persistent current switch of claim 1 , further comprising a thermal switch between the HTS segment and a thermal connection port.

6. 2. The persistent current switch according to claim 1, wherein the LTS segment includes a plurality of LTS sub-segments, the HTS segment includes one or a plurality of HTS sub-segments, and the plurality of LTS sub-segments and one of the plurality of HTS sub-segments are alternately circuited in series.

7. A superconducting magnet comprising an assembly of magnet windings and a persistent current switch according to any one of claims 1 to 6.

8. A superconducting magnet as described in claim 7 when the persistent current switch is dependent on claim 5, comprising a cryogenic system coupled to the magnet winding assembly, and the thermal connection port is thermally coupled to a thermal buffer of the cryogenic system.

9. 8. The method of ramping a superconducting magnet of claim 7 when said persistent current switch is dependent on claim 4, including the step of controlling said controllable switch of said shunt conductor in an open state while said HTS segment is in a normally conductive state.

10. 9. The method of ramping a superconducting magnet of claim 8, including the step of controlling the thermal switch in a closed state during ramp up.

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