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 an electric 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 and a permanent magnet field is generated.

[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. When the switch is heated above its superconducting transition temperature, it becomes resistive. The current in the field-generating coil can then be altered using a power source connected in parallel with the switch.

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

[0006] The magnet persistent current switch known from International Patent Publication WO 2015 / 096567 comprises a single superconducting wire which is part of a magnet winding, i.e. a part of the winding of a superconducting field generating coil is circuited to act as a single 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 PRESENT EMBODIMENT The present invention aims to provide a magnet persistent current switch which 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 superconducting wire having an electrical connection port at an end of the superconducting wire; A heater that supplies heat to a single superconducting wire; In one embodiment, the present invention provides a magnet persistent current switch for a superconducting magnet, the magnet persistent current switch comprising: a first superconducting wire including 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 the 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 material of the single wire. The HTS segment of the single wire is made of a high temperature superconductor material that has a very high electrical resistance in its normal state. Thus, above the critical temperature of superconductivity of the HTS segment, the heat dissipation of the HTS segment, and therefore of the entire single wire, becomes very low during ramping of the magnet. Since 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 a high ramp speed. This high ramp speed is mainly made possible by the higher ramp voltage that is allowed, with almost no (thermal) penalty for dissipation into the cryogen system of the magnet. Conventionally, a ramp voltage of 5V is used in most cases. The present invention allows the ramp voltage to be increased, for example to 20V or even 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 wires (LTS, i.e. also called low temperature superconductors) such as NbTi, and the MPCS is made of the same material. The switch is opened (comes to 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 invention, the 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 at 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 achieves the following: Reduced dissipation in the magnets due to MPCS when energized or discharged (excluding heaters). Less heat load means the magnets heat up less when energized (or discharged). Therefore less time is required to cool down. A magnet power supply with a higher ramp voltage can be used for fast ramp up and / or ramp down. Gradient amplifiers can be used instead of a dedicated magnet power supply. This results in shorter energization and discharge times for ramp up / down. This is a great advantage for a 3T sealed magnet, i.e. in the temperature range (T AutoDischarge -T StartEnergize ) and the ability to terminate automatic discharge before quenching or the appropriate T AutoDischarge Using high-speed current application, 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 e.g. 5 minutes.

[0015] These and other aspects of the invention will be explained in further 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 be superconducting above the boiling point of liquid nitrogen (77 K), near 93 K. Many YBCO compounds are derived from the 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 common family of rare earth barium copper oxides (ReBCO), with other rare earths replacing yttrium. These materials are suitable for the HTS segment. Alternatively, MgB2, with a critical temperature of about 39 K, can be used for the HTS segment.

[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] Since the LTS and HTS segments are connected in series, the currents flowing through them may vary due to drifts and voltage noises of the magnet power supply. In particular, when the HTS segment cools down to recover its superconducting state in the final stage of ramping of the superconducting magnet in the transition state near the critical temperature of the HTS segment, various minimum paths to become superconducting may occur and local dissipation due to the aforementioned current change may weaken the superconducting state. While the HTS segment transitions from its normal state to its superconducting state, the LTS segment remains in its normal (resistive) conductive state. The resistance of the LTS segment lowers the voltage noise of the entire HTS segment of the MPCS and thus reduces the dissipation of the minimum paths in the HTS segment. In practice, the ohmic resistance of the LTS segment 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 arranged. The HTS segment preferably does not include a conductive material as a cladding or matrix. During the cooling at the final stage of the ramp-up process, i.e. when cooling the MPCS, there are moments when the HTS segment is partly superconducting and partly in its normal state. Ceramic superconductor materials are composed of many grains or crystalline domains that may have slightly different ratios of composite elements, slightly different crystal structures, or be at slightly different temperatures and different (stray) magnetic fields, and therefore the effective superconducting critical temperature has a distribution of different values ​​throughout the HTS segment. During the recovery of the superconducting state, in the near-final stage, only some grains or domains are still in the normal resistive state. That is, there are "near-superconducting paths" where a series of grains are in the superconducting state and one or several grains are still in the normal state. Having an LTS segment in series that is still resistive dampens the overall voltage noise across these one or several grains. However, in such a situation, the large dissipation of heat is accommodated by the LTS segment in series with the HTS segment, and therefore the final grains or domains are also efficiently cooled below their individual values ​​of the critical temperature of superconductivity.The voltage applied by the magnet power supply across the length of the wire of the MPCS is divided such that the larger contribution is across the LTS segment, and at most only a portion of the voltage is divided across the HTS segment. Thus, there is at most only low heat dissipation across the HTS segment. Thus, near the superconducting critical temperature of the HTS material, the HTS segment can be efficiently cooled by the cryogenic system to ensure that the LTS segment regains its superconducting state while still in its standard 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 high temperature superconducting material part, which is still in its standard conductive state, and towards the metal or metal oxide. This metal or metal oxide of the HTS segment has a much lower resistance than the electrical resistance of the LTS segment, so the power dissipation across the HTS segment remains low, and therefore the time required for ramping is reduced due to the higher ramping voltage. The HTS segment has a much higher resistance than the resistance of the LTS segment in its standard state. Thus, the addition of the matrix / substrate / cladding still makes this resistance much higher than the resistance of the LTS segment, but it 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 electric voltage source (often called magnet power supply), a capacitance is circuited in parallel with the magnet power supply. It acts as a low pass filter, i.e. a shortcut to high frequency noises that may occur during ramping. The capacitor shortcuts high frequency noises caused by the variations in the conductivity of the HTS segments. The low pass filter avoids the variations in the current through the MPCS caused by EM field variations due to variations caused by external noise sources such as mains power supply, such as passing vehicles, to which the magnet windings are sensitive, and propagating voltage noises 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 / down. The controllable switch is closed while the superconducting magnet is in a persistent mode during cool down. The shunt conductor protects the HTS segment when its superconductivity is quenched, since in this case the current passes through the shunt and the resistive LTS segment where the 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. 4K) structure of the magnet that 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 effectively transporting heat away from the HTS segment to the thermal buffer, thereby reducing the time it takes to cool the HTS segment to 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 a plurality of LTS subsegments, and the HTS segment includes one or more HTS subsegments, with the LTS subsegments and one of the plurality of HTS subsegments alternately circuited in series. Thus, the HTS subsegments are circuited in series between the LTS subsegments. 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 an assembly of superconducting magnet windings and a MPCS comprising a length of 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 and therefore reduces 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 a thermal buffer of the cryogenic system through a thermal switch. When the thermal switch is closed, the thermal connection port is in thermal correspondence with the HTS segment of the MPCS. This provides a thermal path for carrying heat away 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, wherein the thermal switch is closed during cooling of the MPCS, particularly towards the end of the ramping up process. The thermal switch is also closed at the end of the ramping down process to carry away the heat generated during ramping down in a controlled manner. In one implementation, the method for ramping up / down includes controlling a controllable switch in the shunt conductor. In another implementation, the method of the present invention includes controlling a thermal switch between the HTS segment and a thermal connection port that is thermally linked to a thermal buffer in the cryogenic system of the superconducting magnet. The 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 built into the magnet as magnet power supplies 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 higher 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 description 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. [Diagram 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. [Diagram 3] Figure 3 shows a diagrammatic representation of an MPCS incorporating the present invention with an electrical switch in parallel that closes during cooling (restoration 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 is closed during cooling (restoration 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. [Diagram 5] FIG. 5 shows a diagrammatic representation of an MPCS, specifically an MPCS incorporating the present invention with a thermal switch to a thermal buffer to quickly cool the HTS segment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] 1 and 2 show a diagrammatic representation of an MPCS incorporating the present invention, comprising an LTS segment of a specific length of LTS wire and an HTS segment as a part of HTS, with a heater in the HTS segment or also for the LTS segment. In the embodiment of FIG. 1 and FIG. 2, the LTS segment 11 is formed from two LTS sub-segments, and the HTS segment 12 is circuited in series 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 located near the HTS segment so that in 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 sub-segment 11 are in 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, because when the heaters 13, 21 are turned off, the magnet cryogenic system cools the LTS subsegment 13 and the HTS segment 12. In the embodiment of Figure 2, an additional heater 21 is installed near the LTS subsegment 21 to spread the heat more efficiently 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 (restoration to the superconducting state) of the HTS segment. In Figure 3, there is a switch only across the HTS segment, and 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 switch is placed in parallel with the HTS segment 12. In the embodiment of Figure 4, a shunt conductor 41 with 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, particularly with a thermal switch to a thermal buffer for rapid cooling of the MPCS, and in particular the HTS segments. The superconducting wires 11, 12 of a single MPCS 10 are thermally coupled to a thermal conductor 54 made of a highly thermally conductive material. The 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 magnet persistent current switch for a superconducting magnet, the magnet persistent current switch comprising: a single superconducting wire having an electrical connection port at an end of the single superconducting wire; a heater for supplying heat to the one superconducting wire; Equipped with A magnet persistent current switch, wherein the length of superconducting wire includes an LTS segment of low temperature superconducting material and an HTS segment of high temperature superconducting material.

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

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

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

6. 6. The magnet persistent current switch according to claim 1, wherein the LTS segment includes a plurality of LTS sub-segments, the HTS segment includes one or more 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 magnet persistent current switch according to any one of claims 1 to 4.

8. 6. A superconducting magnet comprising: a magnet winding assembly; a cryogenic system coupled to the magnet winding assembly; and the magnet persistent current switch according to claim 5, wherein the thermal connection port is thermally coupled to a thermal buffer of the cryogenic system.

9. 5. The method of ramping a superconducting magnet of claim 4, comprising 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. 8. The method of ramping a superconducting magnet of claim 7 including the step of controlling said thermal switch in a closed state during ramp up.

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