High temperature superconductor (HTS)-based connections between magnet coils

Abstract

A magnet (1) for a magnetic resonance imaging (MRI) device (2) includes at least two coils (12, 14) including a first superconductor having a first superconducting critical temperature; and at least one connector (18) comprising a second superconductor having a second superconducting critical temperature, the at least one connector connecting the at least two coils together. The second superconducting critical temperature is higher than the first superconducting critical temperature.

Description

HIGH TEMPERATURE SUPERCONDUCTOR (HTS)-BASED CONNECTIONS BETWEEN MAGNET COILS

[0001] The following relates generally to the magnetic resonance imaging (MRI) arts, magnet arts, MRI superconductor arts, and related arts.BACKGROUND

[0002] Current MRI devices can include superconductive wires (i.e., MgB2 wires), for example to enable operation of an MRI cryostat at a higher temperature (e.g., around 20 degrees Kelvin (K)) rather than at 4 K as in a liquid helium-cooled cryostat. This offers advantages in terms of robustness to quenches, energy required for cooling, ability to avoid use of liquid helium coolant, and facilitating cost-efficient system designs.

[0003] Since a reliable and economic persistent joint fabrication, which is necessary for persistent magnet operation, is difficult to construct, a driven mode magnet operation can be considered as a practical alternative. In a driven mode magnet configuration, a power supply is permanently connected to the magnet and supplies a constant electric current.

[0004] The following discloses certain improvements to overcome these problems and others.SUMMARY

[0005] In some nonlimiting illustrative embodiments, a magnet for a magnetic resonance imaging (MRI) device includes at least two coils including a first superconductor having a first superconducting critical temperature; and at least one connector comprising a second superconductor having a second superconducting critical temperature, the at least one connector connecting the at least two coils together. The second superconducting critical temperature is higher than the first superconducting critical temperature.

[0006] In some nonlimiting illustrative embodiments, a method of manufacturing a magnet for an MRI device includes electrically connecting a first end of a connector with a first coil; and electrically connecting a second end of the connector opposite from the first end with a second coil. The first coil and the second coil each include a first superconductor having a first superconducting critical temperature, and the connector includes a second superconductor havinga second superconducting critical temperature that is greater than the first superconducting critical temperature.

[0007] In some nonlimiting illustrative embodiments, a MRI device includes a magnet including a first coil including a first superconductor having a first superconducting critical temperature, a second coil including the first superconductor, and a connector connecting the first coil and the second coil together, the connector including a second superconductor having a second superconducting critical temperature that is higher than the first superconducting critical temperature; and a vacuum chamber containing the magnet.

[0008] One advantage resides in providing a magnet for an MRI device with a smaller connector between adjacent coils.

[0009] Another advantage resides in providing a connector to connect adjacent coils where the connector has a critical temperature that is higher than a critical temperature of a superconductor.

[0010] Another advantage resides in weaving a connector through adjacent coils of a magnet of an MRI device to connect the coils.

[0011] Another advantage resides in providing a connector with a U-shaped configuration to connect adjacent coils of a magnet of an MRI device.

[0012] A given embodiment may provide none, one, two, more, or all of the foregoing advantages, and / or may provide other advantages as will become apparent to one of ordinary skill in the art upon reading and understanding the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.

[0014] FIGURE 1 diagrammatically illustrates a sectional view of a portion of a magnetic resonance imaging (MRI) device according to one aspect.

[0015] FIGURE 2 schematically illustrates a perspective view of a magnet of the MRI device of FIGURE 1.

[0016] FIGURE 3 schematically illustrates a perspective view of a magnet of the MRI device of FIGURE 1.

[0017] FIGURE 4 illustrates a flowchart of a method of manufacturing the magnet of FIGURE 1.DETAILED DESCRIPTION

[0018] Connections between the different coils inside an MgEE-based or other MRI magnet influence the residual resistance and thus the heat produced inside the cryostat. These connections are mechanically more challenging than simple wire turns on coil carriers. Cooling of these connections is also more complicated. In addition, MgE wires (or other magnet wires with superconducting critical temperature that is above 4K but still relatively low) can have a limited bending radius, which requires rather large connecting structures without tight bends.

[0019] The following discloses connecting different coils of an MgE magnet (or, more generally, a magnet whose coils comprise a superconducting material with critical temperature that is above 4K but still relatively low) by high temperature superconductors (HTS). These joints may not be superconducting, and hence can require the magnet to be operated in driven mode. However, the use of a HTS wire to connect magnet coils reduces resistive losses overall since the HTS wire itself is superconducting, and this disclosed approach makes cooling of the magnet easier. HTS are substantially more expensive, so using them to build the whole magnet is challenging from a cost standpoint. For short lengths of HTS used to connect magnet coils in the magnet, the cost of the HTS is not as substantial. The connection of the MgEE (or other magnet coil) wire to the HTS can, for example, be realized over a longer distance (i.e., at least 20cm) by soldering, optionally to a thin copper plate. The HTS wire can be routed to the next coil, where it is connected in the same way.

[0020] Since the HTS wire is less sensitive to temperature (as far as the HTS material has a higher critical temperature) it provides a robust solution with respect to heat generated in the imperfect joints as well as potentially lower conduction cooling due to the mechanical routing from one coil to the other. Another advantage is that HTS tape is mechanically flexible. Whereas an MgEE wire, for example, has to be routed in order to limit bending stress, HTS has a smaller bending radius (compared with MgE ) and can be routed with a much shorter 180° bend. A copper reinforcement structure optionally can function as mechanical reinforcement and provide a thermal stabilizer.

[0021] Due to its flexibility, the HTS can also be turned and twisted so that strong magnetic fields through the plane of the HTS tape can be avoided by orienting the plane of the HTS tape parallel with the magnetic field in the vicinity of the HTS tape. Strong magnetic fields oriented perpendicular to the HTS tape substantially reduce the maximum superconducting current the HTS tape can support. Another advantage of being more flexible in routing the connecting wires that the final return conductor after connecting all coils can be a HTS conductor that is routed in close vicinity of the previous coil interconnects, so the additional fields from the axial components of the wires can at least be partially cancelled. In some embodiments the HTS conductor is embedded in a second material that is not a superconductor (e.g., in a fiberglass composite) which provides mechanical support for the connections (including turns, twists, and so forth). This significantly facilitates the mechanical installation, provides stability, and keeps the conductor at the desired position and the orientation (e.g., in order to avoid large magnetic fields).

[0022] With reference now to FIGURE 1, a sectional view is shown of a portion of a magnetic resonance imaging (MRI) device 2 is diagrammatically shown. The Inset of FIGURE 1 illustrates the section plane P through a portion of a cooling vessel or housing 10 of the MRI device 2, which contains two or more (diagrammatically illustrated three) superconducting windings (i.e., coils) 12, 14, 16 of a main magnet 1 of the MRI device 2. The coils 12, 14, 16 are connected by connectors 18 and 19 as will be further described herein. When energized by an electric current flowing therethrough, the superconducting coils 12, 14, 16 produce a static (B0) magnetic field in a bore 13 (see Inset) of the MRI device 2. As seen in the Inset, the cooling vessel 10 is generally cylindrical and surrounds the bore 13. The static (B0) magnetic field produced by the energized superconducting coils 12, 14, 16 produces the B0 field typically oriented parallel (or antiparallel) with the axis of the bore 13. The nominal (i.e., design-basis) strength of the static (B0) magnetic field may, for example, be in a range of 0.2 Tesla to 7 Tesla depending on the MRI device design, although B0 field strengths above or below this range are also contemplated.

[0023] The housing 10 is (or includes) a vacuum container 10 configured to thermally isolate the superconducting coils 12 so the superconducting coils 12 are maintained below the superconducting transition temperature (at the operational electric current) of the superconducting material making up the superconducting coils 12. The cooling vessel 10 is a vacuum vessel or container 10. Outside the vacuum container 10 is typically air at room temperature (300K). Inside the vacuum container 10 is a vacuum (said another way, during operation of the magnet thevacuum container 10 is evacuated). A cryocooler (e.g., a cold head) 20 is operative to cool a diagrammatically indicated cryogenic structure 22 that includes the superconducting coils 12. Although not shown, the cryogenic structure 22 typically includes additional components, such as a former of copper, copper alloy, or another metal or other material on which the superconducting coils 12 are wound or otherwise supported, and endplates that serve as support connection points. The illustrative cryocooler 20 is a two-stage refrigerator with a compressor motor 24 that cyclically compresses and decompresses gas helium or another suitable working fluid to cool a first stage 26 of the cryocooler 20 to a first stage temperature that is below room temperature, and to further cool a second stage 28 of the cryocooler 20 to a lower second stage temperature that is below the superconducting transition temperature (at the operational electric current) of the superconducting material making up the superconducting coils 12. The first stage 26 of the cryocooler 20 is thermally connected to cool a radiation shield 30 disposed inside the vacuum container 10 to about the first stage temperature. During operation of the magnet, the thusly cooled radiation shield 30 reduces radiative thermal losses from the cryogenic structure 22. The second stage 28 of the cryocooler 20 is thermally connected with the cryogenic structure 22 by one or more thermal links 32.

[0024] The magnet coils 12, 14, 16 have terminal connections 33 located outside of the vacuum container 10, which are at room temperature (e.g., -300K). An electrical conductor 34 extends from each terminal connection 33 into the vacuum container 10 through a vacuum feedthrough 42. The terminal connections 33 and conductors 34 are typically made of copper or a copper alloy or the like, and are not superconducting during operation of the magnet. To charge the superconducting coils 12 with an electric current for generating the static B0 magnetic field, a magnet power supply (not shown) is connected with the terminal connections 33 to deliver electric current to the superconducting coils 12 via the electrical conductors 34. In driven mode magnet operation, connection of the magnet power supply with the superconducting coils 12 is maintained during operation of the magnet, to compensate for resistive power losses at non-superconducting joints of the magnet circuit. Alternatively, it is contemplated to operate the magnet (at least part- time) in a persistent mode by disconnecting the power supply from the magnet coils 12, 14, 16 using solenoid driven lead contactors or the like (not shown).

[0025] The magnet 1 of the MRI device 2 may optionally include other components (not shown), such as a superconducting connection across the leads of the interconnected coils 12, 14,16 with a persistent current switch (to support magnet operation in persistence mode), and / or a quench protection circuit to rapidly dissipate the electrical current in the coils 12, 14, 16 in the event of a quench (i.e., the unintended loss of the superconducting property).

[0026] As further shown in FIGURE 1, the MRI device 2 may include various ancillary components, such as an illustrative set of magnetic field gradient coils 38 for encoding NMR induced in a patient or other MRI imaging subject disposed in the bore 13, and / or a whole-body radio frequency (RF) coil 39 for exciting the NMR in the patient or other subject. The RF coil 39 may, for example, be a birdcage coil or a transverse electromagnetic (TEM) coil, as two nonlimiting illustrative examples, and may optionally also include an RF shield (not shown). The gradient coils 38 and optional RF coil 39 are suitably arranged concentrically in the bore 13.

[0027] With continuing reference to FIGURE 1, the superconducting coils 12, 14, 16 include a first coil 12 and a second coil 14 and a third coil 16. More generally, there may be two coils, three coils, four coils, or more. The coils 12, 14, 16 each comprise a first superconductor (that is, a first superconducting material, i.e., a first material that becomes superconducting at below its critical temperature) that has a first superconducting critical temperature. In some nonlimiting illustrative examples, the first superconductor of each of the coils 12, 14, 16 is magnesium diboride (MgEE), for example in the form of a coil of MgEE wire. Other superconducting materials are also contemplated as the first superconductor. In some nonlimiting illustrative embodiments, the first critical temperature of the first superconductor is higher than 4K, enabling use without immersive liquid helium cooling.

[0028] At least one connector (diagrammatically illustrated two connectors 18, 19) interconnect the coils 12, 14, 16 to form the superconducting magnet. The at least one connector 18, 19 comprises a second superconductor (that is, a second superconducting material, i.e., a second material that becomes superconducting at below its critical temperature) having a second superconducting critical temperature that is higher than the first superconducting critical temperature. In some embodiments, the second superconductor is a high temperature superconductor (HTS), so that the connectors 18, 19 are HTS connectors. The connector 18 in the illustrative example connects the first coil 12 and the second coil 14 together; while the connector 19 connects the second coil 14 and the third coil 16 together.

[0029] As used herein, the superconducting critical temperature Tc is the superconducting transition temperature without any ambient magnetic field and with no electric current flowing inthe superconductor. As is known in the art, the superconducting transition temperature may be reduced by an ambient magnetic field and may be reduced by an electric current flowing in the superconductor. In practice, during operation of the magnet 1, the coils 12, 14, 16 are typically cooled to a temperature lower than the (first) critical temperature Tci of the first superconductor to enable the coils 12, 14, 16 to support the superconducting electric current producing the static (Bo) magnetic field of the operating MRI device. Since the (second) critical temperature Tc2 of the second superconductor of the connector(s) 18, 19 is higher than that of the first superconductor, i.e., TC2>TCI, operating the magnet 1 at a temperature below Tci also ensures the superconducting current flowing through the connector 18, 19 is not quenched, i.e., ensures that the connector(s) 18, 19 remain in the superconducting state during magnet operation.

[0030] In some nonlimiting illustrative embodiments, the first superconductor of the coils 12, 14, 16 comprises magnesium diboride (MgB2), and the second superconductor of the connectors 18, 19 is selected from a group consisting of a rare-earth barium copper oxide (REBCO) superconductor or a bismuth strontium calcium copper oxide (BSCCO) superconductor. In some nonlimiting illustrative embodiments, the second superconducting critical temperature of the second semiconductor is at least 77K, which is the boiling point of liquid nitrogen (LN2). Such embodiments advantageously enable the connector(s) 18, 19 to be superconducting when cooled using LN2.

[0031] With continuing reference to FIGURE 1 and further reference now to FIGURE 2, an enlarged view of one nonlimiting illustrative embodiment of the connector 18 and proximate portions of the coils 12 and 14 is shown. The at least one connector 18 in some embodiments is a tape comprising the second superconductor, and can have a length of at least 20 cm. The at least one connector 18 can be soldered to the first superconductor with a solder bond (i.e., a first solder bond connecting a first end of the connector 18 with the first coil 12; and a second solder bond connecting a second end of the connector 18 opposite from the first end of the connector 18 with the second coil 14. As diagrammatically indicated in FIGURE 2, a plane P of the superconducting tape making up the connector 18 is, in some embodiments, oriented parallel with a magnetic field B generated at a location of the tape by a superconducting electric current flowing through the magnet. While the connectors 18, 19 comprise the second superconductor, they may also comprise additional material. In some examples, the connector 18 comprises a composite of the second superconductor and a second material that is not a superconductor (e.g., fiberglass).

[0032] With continuing reference to FIGURE 1 and further reference now to FIGURE 3, an enlarged view of another nonlimiting illustrative embodiment of the connector 18 and proximate portions of the coils 12 and 14 is shown. In this embodiment, the magnet 1 of the MRI device 2 further includes a copper plate 40 disposed in fixed position respective to (e.g., between) the coils 12, 14. The connector 18 is connected to or disposed on the copper plate 40, for example being soldered to the copper plate 40 in one approach. In some embodiments, the connector 18 is weaved between the coil(s) 12, 14 (FIGURE 2), and in other embodiments, the connector 18 has a U-shaped configuration connecting the coil(s) 12, 14 (FIGURE 3).

[0033] FIGURE 4 shows a method 100 of manufacturing a magnet 1 for a magnetic resonance imaging (MRI) device 2. To begin the method 100, at an operation 101, the connector 18 is formed as a fiberglass composite comprising the second superconductor and a fiberglass material. At an operation 102, the connector 18 is arranged with a plane thereof parallel with a magnetic field generated at a location of the connector 18 by a superconducting electric current flowing through the first coil 12 and second coil 14 connected together by the connector 18. At an operation 103, a first end of the connector 18 is electrically connected (e.g., soldered) with the first coil 12. At an operation 104, a second end of the connector 18 is electrically connected (e.g., soldered) with the second coil 14. At an operation 105, the connector 18 is secured to the copper plate 40 that is a fixed position respective to the first coil 12 and the second coil 14. It is contemplated to perform these operations in different order, for example, the connector 18 may be secured to the copper plate 40 (operation 105) which is then placed in fixed position respective to the first coil 12 and the second coil 14 (operation 102), and then the connection operations 103 and 104 may be performed.

[0034] The present disclosure, through one or more of its various aspects, embodiments and / or specific features or sub-components, is thus intended to bring out one or more of the advantages as specifically noted below. For purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, other embodiments consistent with the present disclosure that depart from specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatuses are within the scope of the present disclosure.

Claims

CLAIMS:

1. A magnet (1) for a magnetic resonance imaging (MRI) device (2), comprising: at least two coils (12, 14) comprising a first superconductor having a first superconducting critical temperature; and at least one connector (18) comprising a second superconductor having a second superconducting critical temperature, the at least one connector connecting the at least two coils together; wherein the second superconducting critical temperature is higher than the first superconducting critical temperature.

2. The magnet (1) of claim 1, further comprising: a copper plate (40) disposed between the at least two coils (12, 14); wherein the at least one connector (18) is disposed on the copper plate.

3. The magnet (1) of either one of claims 1 and 2, wherein the at least one connector (18) is a tape comprising the second superconductor.

4. The magnet (1) of claim 3, wherein a plane of the tape is oriented parallel with a magnetic field generated at a location of the tape by a superconducting electric current flowing through the magnet.

5. The magnet (1) of any one of claims 1-4, wherein the at least one connector (18) has a U-shape.

6. The magnet (1) of any one of claims 1-5, wherein the connector (18) comprises a composite of the second superconductor and a second material that is not a superconductor.

7. The magnet (1) of claim 6, wherein the second material that is not a superconductorcomprises fiberglass.

8. The magnet (1) of any one of claim 1-7, wherein a portion of the connector (18) is weaved between the at least two coils (12, 14).

9. The magnet (1) of any one of claim 1-8, wherein: the first superconductor comprises magnesium diboride (MgEh); and the second superconductor (18) is selected from a group consisting of a rare-earth barium copper oxide (REBCO) superconductor or a bismuth strontium calcium copper oxide (BSCCO) superconductor.

10. A method (100) of manufacturing a magnet (1) for a magnetic resonance imaging (MRI) device (2), the method comprising: electrically connecting a first end of a connector (18) with a first coil (12); and electrically connecting a second end of the connector opposite from the first end with a second coil (14); wherein the first coil and the second coil each comprise a first superconductor having a first superconducting critical temperature, and the connector comprises a second superconductor having a second superconducting critical temperature that is greater than the first superconducting critical temperature.

11. The method (100) of claim 10, wherein: electrically connecting the first end of the connector (18) with the first coil (12) comprises soldering the first end of the connector to the first coil; and electrically connecting the second end of the connector with the second coil (14) comprises soldering the second end of the connector to the second coil.

12. The method (100) of either one of claims 10 and 11, further comprising: securing the connector to a copper plate (40) that is in fixed position respective to the first coil (12) and respective to the second coil (14).

13. The method (100) of either one of claims 10 and 11, further comprising: forming the connector (18) as a fiberglass composite comprising the second superconductor and a fiberglass material.

14. The method (100) of any one of claims 10-13, wherein the connector (18) is a tape, and the method further comprises: arranging the connector with a plane of the tape parallel with a magnetic field generated at a location of the tape by a superconducting electric current flowing through the first coil and second coil connected together by the connector.

15. A magnetic resonance imaging (MRI) device (2), comprising: a magnet (1) including a first coil (12) comprising a first superconductor having a first superconducting critical temperature, a second coil (14) comprising the first superconductor, and a connector (18) connecting the first coil and the second coil together, the connector comprising a second superconductor having a second superconducting critical temperature that is higher than the first superconducting critical temperature; and a vacuum chamber (10) containing the magnet.

16. The MRI device (2) of claim 15, further comprising: a copper plate (40) disposed between the first coil (12) and the second coil (14); wherein the connector (18) is secured to the copper plate.

17. The MRI device (2) of either one of claims 15 and 16, wherein: the connector (18) is a tape comprising the second superconductor; and the tape is oriented parallel with a magnetic field generated at a location of the tape by a superconducting electric current flowing through the first coil (12) and second coil (14).

18. The MRI device (2) of any one of claims 15-17, wherein the connecter (18) comprises a fiberglass composite comprising the second superconductor and a fiberglass material.

19. The MRI device (2) of any one of claims 15-18, wherein the magnet (1) furtherincludes: a first solder bond connecting a first end of the connector (18) with the first coil (12); and a second solder bond connecting a second end of the connector opposite from the first end of the connector with the second coil (14).

20. The MRI device (2) of any one of claim 15-19, wherein: the first superconductor comprises magnesium diboride (MgEh).

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