Cryogen-free superconducting magnet system

Abstract

The invention comprises a superconducting coil 1 for generating a magnetic field, a vacuum chamber 2 having a vacuum chamber wall 3 bounding the outside, and a cryocooler 4 for cooling the superconducting coil 1, the cryocooler having a first stage 21 cooled to a first temperature and a second stage 22 cooled to a second temperature, the second temperature being lower than the first temperature, the superconducting coil 1 being disposed within the vacuum chamber 2 at a position away from the vacuum chamber wall 3, the superconducting coil 1 being provided with two superconducting coil connectors 18, the superconducting coil connectors being galvanically connected to two current leads 5, and The current leads are passed through a vacuum chamber wall 3 to provide current to the superconducting coils 1 from outside the vacuum chamber 2, each current lead 5 includes a first section 6 and a second section 7, the cross-sectional area of ​​the first section 6 is smaller than the cross-sectional area of ​​the second section 7, the current leads 5 are galvanically attached to a superconducting coil connector 18 and thermally connected to a first stage 21 of a refrigerator at the corresponding second section 7, and each of the first sections 6 of the current leads 5 includes a cooling arrangement 8 that cools the first section 6 of the current lead 5. In this way, a cryogen-free superconducting magnet system is provided that eliminates the provision of excessive heat load at zero current of the current leads 5 without the risk of thermal runaway.

Description

[Technical Field]

[0001] The present invention relates to the field of cryogen-free superconducting magnets, and in particular to a cryogen-free superconducting magnet system comprising a superconducting coil for generating a magnetic field, a vacuum chamber having a vacuum chamber wall bounding the outside, and a cryocooler for cooling the superconducting coil, the superconducting coil being disposed within the vacuum chamber at a position spaced apart from the vacuum chamber wall, and the superconducting coil being provided with two current leads passing through (penetrating) the vacuum chamber wall for providing current to the coil from outside the vacuum chamber. [Background technology]

[0002] Typically, cryogen-free superconducting magnet systems can be mechanically cooled down to 4.2 K, eliminating the need for liquid cryogens and thus avoiding problems associated with handling liquid cryogens. However, when higher-temperature superconductors are used in superconducting magnets, the operating temperature of the system can be as high as 10-30 K. Therefore, cryogen-free superconducting magnets are advantageous because they are easier to handle, eliminating the need to transfer liquid cryogens, and avoid the need to purchase expensive liquid helium. Furthermore, cryogen-free system operation is easier to handle and requires less maintenance.

[0003] Cryogen-free magnets (also called helium-less magnets), whose coils reside within the insulating vacuum of a cryostat, require permanently installed current leads that cannot be cooled by bleeding gas from a liquid helium reservoir. These leads therefore pose a relatively large heat load to the system's refrigerator. For magnets that operate in a persistent mode with a uniform magnetic field most of the time, such as magnetic resonance imaging (MRI) magnets, the zero-current heat load of the current leads is most problematic.

[0004] No. 5,302,928 describes a current lead for a two-stage type superconducting magnet system. This type of configuration generally operates in such a way that ohmic losses are reduced from ambient temperature to the temperature of the thermal shield and from the temperature of the thermal shield to the temperature of the magnet.

[0005] While the prior art has addressed reducing the heat load on conduction-cooled superconducting magnets, it has not provided a solution that addresses the problem of zero current heat load on the current leads in cryogen-free superconducting magnet systems.

[0006] US Patent Application Publication No. 2019 / 1908932 discloses a magnet system in which permanently installed current leads with variable cross-sectional areas can be additionally cooled via a radiation shield coupled to an upper cooling stage of an active cooling device. Furthermore, the current leads are coupled to a gold reservoir, e.g., in the form of a metal mass. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide a cryogen-free superconducting magnet system that eliminates excess heat load at zero current in the current leads without the risk of thermal runaway. [Means for solving the problem]

[0008] According to the present invention, this object is addressed by the subject matter of the independent claims. Preferred embodiments of the invention are set out in the dependent claims.

[0009] Thus, in accordance with the present invention, there is provided a cryogen-free superconducting magnet system, the system comprising: a superconducting coil that generates a magnetic field; a vacuum chamber having a vacuum chamber wall bounding an exterior; a cryocooler for cooling the superconducting coil, the cryocooler including a refrigerator having a first stage cooled to a first temperature and a second stage cooled to a second temperature, the second temperature being lower than the first temperature; the superconducting coil is disposed in the vacuum chamber at a position spaced apart from the vacuum chamber wall; the superconducting coil is provided with two superconducting coil connectors, the two superconducting coil connectors are galvanically connected to two current leads, the two current leads are passed through a vacuum chamber wall to provide current to the superconducting coil from outside the vacuum chamber; each current lead includes a first section and a second section, the first section having a smaller cross-sectional area than the second section; a current lead galvanically attached to the superconducting coil connector and thermally connected to the first stage of the refrigerator at a corresponding second section; Each of the first sections of the current leads includes a cooling arrangement for cooling the first section of the current lead by heat transfer to the outside, which is at ambient temperature (eg, room temperature).

[0010] Therefore, it is an important aspect of the present invention that each current lead includes at least two different sections with different cross-sectional areas, and the current lead is attached to the superconducting coil connector at the section with the larger cross-sectional area. In this way, the section with the smaller cross-sectional area reduces heat transfer through the current lead. Furthermore, it is important that the first section of the current lead is cooled by the cooling arrangement. In this way, heat leakage to the first stage of the refrigerator can be reduced in situations where the current in the lead is low or zero. The end of the second section of the current lead is thermally fixed to the first stage of the refrigerator, which provides the advantage that the high-temperature superconducting connection is always maintained in their superconducting state.

[0011] Generally, a suitable shape for a current lead for a cryogen-free magnet is a rod strip of conductive material, such as copper, having a constant cross-sectional area along its entire length. In accordance with the present invention, the cross-sectional area of ​​the section of the current lead that is distal to the coil is reduced over a certain length of the lead. In the absence of cooling, when current is passed through the coil, this thinner section of the lead will rapidly heat up to a temperature above the melting point of the material. To prevent such thermal runaway, the thin section of the lead is provided with a cooling arrangement.

[0012] Generally, reducing the cross-sectional area of ​​the current lead at least somewhere along its length is already suitable for reducing the heat load. However, according to a preferred embodiment of the present invention, the current lead is threaded through the wall of the vacuum chamber in the first section. In this way, heat transfer along the current lead is further reduced. Furthermore, according to a preferred embodiment of the present invention, the cross-sectional area of ​​the first section of the current lead is less than 25%, preferably less than 20%, and most preferably less than 15% of the cross-sectional area of ​​the second section. Furthermore, according to a preferred embodiment of the present invention, the cross-sectional area of ​​the second section of the current lead is dimensioned to thermally withstand a predetermined maximum current supplied to the superconducting coil without cooling, while the cross-sectional area of ​​the first section of the current lead is dimensioned to be inadequate for thermally withstanding a predetermined maximum current supplied to the superconducting coil without cooling. Preferably, the predetermined maximum current is 400 A, more preferably 450 A.

[0013] Generally, a current lead may be made of different materials along its length. However, according to a preferred embodiment of the present invention, the current lead is constructed of the same material in the first and second sections. Preferably, the current lead is at least entirely made of copper.

[0014] If the current leads were permanently installed with feedthroughs through the vacuum chamber wall, they would constitute an additional undesirable heat leak into the magnet. The magnet is kept small to avoid additional heat load on the refrigerator. The thin first section of each current lead increases the thermal resistance in the current lead in the zero current situation, i.e. when the magnet is operated in persistent mode where current flows through the superconducting coils in a superconducting state with zero electrical resistance. When the magnet is raised or lowered, current is supplied to or withdrawn from the superconducting coils through the current leads. The thin first section, which has a relatively high electrical resistance, then dissipates heat (I 2R), and the temperature in the narrow first section increases. To avoid so-called thermal runaway of the current lead, which would damage the current lead due to increased heat dissipation in the narrow first section, heat is removed from the narrow first section at the room-temperature end of the current lead. Heat removal is preferably achieved by free convection of ambient air, by providing a sufficient cooling surface, for example, by using cooling fins mounted on the outside of the current lead. Heat transport from the narrow section, through which current flows during up and down, can be enhanced by fluid cooling or, preferably, by using a heat pipe for the narrow section of the current lead. Efficient heat transfer to the outside at room temperature allows heat to be continuously removed from the narrow section, without an upper limit on the cooling period of the narrow section, because the heat capacity of the outside is (virtually) infinite compared to the amount of heat generated by increased dissipation. This is essentially different from conventional enthalpy-driven cooling, which relies on providing a (finite) amount of fluid or solid cooling material, for example, from a buffer or a cooled (He) gas. Due to heat removal (e.g., by fluid cooling or by a heat pipe), heat transport from the thin first section to the room temperature outside is much greater than heat transport along the second section (with a larger cross-sectional area). In the zero-current state, the thermal resistance of the thin upper section of the current lead is much higher than that of the thicker lower section. This strongly reduces unwanted heat leakage through the feedthrough. When heat pipes are used, this is achieved passively, because the working fluid of the heat pipe freezes at its lower end, and the inside of the remaining part of the heat pipe is essentially a vacuum. If the upper section of the heat pipe is cooled by forced fluid flow, stopping this flow causes the coolant to freeze, which has a low thermal conductivity in the frozen state.

[0015] The present invention allows for different designs of the current lead in different sections of the current lead. However, according to a preferred embodiment of the present invention, the length of the first section of the current lead is at least 60%, preferably at least 80%, and most preferably more than 100% of the length of the second section of the current lead. In this way, undesired heat transfer through the current lead can be further reduced.

[0016] Various types of cooling arrangements can be used in the present invention. According to a preferred embodiment of the present invention, each of the cooling arrangements for cooling the first sections of the current leads includes a cooling channel for conducting a coolant in the corresponding first section of the current lead. In this respect, according to a preferred embodiment of the present invention, both current leads include a common cooling channel. This allows for a simple and efficient design of the cooling arrangements. According to a preferred embodiment of the present invention, this common cooling channel is composed of the corresponding cooling channel in the first section of the current lead and a galvanically isolating coupler that interconnects the cooling channels in the first sections of the respective current leads. In this way, the two leads are still galvanically isolated from each other but are thermally connected, i.e., thermally connected for cooling purposes.

[0017] Excess heat can be dissipated in various ways. According to a preferred embodiment of the present invention, each of the cooling arrangements for cooling the first sections of the current leads comprises a heat pipe, with the current leads together with the heat pipe being passed through the vacuum chamber wall, the heat pipe extending into the interior of the vacuum chamber and connected to a heat sink outside the vacuum chamber. In this respect, according to a preferred embodiment of the present invention, the heat pipe is formed by the first section of the current lead.

[0018] The present invention also relates to an MRI system comprising the above-described cryogen-free superconducting magnet system, and to a method of using the above-described cryogen-free superconducting magnet system. [Brief explanation of the drawings]

[0019] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter, although such embodiments do not necessarily represent the full scope of the invention, and reference should therefore be made to the claims and this specification for interpreting the scope of the invention.

[0020] [Figure 1] FIG. 1 shows a schematic diagram of an MRI system according to a preferred embodiment of the present invention. [Figure 2] FIG. 2 shows a first embodiment of the current lead and cooling arrangement in more detail. [Figure 3] FIG. 3 shows schematically a second embodiment of the current lead and cooling arrangement in more detail. DETAILED DESCRIPTION OF THE INVENTION

[0021] 1 in combination with FIG. 2, the MRI system includes a cryogen-free superconducting magnet system having a superconducting coil 1 that generates a magnetic field, a vacuum chamber 2 having an exterior bounding vacuum chamber wall 3, and a cryocooler 4 having a refrigerator that cools the superconducting coil 1. The superconducting magnet system further includes radiation shielding 23 disposed inside the vacuum chamber 2 and surrounding most of the cryogen-free superconducting magnet system. The superconducting magnet system operates in a persistent mode, with the leads to the superconducting coil 1 not carrying current except when necessary to change the magnetic field produced by the superconducting coil 1.

[0022] The refrigerator of the cryocooler 4 has at least two thermal interfaces through which heat is absorbed: a first stage 21, which typically has a temperature of 40 K and is used primarily to cool the radiation shield 23 of the superconducting magnet system via a thermal link 24; and a second stage 22, which typically reaches a temperature of 3-5 K and primarily cools the superconducting coil 1 via a thermal link 26. The superconducting coil 1 is disposed inside the vacuum chamber 2 and is located inside the radiation shield 23 so as not to come into contact with the radiation shield 23. The superconducting coil 1 is provided with two superconducting coil connectors 18 galvanically connected to two current leads 5 passing through the vacuum chamber wall 3 for providing current to the superconducting coil 1 from outside the vacuum chamber 2.

[0023] The superconducting coil connectors 18 are made of high-temperature superconducting (HTS) material and are thermally secured to the first stage 21 of the refrigerator via thermal links 25 so that the temperature of the superconducting coil connectors 18 is significantly lower than the critical temperature of the HTS material. Furthermore, as shown in FIGS. 2 and 3 , each of the current leads 5 includes an electrical connector 15 for connecting to a respective current line. Each current lead 5 includes a first section 6 and a second section 7. The second section 7 is thermally secured to the first stage 21 of the refrigerator. The first section 6, the second section 7, and the superconducting coil connector 18 are always galvanically and thermally connected, regardless of whether current is flowing. The cross-sectional area of ​​the first section of the current leads is sized such that it is not suitable to thermally withstand a predetermined maximum current supplied to the superconducting coil without cooling. Instead, the cross-sectional area is selected so that the heat dissipated by the maximum magnet current can be safely removed from these components by heat pipes or forced-flow cooling. The lower, thicker section of the lead is sized using established methods to provide minimum heat input to the lower end at maximum operating current, assuming that the upper end is at or slightly above room temperature under operating conditions.

[0024] As can be seen in detail in FIG. 2 , the cross-sectional area of ​​the first sections 6 is smaller than that of the second sections 7, and the current leads 5 are attached to superconducting coil connectors 18 at each second section 7. In this way, heat transfer to the first stage 21 of the refrigerator at zero current in the current leads 5 is reduced due to the smaller cross-sectional area of ​​the first sections 6. To prevent damage to the current leads 5 in the smaller cross-sectional area first sections 6 when maximum magnet current flows through the current leads 5, each first section 6 of the current leads 5 includes a cooling arrangement 8 that cools the first section 6 of the current lead 5. As previously explained, this cooling arrangement is active only when there is current flowing through the current leads 5 and is designed not to generate additional heat leakage when the current leads 5 are not carrying current. As shown in FIGS. 2 and 3 , the current leads 5 are threaded through the vacuum chamber wall at the first sections 6. The current leads 5 in the first and second sections are made of the same material, namely, copper.

[0025] According to a preferred embodiment of the present invention, shown in FIG. 2, each of the cooling arrangements 8 for cooling the first sections 6 of the current leads 5 includes a cooling channel 10 for conducting a coolant within the corresponding first section 6 of the current lead 5. These cooling channels 10 together form a common cooling channel 9 for both current leads 5. In this respect, the common cooling channel 9 is composed of the corresponding cooling channel 10 of the first section 6 of the current lead 5 and a galvanically isolating coupler 11 interconnecting the cooling channels of the first sections 6 of the respective current leads 5. In this way, the two current leads 5 are still galvanically isolated from each other but are thermally connected, i.e., thermally connected for cooling purposes. Alternatively, each current lead 5 may have its own supply and return channels for the coolant flow. In this case, an isolating coupler is not required.

[0026] According to a preferred embodiment of the invention shown in Figure 3, each of the cooling arrangements 8 cooling the first section 6 of the current leads 5 forms a heat pipe 12, with which the current leads 5 are threaded through the vacuum chamber wall 3. These heat pipes 12 thus form the first section 6 of the current leads 5, both extending into the inside of the vacuum chamber 2 and both connected to a corresponding heat sink 13 outside the vacuum chamber 2, which is a capacitor at the room temperature end of the current leads 5. In this case, the operation of the system is fully passive and automatic. In the zero current state, the medium of the heat pipes 12 (water, methanol, or other suitable liquid) freezes at the bottom end, and the remaining heat transfer is by conduction through the lead material.

[0027] In accordance with the preferred embodiment of the present invention, an "off-the-shelf" copper heat pipe is used, with an outer diameter of 10 mm and a length of 400 mm. It has a sintered copper core and the heat transport medium is water vapor. Copper blocks are soldered to both ends: a small block at the bottom for electrical connection of the uncooled first section 6 of the current lead 5, and a larger block at the top for other electrical connections and attachment to the heat sinks 12, 13.

[0028] As explained previously, it is important that each current lead 5 includes at least two different sections 6, 7 with different cross-sectional areas. In this way, the section with the smaller cross-sectional area reduces heat transfer through the current lead. In the absence of cooling, when a current is passed through the coil, this thinner first section 6 of the current lead quickly heats up to a temperature above the melting point of its material. To prevent such thermal runaway, the thin section 6 of the current lead 5 is provided with a cooling arrangement 8. In this regard, the cross-sectional area of ​​the second section 7 of the current lead 5 is dimensioned to thermally withstand a predetermined maximum current supplied to the superconducting coil 1 without cooling, while the cross-sectional area of ​​the first section 6 of the current lead 5 is dimensioned such that it is not suitable to thermally withstand a predetermined maximum current supplied to the superconducting coil 1 without cooling.

[0029] As previously mentioned, without cooling, a suitable shape for a current lead for a cryogen-free magnet is a rod strip of conductive material, such as copper, with a constant cross-sectional area along its entire length. According to a preferred embodiment of the invention described herein, the cross-sectional area decreases to about 15-20% of this suitable value over about 40-60% of the length of the lead. Without cooling, this thin first section 6 of the current lead 5 would heat up quickly to a temperature above the melting point of copper, which is used herein as the material for the current lead 5. To prevent such thermal runaway, the thinner first section 5 of the current lead is provided with a cooling arrangement 8. In the example of current leads with a maximum current rating of 450 A, each optimally cooled lead would be approximately 12 mm long. 2 A 300 mm long cooled copper part with a cross-sectional area of ​​55 mm 2 Each lead carries approximately 3.5 W to the cold end. An equivalent uncooled current lead of the same length would be 75 mm long. 2 , each providing a heat load at the cold end of about 12 W.

[0030] While the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive. The present invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the word "a" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims are not to be construed as limiting the scope. Moreover, for the sake of clarity, not all elements in the drawings have been provided with reference signs. [Explanation of symbols]

[0031] Superconducting coil 1 Vacuum Chamber 2 Vacuum chamber wall 3 Cryocooler 4 Current Lead 5 Section 1 6 Second Section 7 Cooling Arrangement 8 Common Cooling Channel 9 Cooling channel in the first section of the current lead 10 Galvanic isolation coupler 11 Heat pipes 12 Heat sink 13 MRI system 14 Electrical Connectors 15 Cooling inlet 16 Cooling outlet 17 Superconducting Coil Connector 18 First stage of freezer 21 Second stage of freezer 22 Radiation Shielding 23 Heat Link 24 Heat Link 25 Heat Link 26

Claims

1. a superconducting coil that generates a magnetic field; a vacuum chamber having a vacuum chamber wall bounding an exterior; a cryocooler for cooling the superconducting coil, the cryocooler including a refrigerator having a first stage cooled to a first temperature and a second stage cooled to a second temperature, the second temperature being lower than the first temperature; Including, the superconducting coil is disposed in the vacuum chamber at a position spaced apart from a wall of the vacuum chamber; the superconducting coil is provided with two superconducting coil connectors, the two superconducting coil connectors are galvanically connected to two current leads, the two current leads are passed through a vacuum chamber wall to provide current to the superconducting coil from outside the vacuum chamber; each current lead includes a first section and a second section, the first section having a smaller cross-sectional area than the second section; the current leads are galvanically attached to the superconducting coil connectors and thermally connected to the first stage of the refrigerator at corresponding second sections; each of the first sections of the current leads includes a cooling arrangement for cooling the first section of the current lead by heat transfer to the outside, which is at ambient temperature; Cryogen-free superconducting magnet system.

2. 2. The cryogen-free superconducting magnet system of claim 1, wherein said current leads are threaded through said vacuum chamber wall in said first section.

3. 3. The cryogen-free superconducting magnet system of claim 1, wherein the cross-sectional area of ​​the first section of the current lead is less than 25% of the cross-sectional area of ​​the second section.

4. 4. The cryogen-free superconducting magnet system according to claim 1, wherein a cross-sectional area of ​​the second section of the current lead is dimensioned to thermally withstand a predetermined maximum current supplied to the superconducting coil without cooling, while a cross-sectional area of ​​the first section of the current lead is dimensioned to be unsuitable for thermally withstanding the predetermined maximum current supplied to the superconducting coil without cooling.

5. 5. The cryogen-free superconducting magnet system of claim 4, wherein the predetermined maximum current is 450A.

6. 6. The cryogen-free superconducting magnet system according to claim 1, wherein the current lead is constructed of the same material in the first section and the second section.

7. 7. The cryogen-free superconducting magnet system according to claim 1, wherein a length of the first section of the current lead is greater than 60% of a length of the second section of the current lead.

8. 8. The cryogen-free superconducting magnet system according to claim 1, wherein each of the cooling arrangements for cooling the first sections of the current leads comprises a cooling channel for conducting a coolant within the first section of the corresponding current lead.

9. 9. The cryogen-free superconducting magnet system of claim 8, wherein both of said two current leads include a common cooling channel.

10. 10. The cryogen-free superconducting magnet system of claim 9, wherein the common cooling channel is comprised of a cooling channel in each of the first sections of the current leads and a galvanically isolating coupler interconnecting the cooling channels in the first sections of each of the current leads.

11. 8. The cryogen-free superconducting magnet system according to claim 1, wherein each of the cooling arrangements for cooling the first sections of the current leads includes a heat pipe, the heat pipe together with the current leads being passed through the vacuum chamber wall, the heat pipe extending into the interior of the vacuum chamber and connected to a heat sink outside the vacuum chamber.

12. 12. The cryogen-free superconducting magnet system of claim 11, wherein the heat pipe is formed by the first section of the current lead.

13. 13. An MRI system comprising a cryogen-free superconducting magnet system according to any one of claims 1 to 12.

14. Use of a cryogen-free superconducting magnet system according to any one of claims 1 to 12 for an MRI system.

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