Magnetic shielding

Abstract

A cryostat comprises a cryogenically cooled interior volume for accommodating an object to be tested or a device operating at a cryogenic temperature, means for reducing the temperature of the interior volume to a cryogenic temperature, at least one coil winding located within the interior volume for generating a magnetic field in the interior volume to counteract the effects of an ambient magnetic field, and a passive magnetic shield located within the interior volume but outside the at least one coil winding.

Description

Technical Field

[0001] The present invention relates to magnetic shielding of confined spaces. In particular, the present invention relates to magnetic shielding of cryogenic cooling environments for cryostats. Background Art

[0002] A cryostat is a device used to test samples or operate equipment while the sample or equipment is contained within a volume maintained at low temperature within the cryostat. Ambient magnetic fields can adversely affect the accuracy of tests. Ambient magnetic fields come from many sources. Ambient magnetic fields include the combined fields of the earth's magnetic field, buildings, utility infrastructure, and instrumentation. Physical objects and components are also potential sources of ambient magnetic fields. The earth's magnetic field has a magnetic field strength of 22×10 -6 T and 67×10 -6 T; the term "ambient magnetic field" used in this article refers to a magnetic field with a strength less than 100 or 200×10 -6 In environments where the ambient magnetic field needs to be eliminated, such as those where sensitive measurements and test procedures are performed, some form of device is used to isolate an area or volume from the external magnetic field. This magnetic shielding can be passive, utilizing a high permeability metal alloy to pull the magnetic field into the alloy, redirecting the magnetic field lines around the shielded volume and making the enclosed space inside the shield relatively free of magnetic flux. This approach is known as flux shunting. Typically, passive shielding requires the shielded volume to be completely enclosed to avoid the ingress of stray fields, and is limited by the magnetic permeability of the material and its ability to effectively shield the local field strength.

[0003] Active shielding can be used as an alternative or supplement to passive shielding in a cryostat. Active shielding cancels the ambient magnetic field within the shielded volume by creating an electromagnetic cancellation field that is substantially opposite to the ambient magnetic field, thereby canceling it. Active magnetic shielding typically requires exciting an electric current through a coil of conductive material (such as copper). Lenz's law in electromagnetism states that a magnetic field proportional to the applied current is induced at 90° to the plane of the coil. The flux density of the induced magnetic field can be controlled by the current applied in the coil to cancel the equal and opposite magnetic field. Helmholtz coils or solenoids are typically used. Alternatively, complex wire patterns and arrangements can be used to fine-tune the cancellation requirements within the space.

[0004] Advances in quantum technology and particle physics research have led to the need for magnetic shielding within cryostats in both laboratory and industrial settings. Passive shielding is typically constructed using high magnetic permeability materials optimized for cryogenic temperatures to create a shielded volume. However, holes and other large openings are often required in the shielding to allow cables to pass through and to allow access to the shielded space. Such gaps in the passive shielding can reduce shielding performance, resulting in unwanted magnetic fields within the shielded environment. In some cases, cryostats require high levels of shielding, and active shielding is used in addition to passive shielding to further reduce residual fields within the shielded enclosure. In a cryostat, this typically takes the form of electromagnetically conductive coils made of copper or similar materials, arranged in a saddle or Helmholtz formation around the outside of the shielded volume, outside the cryostat. It is necessary to place the coils outside the cryostat because the heat generated by the coils counteracts the cryogenic environment inside. However, placing the active shield outside the passive shielding means that its performance is not optimal, as its inductive field will be distorted by the passive shielding material. In addition, high currents are required, as the induced electromagnetic field must both be counteracted by the ambient magnetic field and penetrate the passive shielding material, which increases power costs and raises high current safety issues. Because the coils are farther away from the small area of ​​interest, it is also more difficult to precisely control the magnetic field gradient in the area of ​​interest. Summary of the invention

[0005] There is a need for an apparatus that overcomes the problems associated with prior art cryostat active shielding apparatus.

[0006] Therefore, the present invention provides

[0007] A cryostat comprising: a cryogenically cooled internal volume for containing an object to be tested or a device operating at cryogenic temperatures, a cooling device for reducing the temperature of the internal volume to a cryogenic temperature, at least one coil winding located within the cryogenically cooled internal volume for generating a magnetic field in the internal volume to counteract the effects of an ambient magnetic field, and a passive magnetic shield located within the cryogenically cooled internal volume but external to the at least one coil winding.

[0008] Preferably, at least one coil winding provides active magnetic shielding to eliminate cryogenic cooling within the internal volume less than 100×10 -6 T, more preferably about 22×10 -6 T and 67×10 -6 The influence of the ambient magnetic field between T.

[0009] The passive magnetic shield is ideally located within the interior volume but outside of at least one of the coil windings. This provides an additional degree of magnetic shielding, reducing the background field to be counteracted by the at least one coil winding, thereby reducing current requirements.

[0010] A current source may be provided, located externally to the internal volume, for providing current to the at least one coil winding and adapted to control the current so as to provide active magnetic shielding of the internal volume. Such control systems are known per se from other active shielding field work and may be applied here. Alternatively, the at least one coil may be controlled so as to establish a desired non-zero magnetic field within the internal volume, as the context requires.

[0011] At least one coil winding is preferably mounted on a nylon former; this will allow the coil winding to maintain its desired course or pattern. The former is preferably PA12 nylon, more preferably PA12 nylon with 30% to 50% glass reinforcement by volume. This provides a non-magnetic, non-conductive former with a coefficient of thermal expansion close enough to that of the aluminum bracket, bracket or other form of support to allow cooling to cryogenic temperatures without structural damage due to differences in thermal expansion.

[0012] We prefer that the wire of at least one of the at least one coil windings is supported on a nylon former in a pattern having an arcuate portion, the wire being held in place by an upstanding arcuate ridge located adjacent the arcuate portion, on the inside of the arc. The upstanding arcuate ridge desirably has an elongated channel on its convex side for receiving the wire.

[0013] There are preferably at least three coil windings, oriented in mutually orthogonal directions. They may be mounted on one of three nylon formers, each of which includes a cylindrical portion and is sized so that a second of the three formers is mounted concentrically within a first of the three formers, and a third of the three formers is mounted concentrically within the second former. This provides a compact set of coil windings in which a region having a tightly controlled magnetic field may be defined, which may be actively shielded to near zero.

[0014] A suitable superconducting wire is one of the niobium-titanium alloy family.

[0015] The invention also relates to the above-mentioned coil arrangement itself, for example as a removable or retrofittable component of a cryostat, which can be added when required.

[0016] The above system is significantly less affected by distortion caused by passive shielding materials than active shielding coils external to the cryostat. Even if the active shielding coils are located within the passive shielding, some minor distortion may occur, but the distortion will be much less because the magnetic field does not have to penetrate the passive shielding. In addition, because the area where the active shielding coils operate is shielded, the coils only have to deal with much smaller magnetic fields and can therefore be more precise. The induced magnetic field can be finely tuned to cancel magnetic field gradients within the shielding volume, or to establish a desired background field. In addition, fine field control in the x, y, and z axes can be achieved.

[0017] Compared to active coils arranged externally to the cryostat, the coils of the present invention are smaller and therefore cheaper and faster to manufacture. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0019] Figure 1 An example of the invention is shown in schematic cross-section;

[0020] Figure 2-Figure 4 Three coil formers used in the examples of the present invention are shown, Figure 2 shows a × coil, Figure 3 The y coil is shown, Figure 4 The z-coil is shown;

[0021] Figure 5 shows an enlarged portion of the x and y coils showing the wire supports; and

[0022] Figure 6 A corresponding enlarged portion of the z-coil is shown. DETAILED DESCRIPTION

[0023] Active magnetic shielding is a well-established principle whereby an electric current is excited through a coil of conductive material (e.g. copper) to produce a magnetic field proportional to the applied current at a 90° angle to the plane of the coil according to Lenz's law. By controlling the current applied through the coil, the flux density of the induced magnetic field can be controlled and effectively cancels out an equal and opposite magnetic field when applied to it. Active shielding differs from passive shielding, which utilizes high permeability materials to redirect the magnetic field lines passing through the shield material, making the enclosed space inside the shield relatively free of magnetic flux. This approach is called flux shunting.

[0024] Due to recent advances in quantum technology and particle physics research, cryogenically cooled environments requiring magnetic shielding are becoming more common in both laboratory and industrial settings. (Cryophy is a registered trademark of Aperam AlloysImphy, France) to provide passive shielding optimized for low temperatures (<4 degrees Kelvin). However, holes and other large openings are usually required in the shield design to allow cables to pass through and make the space inside the shield accessible. Due to the presence of these holes, the shielding performance is usually reduced due to flux leakage, and there may be unwanted magnetic field gradients in the shielded environment. Active shielding can then be utilized by assembling electromagnetic conductive coils made of materials such as copper around the outside of the shield shell to further reduce the residual field inside the shield shell and compensate for the negative impact of the arrangement of holes and openings in the shield design.

[0025] In the current state-of-the-art, active shielding coils must be assembled outside the cryostat in a Helmholtz configuration or saddle configuration (CERN, https: / / indico.cern.ch / event / 727555 / contributions / 3452820 / attachments / 1868399 / 3073390 / Active_magnetic_shielding_for_cryomodules.pdf). This is because if the coils were placed inside the cryostat, the resistance in the coils would radiate thermal energy during current excitation and raise the temperature of the cryostat, destroying any superconducting behavior. Placing the coils outside the cryostat creates some limitations:

[0026] The coils must be large, so they are expensive and time-consuming to manufacture

[0027] High currents are required because the induced electromagnetic field must not only be offset by the earth's magnetic field, but must also penetrate passive shielding materials. This increases power costs and raises high current safety issues

[0028] Active shielding performance is not ideal due to the distortion of the induction field caused by the passive shielding material

[0029] Difficult to achieve active shielding in all 3 axes (x, y, and z)

[0030] This approach does not allow for fine field control or fine tuning of the magnetic field within the shielded enclosure.

[0031] We have developed an active magnetic shield that can be placed inside the cryostat thanks to the use of superconducting conductive coil material (in this case, niobium-titanium wire, rather than the copper used previously). The use of niobium-titanium (Nb-Ti) wire means that as the cryostat cools, the active shield coils also begin to superconduct. The resistance of the wire drops to zero and the heat dissipated by the coils becomes negligible, allowing the cryostat to continue operating. The above limitations are thus addressed; the advantages of this system include:

[0032] Because the coil is in a shielded environment, lower currents are required, and therefore less equal and opposite magnetic fields need to be provided. Since the required excitation current is proportional to the induced field, power requirements and safety issues are significantly reduced. In many applications, current requirements can be reduced by up to three orders of magnitude, from several amps to a few milliamps.

[0033] Due to the reduced cross-sectional diameter, the coils are smaller, which reflects the lower current capacity required. Therefore, they are cheaper and less time-consuming to manufacture.

[0034] Active shielding performance is optimized because the induction field does not have to penetrate the passive shielding material.

[0035] Coils mounted around the inner perimeter of a cylindrical passive shield allow for effective shielding in all 3 axes (x, y and z).

[0036] This arrangement allows fine field and field gradient control within a shielded enclosure. Gradient coils can be added to the active assembly, e.g. in more layers.

[0037] The right material had to be chosen to house the superconducting wires while ensuring that the quality of the induced magnetic field and the structural integrity of the system would not be compromised during the cooling process. To maintain the magnetic field quality, nylon was chosen as a non-magnetic and non-conductive material. During the cooling phase in the cryogenic chamber, the material is subjected to extreme thermal changes, resulting in a certain amount of shrinkage. The nylon coil support and the aluminum shielding that holds it naturally have different thermal shrinkage rates; to prevent this from threatening the structural integrity of the shielding and coil support, PA12 nylon with 40% glass filler was used. This material (overall) shrinks at a similar rate to aluminum and reduces the risk to its structural integrity.

[0038] In addition, the coil in the coil support adopts the tension coil winding method, which will be further described below.

[0039] The above active shielding method was tested in the cryogenic laboratory in February 2023. The results showed that when the active shielding method was used, the magnetic noise floor inside the shielded enclosure was reduced by a factor of 2. This result greatly improves the response of the superconducting circuit (device under test) to the magnetic field environment and improves the overall quality of the device for its intended use.

[0040] An application of this device is for shielding superconducting circuits destined for use in quantum computers. However, there are many other applications where this device can improve the quality of the intended use, such as particle physics research, atomic clocks, superconducting magnetometers, superconducting circuits, etc.

[0041] Figures 1 to 6 An example of a cryostat and coil winding according to the invention is shown. Figure 1 , a cryostat is schematically shown, in this case a double walled vessel 10 having an inner wall 12, an outer wall 14 and a vacuum space 16 therebetween, thereby providing a high degree of thermal insulation. A cooling device is provided, in this case a source 18 of liquid helium 20, the helium 20 being located within the vessel 10. The specific form of the cryostat is not particularly important, and a range of various forms of cryostat having different cooling devices are known. Any suitable cryostat and cooling device may be provided. This then defines an internal volume 22 which is maintained at a low temperature.

[0042] In order to provide magnetic shielding for the inner volume 22, a passive shield 24 is provided within the inner volume. This is made of a suitable high magnetic permeability material (e.g. ) (although other materials are known). In our preferred form it also has an inner layer of aluminium to provide superconducting eddy current shielding for AC field sources (such as those caused by a 50Hz mains supply). A passive shield is located within the cryostat and is able to redirect the magnetic field lines through the shielding material and to render the enclosed space within the shield relatively free of magnetic flux. An opening 26 is provided through which equipment to be operated or tested at cryogenic temperatures can be inserted, meaning that the magnetic field therein will not be completely zero as some magnetic flux will enter through the opening. However, the opening is necessary to provide access to the cryostat during use.

[0043] Within the passive shield 24, there are three concentric (or nested) cylindrical coil formers 28, 30, 32, each of which carries a coil winding for providing active magnetic shielding for the interior volume 22. The outermost coil former 28 carries a coil oriented in the x direction. The intermediate coil former 30 is located within the outermost coil former 28 and carries a coil oriented in the y direction. The innermost coil former 32 is located within the intermediate coil former 30 and carries a coil oriented in the z direction. Each coil former carries a corresponding coil on its outer cylindrical surface, which is kept taut (or under slight tension) by winding around ribs formed on the outer cylindrical surface of the coil former, as described later. Each coil former has a lower end face 34 on which they are placed, and the lower end face has a through hole 36 to allow a wire (etc.) to pass through, thereby connecting the coil winding to an external current source (not shown). The current in each coil winding may then be controlled (in a generally known manner) to establish a desired background magnetic field within the interior space 22, or to cancel a magnetic field, thereby reducing the background field to a low field or substantially zero field.

[0044] The coil formers 28, 30, 32 are supported within the cryostat 10 by an aluminum bracket 38 which extends around the coil formers 28, 30, 32 and engages them at the holes 36 at their lower ends and upper edges. The coil formers 28, 30, 32 are made of 3D printed PA12 nylon with a 40% glass filler as described above. The net thermal expansion coefficient of this composite material substantially matches that of the aluminum bracket 38, thereby allowing both to cool to cryogenic temperatures together and maintain their structural integrity. We anticipate that a glass filler content within 10% of the above amount will be sufficient, i.e., between 30% and 50% by volume.

[0045] Figures 2 to 4 Three coil formers and coil windings are shown, Figure 2 The × coil former 28 is shown, Figure 3 The y coil former 30 is shown, Figure 4A z coil frame 32 is shown. Each coil frame (relative to Figure 1 The inverted display) has a generally cup-shaped form, including a hollow cylinder 38x, 38y, 38z, one end of which is partially closed by a circular cover 40x, 40y, 40z. The cover 40 includes a hole 36 and other mounting (etc.) holes, such as a hole 42, to allow the wires connected to the coil to pass through, connect to the aluminum mounting member 38, etc.

[0046] First reference Figure 4 The z-coil former 32 shown in FIG. 1 has a cylindrical body 38z with a series of ridges 44z on the exterior thereof defining a groove 52z therebetween, the groove 52z being sized to receive a superconducting Nb-Ti wire 48z. Figure 6 This is shown in more detail in . The ridges 44z, and thus the wires 48z, extend around the cylindrical body 38z in a circular form centered on and transverse to the axis of the cylindrical body 38z. Thus, the wires can be held snugly in place, taut or under slight tension, and will produce a magnetic field oriented in the z-direction, parallel to the axis of the cylindrical body 38z, with the direction being determined by the general direction of the current in the coil.

[0047] The ridges 44z include gaps (eg 46z) to allow wires to be routed over the surface of the cylindrical body 38z, past each ridge. Alternatively, a single groove or similar recess may be provided, rather than a pair of ridges on either side of the wire.

[0048] Figure 2 and 3 The x-coil former 28 and the y-coil former 30 are shown respectively. They need to generate a magnetic field transverse to the axis of the cylinder 38x, 38y and therefore carry coils of wire which are usually in the form of rings parallel to the axis but projected onto the cylinder sidewalls. They therefore follow arcuate paths on the outer cylindrical surface, some elliptical and others more complex. In order to hold the wire in position along these paths, ridges 44x, 44y are provided on the cylindrical outer surface of the body 38x, 38y, as shown in FIG. Figure 5 They are located on the inside (concave) side of each wire path; thus, for a simple elliptical path it is on the inside of the elliptical path, but for more complex paths it may be on one side or the other of the wire, depending on the direction of the local curvature, e.g. path 50 ( Figure 3 ). Thus, the wires 48x, 48y can be placed on the cylindrical outer surface and pulled taut or slightly tensioned, which will allow them to remain in place against the ridges 44x, 44y.

[0049] To help hold the wires 48x, 48y in place and prevent them from falling off the ridges 44x, 44y, grooves 52x (eg, Figure 5 As shown in FIG. 5 , the groove 52x forms an elongated channel along its outer (convex) side surface into which the wires 48x, 48y can be tightened. Similarly, gaps, such as 46x, 46y, are provided in the ridges 44x, 44y to allow the wires to be routed on the surface of the cylinders 38x, 38y, passing through each ridge.

[0050] The specific coil structure can be designed using conventional design principles, as used in other active shielding environments. Current is excited through the active shielding coil using known electronic components and software, (again) as used in other active shielding environments. The current through the coil can be driven in a variety of ways, and therefore, it, the related software, and the specific shape of the coil are not particularly relevant to the present invention.

[0051] If required, one or more additional coil formers may be provided to further adjust the magnetic field and / or establish gradient fields or the like. Such additional coils and coil formers may be designed using any of the principles described above, depending on the desired field direction.

[0052] The device is not necessarily limited to niobium-titanium wire materials. Any material that is superconducting at atmospheric pressure can be used as a wire as long as it can be processed into a wire shape.

[0053] Of course, it will be appreciated that many variations may be made to the embodiments described above without departing from the scope of the present invention.

Claims

1. A cryostat comprising: a cryogenically cooled internal volume for containing an object to be tested or a device operating at cryogenic temperatures, a cooling device for reducing the temperature of the internal volume to a cryogenic temperature, at least one coil winding located within the cryogenically cooled internal volume for generating a magnetic field in the internal volume to counteract the effects of an ambient magnetic field, and a passive magnetic shield located within the cryogenically cooled internal volume and external to at least one coil winding.

2. The cryostat of claim 2, wherein at least one coil winding provides active magnetic shielding to eliminate magnetic flux below 100×10 -6 T, preferably about 22×10 -6 T and 67×10 -6 The influence of the ambient magnetic field between T.

3. A cryostat according to claim 1 or claim 2, wherein the wire of at least one of the at least one coil windings is supported on a nylon former in a pattern having an arcuate portion, the wire being held in place by upstanding arcuate ridges located adjacent the arcuate portion, located on the inside of the arc.

4. A cryostat according to claim 3, wherein the upstanding arcuate ridge has an elongated channel on its convex side for receiving a wire.

5. A cryostat according to any one of claims 1 to 4, wherein there are at least three coil windings which are oriented in mutually orthogonal directions.

6. A cryostat as claimed in claim 5, wherein each of the at least three coil windings is mounted on one of three nylon coil formers, each of the three coil formers including a cylindrical portion and being sized so that a second coil former of the three coil formers is mounted concentrically within a first coil former of the three coil formers and a third coil former of the three coil formers is mounted concentrically within the second coil former.

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