Shielding for magnetically shielded rooms

Abstract

A magnetically shielded room (MSR) and a shielding structure for shielding an MSR are disclosed. According to one aspect, a shielding structure includes a first layer of magnetizable material, configured to be parallel to a wall of the MSR. The shielding structure also includes a second layer of magnetizable material parallel to and separated by a first gap from the first layer of magnetizable material. The gap may be optimized to obtain a shielding performance gain with less overall thickness of the shielding structure than known shielding structures.

Description

[0001] SHIELDING FOR MAGNETICALLY SHIELDED ROOMS

[0002] TECHNICAL FIELD

[0003] The present invention relates to a method and system for magnetically shielding a room.

[0004] BACKGROUND

[0005] Magnetically shielded rooms (MSRs) are used to provide low magnetic field environments for precise measurements of magnetic fields and / or magnetic signals. For example, and MSR may be used when determining, the magnetic field of the heart of fetus inside a pregnant woman (fetal Magneto-Cardiography, fMCG), or magnetic signals of the brain (Magneto-Encephalography MEG) or for other fundamental measurements. MSRs are typically a few meters along each side and can be entered through a door, which is closed to magnetically seal the inside, while a measurement is performed.

[0006] Superconducting Quantum Interference Devices (SQUIDs) have been used to determine magnetic fields at the level of Femto-Tesla to Nano-Tesla. Recently, optically Pumped Magnetometers (OPMs) have become an economic solution to perform such measurements. OPMs do not use liquid helium and do not required the same high-maintenance required of SQUIDs, OPMs are operated at room-temperature and are basically plug-and-play solutions. Recently, even commercial sensors are available from vendors such as Quspin Inc. (http: / / quspin.com), Fieldline Inc. (https: / / fieldlineinc.com) or TwinLeaf LLC (https: / / twinleaf.com).

[0007] Due to this paradigm change, the need for MSRs is increasing. MSRs, however, are very expensive: on the order of a half million United States dollars (USD) or more, depending on the actual performance requirements.

[0008] For example, there is an increasing market for systems capable of magnetoencephalography (MEG) measurements. However, current costs for such systems are on the order of 0.5-1 million USD, which provides an entry barrier to get started in this field.

[0009] MSRs should be designed to shield magnetic fields in a range of frequencies and in addition provide a low static magnetic residual field to enable proper operation of 0PM sensors. The requirement for a static field is typically less than 5 nT (Nanotesla) over an extended volume in space with gradients of a few nT / m. The requirement for damping of time-dependent external excitations (the Shielding Factor SF) is on the order of 100 at 0.01 Hz, increasing to about 10.000 at 100-1000 Hz. This requirement is typical but also depends on the external conditions.

[0010] Shielding may he achieved using two physical processes. In the case of slow and static magnetic fields, shield may be achieved by guiding magnetic field lines through magnetizable material, so-called mu-metal. For the case of alternating current (AC) fields in the range from 1 - 1000 Hz, the induction of currents in conducting materials increases with increasing frequency. Therefore, MSRs are constructed in a nested structure of mu-metal and aluminum, often stacked as nested shells. For example, a first shell may be a mu-metal sheet of 1-3 millimeters (mm) in thickness, an second shell encompassed by the first shell may be a good conductor (such as copper or aluminum) of 4-10 mm in thickness, and third shell encompassed by the first and second shells may be a mu-metal of about 25 cm in thickness. The three shells are adjacent and in contact.

[0011] To actually achieve low residual fields, MSRs need should also be degaussed.

[0012] Because of the high costs of mu-metals, expensive processing of the conductive layer components and elaborate engineering, there is strong interest in the cost-optimization of MSRs. In addition, the optimization of the performance of MSRs for use with 0PM sensors is a current field of research.

[0013] About half the cost of an MSR is for material and about half the cost if for labor. An additional cost driver is risk contingency, in case the performance specifications are not reached. There are performance issues at the frequency range from 10-1000 in MSRs by almost all manufacturers. Low-frequency shielding performance issues arise typically from annealing issues and can be controlled with proper design and quality control. However, for the frequency range of 10-1000 Hz, performance degrades. FIG. 1 is a graph of shielding factors versus frequency for shielding provided by a plurality of manufacturers. As can be seen, there is a large variation between the performances of the different shields. The performance depends on the quality of the good conductor used in the intermediate shell between the inner and outer shells of mu-material. The performance also depends on the quality of electrical connections between parts of the layer of good conductor. The performance may also depend on other general design factors, such as hole sizes and wave guide configurations to block penetration of the shield by AC signals.

[0014] SUMMARY

[0015] Shielding structures for magnetically shielding a room are disclosed. According to one aspect, a shielding structure for providing magnetic shielding to a magnetically shielded room (MSR) is provided. According to this aspect, in some embodiments, a shielding structure is formed by spacing a first magnetizable material layer apart from a second magnetizable material layer by a gap. In some embodiments, the gap is selected to tune the shielding structure to maximize a shielding performance of the shielding structure in a frequency range between 10 and 1000 Hz. In some embodiments, the first and second magnetizable material layers may be configured to be parallel to a wall of a magnetically shielded room. In some embodiments, the gap may include a metal layer positioned between and spaced apart from the first and second magnetizable material layers. In some embodiments, the metal layer may be configured to have a thickness that does not exceed 1 millimeter (mm). In some embodiments, the first and second magnetizable material layers may each have a thickness that does not exceed 2 mm. In some embodiments, the first and second magnetizable material layers may each have a thickness that does not exceed 1 mm. In some embodiments, the gap between the first and second magnetizable material layers is not less than 1 mm. In some embodiments, the gap is at least partially filled with a non-conductive material. In some embodiments, the gap is partially filled with a conductive material. In some embodiments, the gap is at least 2 mm. In some embodiments, the gap is between 1 and 4 mm. In some embodiments, a third magnetizable layer is separated by a second gap from the second magnetizable material layer. In some embodiments, at least one of the first and second layers of magnetizable material includes multiple sublayers of magnetizable material.

[0016] According to another aspect, a magnetically shielded room (MSR) configured to shield magnetic fields from penetrating into an interior of the MSR is provided. The MSR includes on each of at least one wall of the MSR, a first shielding structure. The first shielding structure includes first and second magnetizable layers parallel to each of the at least one wall of the MSR. The first and second magnetizable layers are separated by a gap. According to this aspect, in some embodiments, the MSR also includes, on the at least one wall, a second shielding structure parallel to the first shielding structure. In some embodiments, the MSR also includes a second gap between the first shielding structure and the second shielding structure. In some embodiments, the MSR also includes a metal layer configured to lie between and parallel to the first and second layers of magnetizable material. In some embodiments, the first gap is at least 2 millimeters. In some embodiments, the first gap is between 1 and 4 millimeters. In some embodiments, each of the first and second layers of magnetizable material have a thickness that does not exceed 2 millimeters. In some embodiments, the first gap is selected to tune the first shielding structure to maximize a shielding performance of the first shielding structure in a frequency range between 10 and 1000 Hz. In some embodiments, the first and second layers of magnetizable material are mechanically compressed toward each other. In some embodiments, the first and second layers of magnetizable material are electrically connected.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

[0019] FIG. 1 illustrates shielding performance for various known MSRs;

[0020] FIG. 2 illustrates an MSR with shielding structures constructed according to principles disclosed herein;

[0021] FIG. 3 is a first example embodiment of a section of a shielding structure constructed according to principles disclosed herein;

[0022] FIG. 4 is a second example embodiment of a section of a shielding structure constructed according to principles disclosed herein;

[0023] FIG. 5 is a third example embodiment of a section of a shielding structure constructed according to principles disclosed herein;

[0024] FIG. 6 is a fourth example embodiment of a section of a shielding structure constructed according to principles disclosed herein; FIG. 7 is a fifth example embodiment of a section of a shielding structure constructed according to principles disclosed herein;

[0025] FIG. 8 is a sixth example embodiment of a section of a shielding structure constructed according to principles disclosed herein; and

[0026] FIG. 9 is a graph of shielding performance of two different shielding structures constructed according to principles disclosed herein.

[0027] DETAILED DESCRIPTION

[0028] Shielding structures and MSRs are disclosed. As used herein, the term “multilayer shielding structure” refers to a shielding structure having a least two layers of magnetizable material separated by a gap, with or without a metal center layer between any two of the at least two layers. A multi-layer shielding structure may include a stack of shielding structures, each shielding structure in the stack having two or more magnetizable material layers, any two of which are separated by a gap. In some embodiments, each of at least one layer of the at least two layer of magnetizable material includes multiple layers of magnetizable material that are separated at most by a thin film or adhesive layer. As used herein, the term “magnetizable material” in includes a mu-metal. As used herein, the term “wall” may include not only walls, but ceilings and floors as well.

[0029] FIG. 2 is an example of a magnetically shielded room (MSR) 10 having walls 11 that are shielded by shielding structures 12 constructed according to principles disclosed herein. In some embodiments, each shielding structure 12 may be constructed of a plurality of shielding structure sections, for example, shielding structure sections 12a and 12b, placed side by side. There may be more than two shielding structure sections on each wall 11 of the MSR 10. Each shielding structure section may be electrically and / or magnetically connected to at least one other shielding structure section. In some embodiments, the shielding structures 12 may be constructed as nested shells, each shell having a shielding structure 12 and each shell being electrically and / or magnetically connected to the other shells.

[0030] FIG. 3 illustrates an example section of an embodiments of a shielding structure 12 that includes two magnetizable material layers 14 and 16 that may have the same or different thicknesses tl and t2 and are separated by a gap gl. The size of the gap gl may be selected to optimize performance (as measured by, for example, the shielding factor) of the shielding structure 12. The space in the gap gl may be air or a non-conductive material, such as a permeable filler or dielectric filler. The magnetizable material layers 14 and 16 may be or include mu-metals.

[0031] In some embodiments, layer thicknesses tl and t2 of first and second magnetizable material layers 14 and 16 of a shielding structure 12 is approximately 1 mm and the gap gl between the first and second magnetizable material layers 14 and 16 is between a fraction of a millimeter and 4 mm. In some embodiments, the gap may be at least partially filled with air and / or partially filled with aluminum or copper.

[0032] In some embodiments, the gap gl between the first and second magnetizable material layers 14 and 16 may be approximately 3 mm in size. In some embodiments, a 1 mm metal layer 18 is placed inside the gap gl and there is a gap between the metal layer 18 and each of the first and second magnetizable material layers 14 and 16. The metal layer 18 may be a good conductor such as aluminum or copper.

[0033] FIG. 4 is an example section of another embodiment of a shielding structure 12 constructed according to principles disclosed herein. The shielding structure 12 of FIG. 4 is similar to the shielding structure 12 of FIG. 3 with an additional inner conductive or metal layer 18 having a thickness t3 which may be the same or different from the thicknesses tl and t2. In the embodiment of FIG. 4, a gap exists on either side of the center metal layer 18. These gaps may be the same size or different from each other and may be filled with air or the same or different non-conductive materials.

[0034] FIG. 5 is an example section of yet another embodiment of a shielding structure 12 constructed according to principles disclosed herein. The embodiment of FIG. 5 is similar to the embodiment of FIG. 4 except that in FIG. 5 is that magnetizable material layers 14 and 16 include a plurality of sublayers, such as sublayers 14a and 14b of magnetizable material layer 14 and sublayers 16a and 16 be of magnetizable material layer 16. There may be more than two sublayers for one or both of magnetizable material layers 14 and 16. Sub layering of one, some or all of the magnetizable material layers may be applied to one or more other embodiments shown in the drawing figures. In some embodiments, a magnetizable material layer 14, 16 may include sublayers of magnetizable material layers and sublayers of non- magnetizable material or weakly magnetizable material.

[0035] FIG. 6 is an example section of another embodiments of a shielding structure 12 constructed according to principles disclosed herein. In FIG. 5, the outer layers 14 and 22 and the central layer 16 include magnetizable material and the in-between layers 18 and 20 include conductive material such as copper or aluminum. The size of the gaps g2, g3, g4 and g5 between the layers 14, 16, 18, 20 and 22 may be the same or different and may be selected to optimize performance in the 10-1000 Hz frequency band, for example. Also, the thicknesses of the layers 14, 16, 18, 20 and 22 may be the same or different.

[0036] FIG. 7 is another example embodiment of a shielding structure 12 constructed according to principles disclosed herein which includes two sets of shielding structures 12-1 and 12-2 that are themselves layered or separated by a gap g6. The layers 12-1 and 12-3 may have the same or identical materials and may have the same gap sizes between layers 14, 16 and 18. Thus, in some embodiments, a plurality of multi-layer shielding structures constructed according to principles disclosed herein may each be parallel to a same wall 1 1 of the MSR 10 and laterally spaced apart. In some embodiments, at least two laterally spaced apart shielding structures may be electrically connected.

[0037] FIG. 8 is another example of a section of shielding structure 12 constructed with two outer layers of magnetizable material 14 and 16. In the gap between them are two metal layers 18a and 18b. The metal layers 18a and 18b may be separated by a gap g7. Metal layer 18a may be separated by a gap g8 from the magnetizable material layer 14. The gap g8 may be selected to optimize performance of the shielding structure 12 and may be zero in some embodiments. Metal layer 18b may be separated by a gap g9 from the magnetizable layer 16. The gaps g7, g8 and g9 may be the same or different and may be selected to optimize shielding performance of the shielding structure 12.

[0038] In the near- magnetostatic case, in some embodiments, an aluminum center layer may not significantly alter performance. Using a multi-layer shielding structure 12, as disclosed herein, the magnetic field may be reduced significantly with an overall shielding structure thickness that is smaller than a conventional shielding structure thickness. In some embodiments, small geometries of shielding structure 12 measuring approximately 0.5 meter on a side may achieve a 40% improvement in shielding effectiveness. At MSR geometries, a slight gap gl between the sheets used to form a shielding structure 12 may reduce the residual magnetic field by about 3%. With degaussing, the residual magnetic field may be reduced by a factor of 3 in the interior of the MSR 10.

[0039] Thus, instead of building shielding structure 12 from two sheets of mu-metal directly touching each other, or simply from 1 sheet having the twice the thickness of two, the performance at 10-1000 Hz may be substantially improved by arranging the sheets 14 and 16 of mu-metal with a gap gl, with or without a thin conductive sheet 18 between them. In some embodiments, performance is enhanced regardless of how the thin conductive sheet 18 is positioned between the mu-metal layers 14 and 16. In some embodiments, the thin conductive sheet 18 between the two mu-metal layers 14 and 16 may be positioned closer to one of the two mu-metal layers 14 and 16 than the other of the two mu-metal layers 14 and 16. In some embodiments, multiple thin conductive sheets 18, 20 may be configured between two mu-metal layers 14, 16 and 22, and in some embodiments, the multiple thin conductive sheets may 18, 20 be separated by a gap g4, that is the same or different than the gap between an outermost one of the multiple thin conductive sheets 18 and 16 and a nearest one of the two mumetal layers.

[0040] In some embodiments, the performance of a shielding structure constructed according to principles disclosed herein is relatively insensitive to the manner in which different shielding structure sections are electrically connected. Any two shielding structure sections may be parallel to the same wall 11 of the MSR 10 or may be parallel to different walls 11 of the MSR 10.

[0041] Some embodiments are applicable to MSRs 10 that are optimized for operation with OPMs or other sensors for bio signal detection, where the 10-1000 Hz performance is the relevant frequency range. Some embodiments mitigate risk of variations in expected or predicted performance of the MSR 10 from the actual performance of an MSR 10 constructed according to principles disclosed herein. In some embodiments, improvement of the performance for DC residual fields may exceed 10%. FIG. 9 is a graph of the relative strength of the field that penetrates through a shielding structure configured with and without a copper layer between the first and second magnetizable material layers.

[0042] According to one aspect, a shielding structure 12 for providing magnetic shielding to a magnetically shielded room (MSR 10) is provided. According to this aspect, in some embodiments, a shielding structure 12 is formed by spacing a first magnetizable material layer 14 apart from a second magnetizable material layer 16 by a gap. In some embodiments, the gap is selected to tune the shielding structure 12 to maximize a shielding performance of the shielding structure 12 in a frequency range between 10 and 1000 Hz. In some embodiments, the first and second magnetizable material layers 14 and 16 may be configured to be parallel to a wall of a magnetically shielded room 10. In some embodiments, the gap may include a metal layer 18 positioned between and spaced apart from the first and second magnetizable material layers 14 and 16. In some embodiments, the metal layer 18 may be configured to have a thickness that does not exceed 1 millimeter (mm). In some embodiments, the first and second magnetizable material layers 14 and 16 may each have a thickness that does not exceed 2 mm. In some embodiments, the first and second magnetizable material layers 14 and 16 may each have a thickness that does not exceed 1 mm. In some embodiments, the gap between the first and second magnetizable material layers 14 and 16 is not less than 1 mm. In some embodiments, the gap is at least partially filled with a non-conductive material. In some embodiments, the gap is partially filled with a conductive material. In some embodiments, the gap is at least 2 mm. In some embodiments, the gap is between 1 and 4 mm. In some embodiments, a third magnetizable layer 22 is separated by a second gap from the second magnetizable material layer 16. In some embodiments, at least one of the first and second layers of magnetizable material 14, 16 includes multiple sublayers 14a, 14b, 16a, 16b of magnetizable material.

[0043] According to another aspect, a magnetically shielded room (MSR 10) configured to shield magnetic fields from penetrating into an interior of the MSR 10 is provided. The MSR 10 includes on each of at least one wall 11 of the MSR 10, a first shielding structure 12. The first shielding structure 12 includes first and second magnetizable layers 14, 16 parallel to each of the at least one wall 11 of the MSR 10. The first and second magnetizable layers 14, 16 are separated by a gap. According to this aspect, in some embodiments, the MSR 10 also includes, on the at least one wall 11, a second shielding structure 12-2 parallel to the first shielding structure 12-1. In some embodiments, the MSR 10 also includes a second gap between the first shielding structure 12 and the second shielding structure 12. In some embodiments, the MSR 10 also includes a metal layer 18 configured to lie between and parallel to the first and second layers of magnetizable material 14, 16. In some embodiments, the first gap is at least 2 millimeters. In some embodiments, the first gap is between 1 and 4 millimeters. In some embodiments, each of the first and second layers of magnetizable material 14, 16 have a thickness that does not exceed 2 millimeters. In some embodiments, the first gap is selected to tune the first shielding structure 12 to maximize a shielding performance of the first shielding structure 12 in a frequency range between 10 and 1000 Hz. In some embodiments, the first and second layers of magnetizable material 14, 16 are mechanically compressed toward each other. In some embodiments, the first and second layers of magnetizable material 14, 16 are electrically connected. In some embodiments, a first shielding structure section 12a is electrically connected to a second shielding structure section 12b. In some embodiments, the first shielding structure section 12a is electrically connected to one or more other shielding structure sections which may or may not be adjacent to each other or to the first shielding structure section 12a. Also, in some embodiments, one or more shielding structure sections on one wall 11 of the MSR 10 may be electrically connected to one or more shielding structure sections on another wall 11 of the MSR 10.

[0044] It will be appreciated by persons skilled in the art that the scope of the following claims is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.

Claims

What is claimed is:

1. A shielding structure (12) for providing magnetic shielding to a magnetically shielded room, MSR (10), the shielding structure (12) comprising: a first layer (14) of magnetizable material, configured to be parallel to a wall of the MSR (10); and a second layer (16) of magnetizable material parallel to and separated by a first gap from the first layer (14) of magnetizable material.

2. The shielding structure (12) of Claim 1, further comprising a metal layer configured to lie between and parallel to the first and second layers (14, 16) of magnetizable material.

3. The shielding structure (12) of Claim 2, wherein the metal layer has a thickness that does not exceed 1 millimeter.

4. The shielding structure (12) of any of Claims 1 -3, wherein the first gap is at least 2 millimeters.

5. The shielding structure (12) of any of Claims 1-3, wherein the first gap is between 1 and 4 millimeters.

6. The shielding structure (12) of any of Claims 1-5, wherein each of the first and second layers (14, 16) of magnetizable material have a thickness that does not exceed 2 millimeters.

7. The shielding structure (12) of any of Claims 1-6, wherein the first gap is at least partially filled with a non-conductive material.

8. The shielding structure (12) of any of Claims 1-7, wherein the first gap is at least partially filled with a conductive material (18).

9. The shielding structure (12) of any of Claims 1-8, wherein the first gap is selected to tune the shielding structure (12) to maximize a shielding performance of the shielding structure (12) in a frequency range between 10 and 1000 Hz.

10. The shielding structure (12) of any of Claims 1-9, further comprising a third layer of magnetizable material separated by a second gap from the second layer (16) of magnetizable material.

11. The shielding structure (12) of any of Claims 1-10, wherein each of at least one of the first and second layers (14, 16) of magnetizable material includes multiple sublayers of magnetizable material.

12. A magnetically shielded room, MSR (10), configured to shield magnetic fields from penetrating into an interior of the MSR (10), the MSR (10) comprising: on each of at least one wall of the MSR (10), a first shielding structure (12) that includes: a first layer (14) of magnetizable material, configured to be parallel to the wall of the MSR (10); and a second layer (16) of magnetizable material parallel to and separated by a first gap from the first layer (14) of magnetizable material.

13. The MSR (10) of Claim 12, further comprising, on the at least one wall, a second shielding structure (12) parallel to the first shielding structure (12).

14. The MSR (10) of Claim 13, further comprising a second gap between the first shielding structure (12) and the second shielding structure (12).

15. The MSR (10) of any of Claims 12-14, further comprising a metal layer configured to lie between and parallel to the first and second layers (14, 16) of magnetizable material.

16. The MSR (10) of any of Claims 12-15, wherein the first gap is at least 2 millimeters.

17. The MSR (10) of any of Claims 12-15, wherein the first gap is between1 and 4 millimeters.

18. The MSR (10) of any of Claims 12- 17, wherein each of the first and second layers (14, 16) of magnetizable material have a thickness that does not exceed2 millimeters.

19. The MSR (10) of any of Claims 12-18, wherein the first gap is selected to tune the first shielding structure (12) to maximize a shielding performance of the first shielding structure (12) in a frequency range between 10 and 1000 Hz.

20. The MSR (10) of any of Claims 12-19, wherein the first and second layers (14, 16) of magnetizable material are mechanically compressed toward each other.21 The MSR (10) of any of Claims 12-20, wherein the first and second layers (14, 16) of magnetizable material are electrically connected at multiple points.

Images