JP2016049159A - Superconducting magnet and magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an MRI apparatus using a superconducting coil capable of maintaining a temperature of superconducting state while suppressing the power consumption of a refrigerator by cooling the superconducting coil to a temperature required for a superconducting state with heat removal means using a refrigerator comprising a heat transfer cooling member without using a liquid refrigerant and reducing the heat resistance of the contact face between the heat transfer cooling member and the superconducting coil.SOLUTION: The present invention relates to a superconducting magnet including a superconducting coil which uses a superconducting material for magnetic poles generating a static magnetic field and an MRI apparatus, the superconducting coil being annularly constructed of a good thermal conductive material such as a metal and installed contacting a heat transfer cooling member 11b which has a heat transfer path between a heat removal apparatus such as a refrigerator and the superconducting coil. The heat transfer cooling member 11b has a lattice-like slits 15 in a circumferential direction of the annular ring to allow for superior heat transfer in both circumferential and radial directions. In addition, the superconducting coil and the heat transfer cooling member 11b are integrally formed by resin hardening or adhesion.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a superconducting coil cooling structure that does not use a refrigerant or can reduce the amount of use in a superconducting magnet or magnetic resonance imaging apparatus using a superconducting coil.

  A superconducting coil magnet device is a device that generates a desired magnetic field by passing a current through a coil made of a superconducting material cooled to a cryogenic temperature. Superconducting material is a substance whose electric resistance becomes zero when the temperature falls below a certain temperature, and it can pass a larger current than a normal conductive metal at normal temperature, so it is a device that requires a strong magnetic field, especially magnetic It is used in a resonance imaging apparatus (hereinafter referred to as an MRI apparatus). An MRI device is a device that obtains a cross-sectional image showing the physical and chemical properties of a subject by utilizing the nuclear magnetic resonance phenomenon that occurs when a subject placed in a uniform static magnetic field is irradiated with a high-frequency pulse. In particular, it is used for medical purposes.

  An MRI apparatus generates a magnetic field that generates a uniform static magnetic field mainly in an imaging space into which a subject is inserted, and a magnetic field that is spatially gradient in intensity to give positional information to the imaging space. A gradient magnetic field coil, an RF coil that irradiates a subject with a high frequency pulse, a receiving coil that receives a magnetic resonance signal from the subject, and a computer system that processes the received signal and displays an image.

  As a main means for improving the performance of the MRI apparatus, there is an improvement in the strength of the static magnetic field generated by the magnet apparatus. Since the stronger the static magnetic field, the clearer the image, the MRI system is being developed to improve the magnetic field strength. In particular, in an MRI apparatus having a static magnetic field strength of 0.5 Tesla or more, a magnet apparatus using a superconducting coil has become mainstream.

  The superconducting coil is made of a superconducting material whose electric resistance becomes zero when cooled to a very low temperature. The temperature varies depending on the material, but it must be cooled from 4 to 77 Kelvin in absolute temperature. For this reason, a superconducting coil made of niobium / titanium, which is a material of a superconducting coil currently used in a general MRI apparatus, is immersed in liquid helium in order to maintain a state cooled to 4 Kelvin. . In addition, since helium maintains a liquid state, the superconducting coil and liquid helium are a metal container called a helium container, a radiation shield that surrounds it and shields heat transfer by radiation, and heat conduction from the outside with a vacuum inside. It is housed in a vacuum vessel that reduces heat intrusion due to. Furthermore, the cryogenic state is maintained by suppressing evaporation of liquid helium by a refrigerator.

  Liquid helium is expensive because it is difficult to collect, and the demand for downsizing of the apparatus has led to the development of MRI apparatuses that reduce or do not use liquid helium.

As a background art in this technical field, there is a publication of JP-A-10-189328 (Patent Document 1). This publication discloses a technique for suppressing the loss of the superconducting state due to heat generation of the superconducting coil by having a member with good heat conduction for cooling the coil surface to which the electromagnetic force generated between the superconducting coils is applied. Yes. Japanese Patent Application Laid-Open No. 2011-222729 (Patent Document 2) discloses a technique for realizing a small size of a helium vessel and a vacuum vessel by winding a metal linear member around the outer periphery of a coil.

JP 10-189328 A JP2011-222729

  As shown in Patent Document 1, in order to cool a superconducting coil without using a liquid refrigerant, it is necessary to use a good heat conductor as a heat path to a refrigerator that is a cooling source. On the other hand, since a superconducting coil is generally a composite member of a metal and a resin, it has a low thermal conductivity with respect to a good heat conductor such as a metal, and in order to make the whole superconducting coil be in a superconducting state or less, the whole Must be cooled uniformly.

  For this reason, for the purpose of uniformly cooling the superconducting coil in the circumferential direction, a heat transfer cooling member that is a good heat conductor is arranged in the circumferential direction of the superconducting coil. However, depending on the contact state between the superconducting coil and the heat transfer cooling member, the thermal resistance between them increases, and a sufficient cooling effect of the superconducting coil may not be obtained. For this reason, Patent Document 1 is a technique in which the superconducting coil is held against the heat transfer cooling member by electromagnetic force in the coil axial direction generated in the superconducting coil, thereby reducing the thermal resistance of the contact portion.

  However, in this technology, when no electromagnetic force is generated before the coil is energized, the thermal resistance of the contact portion with the heat transfer cooling member is large, and the position of the heat transfer cooling member is also suppressed by the superconducting coil with electromagnetic force. There was a problem that it had to be installed in the direction to be able to. Further, as shown in Patent Document 2, when a heat transfer cooling member made of a linear metal is wound around the outer periphery of the superconducting coil, the winding of the linear body with the metal is not dependent on the direction of the electromagnetic force of the superconducting coil. The superconducting coil and the heat transfer cooling member can be brought into close contact with each other by the tightening force. However, in general, a superconducting coil and a metal heat transfer cooling member have different heat shrinkage rates due to cooling, and when cooled to an extremely low temperature, the superconducting coil contracts more than the heat transfer cooling member, so that the effect of close contact due to the tightening force is sufficient. In some cases, it could not be obtained.

  Thus, in the prior art, the superconducting coil can be cooled by the heat transfer cooling member regardless of the liquid refrigerant such as liquid helium. Was not enough. As another method, if the heat transfer cooling member and the superconducting coil are integrally molded with resin, or if the heat transfer cooling member is attached to the superconducting coil by adhesion, the thermal resistance of the contact portion can be reduced, but at room temperature. In order to cool to a very low temperature after molding, there is a problem in that the contact surface is peeled off due to the difference in thermal shrinkage between the heat transfer cooling member and the superconducting coil, and the thermal resistance increases.

The present invention has been made in view of such circumstances, and the superconducting coil is cooled to a temperature necessary for the superconducting state by a heat removal means by a refrigerator via a heat transfer cooling member, regardless of the liquid refrigerant, By reducing the thermal resistance of the contact surface between the heat transfer cooling member and the superconducting coil, an MRI system using a superconducting coil that can maintain the temperature of the superconducting state while reducing the power consumption of the refrigerator is realized. .

  In order to solve the above problems, various aspects can be considered in the present invention.As an example, a superconducting magnet according to an embodiment of the present invention includes a superconducting coil, a bobbin around which the superconducting coil is wound, An annular heat transfer cooling member provided between the superconducting coil and the bobbin flange, the heat transfer cooling member is made of a good heat conductive material, and a plurality of voids are formed over the entire surface. Has been.

  According to the superconducting magnet of this configuration, a good superconducting coil cooling performance can be obtained regardless of whether or not the superconducting coil is energized, and even when cooling to a cryogenic temperature at which the superconducting state is achieved. That is, since the power consumption of the refrigerator as a heat removal means is suppressed and the superconducting coil is maintained at a lower temperature, the superconducting magnet that can maintain a stable magnetic field generation state by suppressing the transition to the normal state, And the MRI apparatus using it is realized.

FIG. 2 is a front view showing the structure of a heat transfer cooling member used in the MRI apparatus that is the first embodiment of the present invention. 1 is a vertical external perspective view of an MRI apparatus according to an embodiment of the present invention. It is a vertical external perspective view which shows the other system of the MRI apparatus which is embodiment of this invention. FIG. 3 is a partial cross-sectional view showing an arrangement configuration of a superconducting coil and a heat transfer cooling member used in the MRI apparatus according to the first embodiment of the present invention. It is a figure explaining the state of the thermal contraction accompanying cooling of the heat transfer cooling member and superconducting coil in 1st Embodiment of this invention (a) conventional structure. It is a figure explaining the state of the thermal contraction accompanying cooling of the heat-transfer cooling member and superconducting coil of 2nd Embodiment. It is a front view which shows the structure of the heat-transfer cooling member of 3rd Embodiment. It is a front view which shows the structure of the heat-transfer cooling member of 4th Embodiment. It is a front view which shows the structure of the heat-transfer cooling member of 5th Embodiment. It is a perspective view which shows the structure of the heat-transfer cooling member of 6th Embodiment. It is a fragmentary sectional view which shows the structure of the heat-transfer cooling member of 6th Embodiment, a superconducting coil, and a bobbin. It is the fragmentary sectional view (a) which shows the structure of the heat-transfer cooling member of 7th Embodiment, a superconducting coll, and a bobbin, and a figure (b) explaining the effect at the time of cooling. It is a fragmentary sectional view which shows the structure of the heat-transfer cooling member of 8th Embodiment, a superconducting coil, and a bobbin.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 2, a general MRI apparatus has a cylindrical magnetic pole 1 made of a superconducting coil, and generates a static magnetic field in an imaging space 2 in a direction indicated by an arrow 3. The subject 4 is carried to the imaging space 3 by the movable bed 5 and an image is acquired. The MRI apparatus has a concentric cylindrical gradient magnetic field coil 6 and a high-frequency irradiation coil 7 inside a magnetic pole 1 made of a superconducting coil. The gradient magnetic field coil 6 gives positional information by a magnetic field to a living body, and the high-frequency irradiation coil. 7 generates a magnetic resonance in the element (hydrogen) constituting the living body to acquire a signal. Both are normal conducting coils.

  These are covered with a cover (not shown) integrally with the magnetic pole 1. The MRI apparatus includes a power supply apparatus that supplies current to the gradient magnetic field coil 6 and the high-frequency irradiation coil 7 and a computer system for displaying operations and images as other main components. It is omitted.

  FIG. 3 shows a structure of an MRI apparatus called an open type, which is another general method of the MRI apparatus. Disk-shaped upper and lower magnetic poles 1 are arranged above and below the imaging space 2, and a static magnetic field is generated in the direction of the arrow 3 in the imaging space. The subject 4 is carried to the imaging space 3 by the movable bed 5 to acquire an image, as in FIG.

  In the MRI apparatus of this embodiment, the upper and lower magnetic poles are supported by a structure such as a column, and may be connected by a magnetic return yoke 8 having a substantially C shape as shown in FIG. Can be seen in devices with a magnetic field strength of 1 Tesla or less. In the open-type MRI apparatus, the gradient magnetic field coil 6 and the high-frequency irradiation coil have a disk shape arranged on the top and bottom of the imaging space 3 like the magnetic pole, and are covered with a cover (not shown) integrally with the magnetic pole.

  As shown in FIG. 4, the magnetic pole 1 is composed of an annular superconducting coil 9, and a desired static magnetic field is generated in the imaging space 2 by combining one or more superconducting coils 9 so as to be concentric with each other. Let The superconducting coil 9 is a composite member obtained by molding a wound superconducting wire with a resin such as epoxy or wax. The superconducting wire is composed of a compound such as niobium-titanium, bismuth and magnesium and a metal such as copper and nickel.

  The superconducting coil 9 is usually wound around a winding frame 10 called a bobbin made of metal such as stainless steel, aluminum, or copper, and maintains its position and shape.

  The superconducting coil 9 is cooled to a temperature at which the electric resistance of the superconducting wire becomes zero (depending on the material, usually 77 to 4 Kelvin or less), and in order to maintain that temperature, liquid helium (4 Kelvin) or liquid nitrogen (77 Kelvin) The superconducting coil may be cooled by being immersed in a liquid refrigerant such as) or by a heat removal device such as a refrigerator through a heat transfer member made of a structure. Although both may be used together, the former requires a container for enclosing the superconducting coil in liquid helium for each bobbin, and therefore the magnetic pole tends to be larger than the latter.

  On the other hand, the latter does not require a helium vessel, but requires a heat transfer cooling member 11 for cooling the coil. The heat transfer cooling member 11 is generally made of a metal having good thermal conductivity such as copper or aluminum. In order to cool the superconducting coil 9 to a desired temperature by the heat transfer cooling member 11 and maintain the superconducting state, the thermal resistance between the heat transfer cooling member and the superconducting coil needs to be sufficiently small. In particular, there is a gap at the interface between the superconducting coil and the heat transfer cooling member, which tends to increase the thermal resistance.

  FIG. 5 is an enlarged view of the cross section of the superconducting coil 9 and the conductive cooling member 11 of FIG. The superconducting coil 9 is a composite of a superconducting wire and a resin, and one heat transfer cooling member 11 is a metal. For this reason, the thermal contraction rate of both is different, and the superconducting coil having resin generally has a higher shrinkage rate due to cooling.

  For this reason, as shown in FIG. 5 (a), in the structure having a conventional heat transfer cooling member, when it is cooled to a very low temperature, a shearing force 12 due to the difference in thermal shrinkage occurs at the interface between the two, A gap is generated starting from the unevenness. Dashed lines 13 and 14 show the cross-sectional shape at normal temperature, which is the time when the superconducting coil 9 and the heat transfer cooling member 11 are manufactured.

  For this reason, conventionally, in order to suppress an increase in thermal resistance between the superconducting coil and the heat transfer cooling member, a structure in which they are pressed against each other has been required. Further, although there is an idea that the superconducting coil 9 and the heat transfer cooling member 11 are integrally cured or bonded with a resin, even if there is no gap at the time of manufacture, it is not possible to suppress separation from each other with cooling.

  Therefore, in this embodiment, as shown in FIG. 1, a heat transfer cooling member 11b having an annular shape like the superconducting coil 9 and a structure in which circumferential slits 15 are alternately arranged in the radial direction is provided. It is installed.

  More specifically, the heat transfer cooling plate 11b in the present embodiment basically has a structure in which four slits 15 are arranged at equal intervals in the circumferential direction at one radial position. There are four locations in the radial direction. The circumferential length of each slit has a length of about a quarter of the circumference when the distance between the installation location and the z axis is a radius. Note that the length of the slit 15 in the circumferential direction is merely an example, and any length can be adopted.

  Further, the center positions of the slits 15 in the circumferential direction are provided so that adjacent slits in the radial direction are different from each other. This arrangement relationship will be described with reference to FIG. 1. If the arrangement position of the slit 15 whose position in the radial direction as viewed from the z axis is closest to the z axis is the first layer, the center position of the first layer and the third layer slit 15 is assumed. Are the same, but the center positions of the second and fourth layers are different.

  In addition, although the heat-transfer cooling member 11b in a present Example has an annular shape, it is not restricted to this, For example, the outline may be a polygon. Further, the heat transfer cooling member 11b is inserted between the superconducting coil 9 and the flange portion of the bobbin 10, as shown in FIG.

  As shown in FIG. 5B, since the gap is formed in the heat transfer cooling member 11b in accordance with the thermal contraction of the superconducting coil by forming a gap over the entire surface of this configuration, that is, the electrothermal cooling member 11b. In addition, the superconducting coil 9 and the heat transfer cooling member 11b are integrated or bonded with a resin, and even when cooled at a cryogenic temperature, they do not peel and no gap is formed between them.

  Thereby, the thermal resistance between the heat transfer cooling member 11b and the superconducting coil 9 is kept small. In addition, the slits 15 are arranged in a lattice shape, that is, as described above, the slits 15 that are adjacent to each other in the radial direction with respect to the central axis have different superconductivity by changing the center positions in the circumferential direction. Heat transfer not only in the circumferential direction of the coil 9 but also in the radial direction can be improved, and the superconducting coil can be cooled uniformly. Further, the superconducting coil 9 and the heat transfer cooling member 11b are more firmly integrated by the resin entering the slit.

  FIG. 6 shows the shape of the heat transfer cooling member 11b in the second embodiment of the present invention. In the present embodiment, the heat transfer cooling member 11b has at least one cutting portion 16 in the circumferential direction. As shown in FIG. 6, the cut portion 16 here refers to a gap reaching from the inner diameter to the outer diameter provided in the heat transfer cooling member 11b.

  In a superconducting coil, an eddy current is generated in a metal heat transfer cooling member due to a magnetic field change caused by excitation or the like, which may cause unnecessary heat generation or electromagnetic force. However, the generation of eddy current can be reduced by providing the cut portion 16 as in the present embodiment. In this case as well, the slits 15 are alternately arranged in the radial direction, so that the heat conduction in the radial direction is improved, so that the temperature of the superconducting coil becomes uniform.

  In FIG. 7, the shape of the heat-transfer cooling member 11b in 3rd Embodiment of this invention is shown. In the present embodiment, the heat transfer cooling member 11b has a slit 17 extending in the radial direction in addition to the slit 15 extending in the circumferential direction.

  The slit 17 can suppress the separation by the expansion and contraction of the heat transfer cooling member 11b even when the thermal contraction in the circumferential direction of the superconducting coil 9 is large.

  In FIG. 8, the shape of the heat-transfer cooling member in 4th Embodiment of this invention is shown. In the present embodiment, holes 18 are provided in the heat transfer cooling member 11b in place of the slits 15 and 17. Thus, even if it is the structure which provides a hole in the whole surface of the heat-transfer cooling member 11b, the effect similar to Example 1 can be acquired.

    Although FIG. 8 shows a uniform round hole, it need not be a round hole in particular, and may be a polygon or a hole having a plurality of sizes and shapes. According to this embodiment, it is particularly suitable for a manufacturing method using punching or the like.

  FIG. 9 shows the structure of the heat transfer cooling member 11b in the fifth embodiment of the present invention. In the present embodiment, the heat transfer cooling member 11b is constituted by a wire mesh or a braided wire molded with a good heat conductor such as copper or aluminum.

    According to the present embodiment, the heat transfer cooling member 11b can be expanded and contracted in any direction parallel to the contact surface with the superconducting coil, and only peeling from the superconducting coil due to thermal contraction during cooling can be suppressed. However, since the surface area increases, the adhesion between the superconducting coil and the heat transfer cooling member or the integration by the resin becomes stronger.

  In FIG. 10, the structure of the heat-transfer cooling member in 6th Embodiment of this invention is shown. In this embodiment, the heat transfer cooling member 11b has a cylindrical shape concentric with the superconducting coil, and is installed on the outer peripheral surface in the radial direction of the superconducting coil 9 as shown in FIG. 11 (a), and as shown in FIG. 11 (b). As described above, there are two types of installation methods when the superconducting coil 9 is installed on the radially inner peripheral surface. Moreover, even when the heat transfer cooling member is installed on the radially inner or outer peripheral surface of the superconducting coil as in this embodiment, the structure of the heat transfer cooling member described in the second to fifth embodiments is used in combination. Is also possible.

  In FIG. 12, as a seventh embodiment of the present invention, when the heat transfer cooling member 11b is installed on the outer peripheral surface of the superconducting coil 9, a resin bind 19 is installed on the outer peripheral surface of the heat transfer cooling member 11b. Shows the case.

  According to the present embodiment, during the cooling, the resin-made bind 19 is more thermally contracted than the superconducting coil 9 and the heat transfer cooling member 11b, so that the space between the superconducting coil 9 and the heat transfer cooling member 11b is made stronger. It is possible to make contact and to keep the thermal resistance between the two small.

  By using resin composite material (FRP) using glass fiber or carbon fiber, resin-made binding can be contracted more than the superconducting coil by cooling, and the heat transfer cooling member 11b can be used as the superconducting coil 9. It is possible to have sufficient strength to hold down. The resin-made bind 19 has a width equal to or less than the width in the z-axis direction of the superconducting coil 9 or the heat transfer cooling member 11b so as not to interfere with the bobbin 10 during heat shrinkage.

  As shown in FIG. 12B, the pressing force 20 accompanying the heat shrinkage of the resin bind 19 has the effect of deforming the heat transfer cooling member 11b into a shape accompanying the shrinkage difference of the superconducting coil 9 in the axial direction. In addition, since the resin binding does not require the heat transfer cooling member to be fastened with a large fastening force in advance at room temperature, it has an effect of preventing damage caused by generating unnecessary force on the superconducting coil.

In FIG. 13, the structure of the heat-transfer cooling member in the 8th Embodiment of this invention is shown. In the present embodiment, the bobbin 10 supports the superconducting coil 9 from the outside in the radial direction. In this configuration, when the heat transfer cooling member 11b is installed on the radially inner peripheral surface of the superconducting coil 9, a bind 21 having a smaller heat shrinkage than the superconducting coil 9 is installed on the inner peripheral surface side of the heat transfer cooling member 11b. Is done. ,
When cooling from a steady state to a very low temperature, the superconducting coil 9 having an annular shape is thermally contracted toward the central axis. On the other hand, the bind 21 is a member having a thermal contraction rate smaller than that of the superconducting coil 9, that is, a member that is not easily heat shrunk. As a result, the bind 21 has a structure in which the superconducting coil 9 is stretched from the inner diameter side, and the heat transfer cooling member 11b can be pressed against the superconducting coil 9 from the inner peripheral side. Examples of the material of the bind 21 installed on the inner periphery include a material having a smaller thermal contraction rate than a resin, such as ceramic, in addition to a metal such as SUS or aluminum.

DESCRIPTION OF SYMBOLS 1 Magnetic pole 2 Imaging space 3 Arrow which shows a static magnetic field and its direction 4 Subject 5 Movable bed 6 Gradient magnetic field coil 7 High frequency irradiation coil 8 Return yoke 9 Superconducting coil 10 Bobbin 11 Heat transfer cooling member 11a Conventional heat transfer cooling member 11b Heat transfer cooling member 12 of the present invention Direction of shear force during cooling 13 Cross section shape of superconducting coil at room temperature 14 Cross section shape of heat transfer cooling member at room temperature 15 Slit 16 Cutting portion 17 Radial slit 18 Hole 19 Made of resin Binding 20 Pushing force 21 by binding Binding on the inner circumference side of the coil

Claims (13)

A superconducting coil;
A bobbin around which a superconducting coil is wound;
An annular heat transfer cooling member provided between the superconducting coil and a flange of the bobbin,
The superconducting magnet, wherein the heat transfer cooling member is made of a good heat conductive material, and a plurality of voids are formed over the entire surface. The gap is a slit extending in the circumferential direction,
The superconducting magnet according to claim 1, wherein a plurality of the slits are provided at different positions in the radial direction of the heat transfer cooling member.   The superconducting magnet according to claim 1, wherein the heat transfer cooling member is made of a metal such as a copper material or an aluminum material.   The superconducting magnet according to any one of claims 1 to 3, wherein the heat transfer cooling member and the superconducting coil are integrated by integral molding or adhesion using a resin.   The superconducting magnet according to any one of claims 1 to 4, wherein the heat transfer cooling member has a substantially annular shape and has a cut portion in a circumferential direction.   The superconducting magnet according to any one of claims 1 to 5, wherein the heat transfer cooling member has a substantially annular shape and has a slit extending in a radial direction.   The superconducting magnet according to claim 1, wherein the gap is a hole.   The superconducting magnet according to claim 1, wherein the heat transfer cooling member is a mesh or a braided wire of a good heat conductor. A pair of gradient coils for generating a magnetic field with a gradient intensity in the imaging space;
10. A magnetic resonance imaging apparatus having a superconducting magnet according to claim 1, further comprising: a pair of RF coils that irradiate the imaging space with a high-frequency magnetic field. 11. A superconducting coil;
A substantially cylindrical heat transfer cooling member,
The heat transfer cooling member is installed coaxially with and in contact with the superconducting coil,
Further, the heat transfer cooling member is made of a good heat conductive material, and slits extending in the circumferential direction are alternately arranged in the axial direction.   11. The superconducting magnet according to claim 10, wherein the substantially cylindrical heat transfer cooling member is installed on an outer peripheral surface of the superconducting coil, and a resin binding is wound around the outer peripheral surface.   The substantially cylindrical heat transfer cooling member is installed on an inner peripheral surface of the superconducting coil, and further, a binding made of a member having a thermal contraction rate smaller than that of the superconducting coil is arranged on the inner peripheral surface. The superconducting magnet according to claim 10. A pair of gradient coils for generating a magnetic field with a gradient intensity in the imaging space;
A magnetic resonance imaging apparatus comprising the superconducting magnet according to claim 10, further comprising a pair of RF coils that irradiate the imaging space with a high-frequency magnetic field.