RU221707U1 - Multilayer cylindrical magnetic screen for magnetoencephalography - Google Patents

Abstract

The utility model relates to multilayer magnetic screens and can be used when conducting biomagnetic research in the field of neurotechnology, in particular magnetoencephalography. The magnetic screen contains two coaxial cylinders of different diameters, which differ by ΔD, made of soft magnetic material. The inner cylinder of the screen has a shorter length than the outer one by ΔL. A value of ΔL equal to twice the distance from the edge of the outer cylinder to the edge of the inner cylinder must satisfy the following inequality 2ΔD≤ΔL≤4ΔD. The geometric centers of the inner and outer cylinders must coincide. The useful model makes it possible to reduce the area of magnetic field inhomogeneity. 3 ill.

Description

The utility model relates to multilayer magnetic screens and can be used when conducting biomagnetic research in the field of neurotechnology, in particular magnetoencephalography.

Magnetoencephalography is a method of non-invasive mapping of brain activity, recording its very weak magnetic fields [1]. Measurements of weak magnetic fields of the brain by this method are made using compact optically pumped magnetometers (hereinafter referred to as MOPs) placed on the patient’s scalp. The most sensitive are MONs based on the Hanle effect, but they can be used to make measurements only in low magnetic fields (with a strength of no more than 100 nT). There is a need to protect MONs and the patient on whose scalp they are located from the Earth’s magnetic field and industrial electromagnetic noise. An effective way to weaken magnetic fields and ensure correct operation of the MON is to use shielding [2]. Magnetic screens are used in devices sensitive to constant, alternating and quasi-static magnetic fields, including fiber-optic and nuclear gyroscopes [3], quantum electronics devices [4]. Large-sized magnetic screens are used in physical and biological experiments.

The effectiveness of a magnetic shield depends on the characteristics of the material from which it is made (magnetic permeability), the amplitude and frequency of the magnetic field surrounding it, as well as the number and configuration of layers used in the shield construction.

A multilayer electromagnetic screen is known from the prior art, containing alternating electrically conductive layers of copper and magnetic layers of a nickel-iron alloy with an optimal combination of the thickness of magnetic and non-magnetic layers [5], with a total number of 100 to 900 layers. A multilayer electromagnetic screen is also known, which is a coating of sequentially applied magnetic (nickel-iron alloy containing 15-25 at.% iron) and non-magnetic layers (copper, gold, silver), which are applied by electrolytic deposition of non-magnetic and magnetic layers; number of layers - up to 99 [6].

The general disadvantages of the screens listed above include the fact that layers of such screens are applied directly to the rigid base of the device body that needs to be shielded, which reduces its operational reliability, since adjustment of the shielded sensor or device without breaking the shielding layer becomes impossible. In addition, the non-magnetic layers of such screens are made on the basis of copper, gold and silver, which does not allow achieving high shielding coefficients in the low-frequency region and makes the screens expensive. Also, applying up to 900 magnetic and non-magnetic layers with uniform coating thicknesses over the entire screen area is a complex and time-consuming task for the implementation of large screens that are supposed to be used for magnetoencephalography.

The listed problems are solved by technical solutions protected by patents [7, 8]. They offer a composition in the form of a single-layer screen of different configurations (lattice, weaving of ribbons), fixed in a certain way into a single mechanical system. However, the efficiency of such single-layer screens is low (the shielding coefficient does not exceed 100), and is usually used for a narrow range of field intensities: 500-600 A/m, 20-50 A/m.

Known from the prior art is a magnetic screen made of soft magnetic material, containing upper and lower parts covering the body of the device, characterized in that the upper and lower parts of the magnetic screen are made in the form of hollow cylinders without lower and upper bases, respectively, and the upper part of the magnetic screen is made with a bend installed with a tight fit between the inner surface of the upper section of the lower part of the magnetic screen in a groove in the upper part of the device body, made to the height and thickness of the bend of the upper part of the magnetic screen, and permalloy is used as a soft magnetic material [9]. However, the design of such a screen requires complete spatial isolation of the device placed in it from the external environment, which is unacceptable within the framework of magnetoencephalography, because The patient placed in the screen must be provided with natural ventilation.

The magnetic shield [10], chosen as a prototype, is made in the form of two coaxial shells made of ferromagnetic material in the form of cylinders of different diameters, characterized in that in order to increase the degree of shielding and manufacturability of the design, the shell cylinders are made of composite parts of the cylinders overlapping each other and equipped with threaded bushings of the screw-nut type made of non-magnetic material placed between the shells, parts of the cylinders of each shell are connected to each other by the specified threaded bushings, and the internal shells are connected to each other and secured to the outer shell by means of the specified threaded bushings, which are located coaxially with each other and oriented radially direction relative to the shells.

The disadvantage of the prototype screen is that all its layers (cylinders) have the same length and the turbulence of the tension vector, caused by edge effects at the ends of the outer cylinders, magnetizes the edges of the inner cylinders of the magnetic screen, which leads to a violation of the uniformity of the magnetic field in the inner part of the screen, due to which reduces the volume of the working area.

The technical problem being solved is to improve the design of the magnetic screen while maintaining its external dimensions to reduce the area of magnetic field inhomogeneity caused by edge effects.

The achieved technical result of the claimed technical solution is to reduce the area of magnetic field inhomogeneity caused by edge effects, which makes it possible to increase the volume of the internal working area with a uniform magnetic field without increasing the external dimensions of the magnetic screen.

The essence of the utility model is as follows.

A multilayer magnetic screen (hereinafter referred to as the screen) is an assembly of two cylinders, the diameters of which differ by ΔD, with open ends, made of soft magnetic material with a high initial relative magnetic permeability (more than 1000) and nested inside each other.

An air gap is left between the cylinders or a non-magnetic layer (insulator) with a relative magnetic permeability close to 1 is placed. Thanks to the gap ΔD/2, the transverse shielding coefficient of the entire screen is determined by the formula [10]:

, (1)

Where - shielding coefficients of the outer and inner cylinders of the shells;

- cross-sectional area of the shells.

In this case, the inner cylinder has a length shorter than the outer one by ΔL, and is located so that its geometric center coincides with the geometric center of the outer cylinder (Fig. 1). A value of ΔL equal to twice the distance from the edge of the outer cylinder to the edge of the inner cylinder must satisfy the following inequality:

(2)

In a non-zero magnetic field, the outer cylinder of the screen creates turbulence in the intensity vector, which leads to an increase in the inhomogeneity of the magnetic field in the area of the ends. With the same cylinder lengths or with this heterogeneity enhances the edge effects at the ends of the inner cylinder. When making the inner cylinder of the screen shorter than the outer one ( ), its ends are not influenced by edge effects created by the outer cylinder and depending on the strength of the external magnetic field. As a result of this, the inner cylinder, being in a field close to zero, creates insignificant edge effects of its own and a smaller zone of inhomogeneity. By reducing the zone of inhomogeneity at the edges of the screen and maintaining its external dimensions, the volume of the internal zone with a uniform magnetic field increases. In case of further reduction in the length of the inner cylinder of the screen ( ) the volume reduction of the region of a uniform magnetic field begins again due to the reduction of the zone protected by two layers of the screen. Thus, inequality (2) determines the optimal difference in the lengths of the screen cylinders.

Brief description of the drawings.

Figure 1 shows a general cross-sectional view of the proposed two-layer magnetic screen, where:

1 - outer cylinder,

2 - inner cylinder,

3 - gap between the walls of the inner and outer cylinders;

4 - distance between the edges of the inner and outer cylinders;

5 - longitudinal axis of symmetry of the outer cylinder x with the beginning at its end;

6 - geometric center of the outer cylinder.

In fig. Figure 2 shows the dependences of the values of magnetic induction field B on the distance x from the edge of the outer cylinder for various distance values .

In fig. Figure 3 shows the dependence of the distance l, which defines the boundaries of the magnetic field inhomogeneity zone, on the displacement , where l is the distance from the edge of the outer cylinder to the beginning of the zone in which the field heterogeneity does not exceed a specified value.

An example of the implementation of the proposed utility model.

Prototype screen with outer cylinder diameters mm and internal cylinder mm and lengths mm and mm, respectively, is made of 79NM permalloy material. The air gap between the cylinders is provided by guides made of non-magnetic, non-conductive material. Its parameters satisfy inequality (2). In this case, the difference in diameters ΔD was 6.7 cm, and the difference in lengths was 20 cm.

To substantiate this design, the following experiment was performed. The inner cylinder was displaced relative to the outer cylinder along the symmetry axis x and for each displacement value The dependence of the values of the magnetic induction of the field B on the distance x from the edge of the outer cylinder was determined (Fig. 2). Next, the distance l was determined from the edge of the outer cylinder to the beginning of the zone in which the value of the magnetic field induction does not exceed 100 nT, i.e. zones in which MON can operate based on the Hanle effect. The distance l for each displacement value ∆L/2 was determined according to the data in Fig. 2 as the minimum value along the x axis at which magnetic induction B becomes less than 100 nT. Graph of distance l, which defines the boundaries of the heterogeneity zone, as a function of displacement is presented in Fig. 3.

The graph shows that in the first section ( ) there is a rapid reduction in the zone of inhomogeneity of the magnetic field due to a decrease in the influence of edge effects, in the second section, satisfying inequality (2), the minimum value of the inhomogeneity zone is achieved, in the third section, with There is a linear growth of the heterogeneity zone caused by a reduction in the size of the inner cylinder.

For the case when the ends of the outer and inner cylinders are in the same plane ( ) the zone of inhomogeneous magnetic field extends 12 cm from the end into the depth of the screen and, taking into account the internal diameter of 530 cm, has a volume of 0.265 m ³ , and when displaced , the heterogeneity zone is reduced to 6.5 cm, and its volume to 0.143 m ³ . Thus, the fulfillment of inequality (2) ensured a reduction in the zone of magnetic field inhomogeneity by 45%.

Thus, what has been considered shows that the claimed technical solution is feasible and ensures the achievement of the declared technical result.

Information sources

1. Borna A. et al. A 20-channel magnetoencephalography system based on optically pumped magnetometers //Physics in Medicine & Biology. 2017. T. 62. No. 23. P. 8909.

2. Yu.Ya. Reutov. Classic protective screens. Ekaterinburg: Ural Branch of the Russian Academy of Sciences, 2006.

3. Peshekhonov V. G. Prospects for gyroscopy. // XIII All-Russian meeting on management problems VSPU-2019, 2019. pp. 36-38.

4. Dukhina N. G. et al. Study of the shielding efficiency of precision on-board cesium ALT-LN //Electronic technology. Series 1: Microwave technology. 2020. no. 4. pp. 37-52.

5. RF Patent No. 2646439.

6. RF Patent No. 2474890.

7. RF Patent No. 2381601.

8. RF Patent No. 2274914.

9. RF Patent No. 175603.

10. RF Patent No. 2012175.

Claims (1)

Multilayer cylindrical magnetic screen for magnetoencephalography, containing two cylinders of different diameters, which differ by ΔD, made of soft magnetic material with high initial magnetic permeability, separated by an insulator, characterized in that the ends of the cylinders are open, and the inner cylinder of the screen has a shorter length than the outer one by ΔL, and the value of ΔL, equal to twice the distance from the edge of the outer cylinder to the edge of the inner cylinder, must satisfy the following inequality 2ΔD≤ΔL≤4ΔD, and the geometric centers of the inner and outer cylinders must coincide.