Single-end open type high-performance magnetic shielding system and design method thereof

Abstract

The invention discloses a single-end open type high-performance magnetic shielding system and a design method thereof. The system is formed by combining a passive magnetic shielding subsystem and an active magnetic compensation subsystem. The passive magnetic shielding subsystem is formed by alternately nesting single-end open type high-permeability and high-conductivity metal barrels, and the structure is beneficial to synchronously improving the shielding capability for full-band magnetic interference; the active magnetic compensation subsystem is composed of a group of axial nonlinear gradient coils, two groups of radial nonlinear gradient coils with orthogonal magnetic fields and a magnetic compensation control circuit, and three groups of magnetic compensation coil windings are obtained based on specific nonlinear distributed magnetic interference reverse design in a single-end open type magnetic shielding barrel; different from conventional shimming and linear gradient coils, accurate compensation of magnetic interference in a working area in the barrel can be realized. According to the invention, the shielding capability of the single-end open type magnetic shielding system is improved, and a required near-zero magnetic environment is provided for extremely weak magnetic measurement.

Description

Technical Field

[0001] The invention belongs to the technical field of magnetic compensation, and in particular relates to a single-end open-type high-performance magnetic shielding system and a design method thereof. Background Art

[0002] Spin-Exchange Relaxation-Free (SERF) atomic spin magnetic field measurement technology, with its advantages of no radiation, non-invasiveness, high resolution and high sensitivity, provides an effective means for the research of cardio-cerebral functional imaging and cardio-cerebral medicine. It is expected to serve the people's life and health areas such as the diagnosis of major and difficult cardio-cerebral diseases and promote the leapfrog development of medical equipment.

[0003] A near-zero magnetic measurement environment is a necessary condition for realizing ultra-high sensitivity and extremely weak magnetic field measurement based on the SERF effect. In order to construct the required magnetic field environment, the traditional method is to shield the external environmental magnetic field interference through a passive magnetic shielding barrel. However, considering some practical application needs, such as to alleviate the discomfort of claustrophobic patients, one end of the passive magnetic shielding barrel must be kept open, so the external magnetic field will enter the magnetic shielding barrel, causing a large magnetic field around the magnetometer air chamber in the barrel, causing it to fail to work normally. The combination of the passive magnetic shielding subsystem and the active magnetic compensation subsystem helps to improve the magnetic shielding performance of the device, which is the main way to achieve a near-zero magnetic measurement environment. Therefore, it is crucial to improve the shielding performance of the passive magnetic shielding subsystem under the limitation of a single-ended open structure, and to design an active magnetic compensation subsystem that can accurately compensate for the residual magnetism in the barrel. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention proposes a single-ended open-type high-performance magnetic shielding system and a design method thereof. The invention first proposes a scheme for optimizing the passive magnetic shielding system under the limitation of the single-ended open-type structure, and then proposes a scheme for reversely designing the corresponding magnetic compensation coil based on the unique nonlinear distributed magnetic field in the single-ended open-type magnetic shielding barrel. The scheme does not adopt conventional uniform field and linear gradient coils, and can achieve accurate compensation for magnetic interference in the working area of ​​the barrel, and specifically solves the problem of large nonlinear gradient magnetic field in the single-ended open-type barrel, which helps to improve the shielding capability of the single-ended open-type magnetic shielding system and provide the required near-zero magnetic environment for extremely weak magnetic measurement.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A single-ended open-type high-performance magnetic shielding system, comprising: a passive magnetic shielding subsystem and an active magnetic compensation subsystem;

[0007] The passive magnetic shielding subsystem is composed of single-end open-type high magnetic permeability and high electrical conductivity magnetic shielding barrels alternately nested;

[0008] The active magnetic compensation subsystem consists of a group of axial nonlinear gradient coils, two groups of radial nonlinear gradient coils with orthogonal magnetic fields, and a magnetic compensation control circuit. Among them, the three groups of nonlinear gradient coil windings are all designed based on the reverse nonlinear distributed magnetic field unique to the single-ended open-ended magnetic shielding barrel, and cooperate with the magnetic compensation control circuit to realize magnetic interference compensation in the working area of ​​the barrel.

[0009] The present invention also discloses a design method for a single-ended open-type high-performance magnetic shielding system, which is implemented by applying the single-ended open-type high-performance magnetic shielding system, and includes the following steps:

[0010] S1: Determine the dimensional constraints of each layer of the single-ended open-end magnetic shielding barrel, such as the maximum and minimum values ​​of the diameter and length. Under the condition of meeting the dimensional requirements of the test and support devices, construct a three-dimensional model of alternating nesting of high magnetic permeability and high electrical conductivity single-ended open-end alloy barrels in the finite element software. Preliminary parametric scanning is used to analyze the influence of various dimensions, such as length, radius, radial spacing, axial spacing and other parameters on the magnetic shielding effectiveness. Then, the Nelder-Mead local optimization method is used to obtain the structural parameters with the optimal magnetic shielding effectiveness under dimensional constraints.

[0011] S2: Determine the target working area, select multiple target field points on the central axis of the target area, test and analyze the magnetic field strength of the target field points, and normalize the magnetic field strength to obtain the expression of the target magnetic field;

[0012] S3: Design corresponding axial and radial nonlinear gradient coils. Based on the Fourier-Bessel expansion of the magnetic vector potential in the cylindrical coordinate system, combined with the boundary value equations at the interface of high magnetic permeability materials and the mirror method, obtain the analytical equations of the magnetic field components generated by the axial and radial nonlinear gradient coils in the internal space of the magnetic shielding barrel under the coupling of high magnetic permeability and high electrical conductivity materials. Then use the particle swarm optimization algorithm to globally optimize and obtain the coil parameters that accurately compensate for the magnetic field in the magnetic shielding barrel.

[0013] Preferably, the method of normalizing the magnetic field intensity to obtain the target magnetic field includes:

[0014]

[0015] in, represents the target magnetic field along the x, y and z directions at the jth target point, B x,y,z,j is the actual measured magnetic field along the x, y and z directions at the jth target point, B x,y,z,1 are the actually measured magnetic fields at the first target point along the x, y and z directions respectively, where j = 1, 2, …Num.

[0016] Preferably, the method for obtaining the magnetic field components generated by the axial and radial nonlinear gradient coils in the internal space under the coupling effect of the high magnetic permeability and high electrical conductivity materials of the magnetic shielding barrel comprises:

[0017]

[0018] in,

[0019]

[0020] α b =(μ r μ 0 -μ 0 )I′ m (ka)K m (kb)

[0021] β b =μ r μ 0 I m (kb)K′ m (kb)-μ 0 I′ m (kb)K m (kb)

[0022] γ b =(μ r μ 0 -μ 0 )I m (kb)K′ m (kb)

[0023] α c =(μ r μ 0 -μ 0 )I′ m (ka)K m (kc)

[0024] β c =(μ r μ 0 -μ 0 )I′ m (kb)K m (kc)

[0025] γ c =μ r μ 0 I′ m (kc)K m (kc)-μ 0 I m (kc)K′ m (kc)

[0026] Among them, μr is the relative magnetic permeability of the material, ρ, φ and z represent the radial distance, azimuth and height of the target point in the cylindrical coordinate system; B ρ (ρ,φ,z),B φ (ρ,φ,z) and B z (ρ, φ, z) represents the magnetic field components of the target field point along the radial distance, azimuth and altitude in the cylindrical coordinate system, which are converted into the magnetic field generated by the coil at the target point in the Cartesian coordinate system by the coordinate system conversion formula μ 0 is the vacuum magnetic permeability; a is the coil radius; b is the inner radius of the innermost shielding material of the magnetic shielding barrel; c is the outer radius of the innermost shielding material of the magnetic shielding barrel; I m , K m are the first and second modified Bessel functions, respectively, and I′ m and K′ m is the corresponding first-order derivative; k and m are integral parameters and have no practical meaning; i is an imaginary number and has no practical meaning; R m , α b , β b , γ b , α c , β c and γ c It is an intermediate variable and has no practical significance.

[0027] Preferably, is the Fourier transform expression of the current density of the coil, which is related to the coil configuration. It is expressed as:

[0028]

[0029] The axial nonlinear coil is composed of Q current-carrying rings, the planes of each ring are parallel to the xoy plane, and the centers of the rings are located on the axis of the magnetic shielding barrel; the distance between the qth ring and the bottom of the magnetic shielding barrel is d q , the number of turns is n q , the current flowing through is I q , where q = 1, 2…Q;

[0030] Radial nonlinear gradient coils It is expressed as:

[0031]

[0032] The radial nonlinear coil consists of T pairs of saddle-shaped windings, and the distance between the inner and outer planes of the tth pair of windings and the bottom of the barrel is l t1 and l t2 , the winding arc angle is The number of turns is n t, the current flowing through is I t , where t=1,2…T.

[0033] Preferably, the method of using a particle swarm optimization algorithm to globally optimize and obtain coil parameters for accurately compensating the magnetic field in the magnetic shielding barrel includes:

[0034]

[0035] std min ≤d q ≤d max

[0036] l min ≤l t1 ≤l max

[0037] l min ≤l t2 ≤l max

[0038]

[0039] n q ,n t ∈N +

[0040] I q ,I t ∈N +

[0041] Among them, d min and d max is the minimum and maximum value of the current-carrying ring position of the axial nonlinear gradient coil, l min and l max are the minimum and maximum values ​​of the saddle winding height of the radial nonlinear gradient coil.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] (1) The present invention proposes a solution to optimize the passive magnetic shielding system under the limitation of a single-end open structure, which helps to simultaneously improve the shielding capability of the magnetic shielding barrel against full-band magnetic interference;

[0044] (2) The present invention is the first to propose a solution of reversely designing a corresponding magnetic compensation coil based on the unique nonlinear distributed magnetic field in a single-ended open-end magnetic shielding barrel, instead of using conventional uniform field and linear gradient coils, so as to achieve accurate compensation of magnetic interference in the working area of ​​the barrel. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0046] Figure 1 This is a schematic structural diagram of a passive magnetic shielding barrel according to an embodiment of the present invention;

[0047] Figure 2 Schematic diagram of the structure of axial and radial nonlinear coils in an embodiment of the present invention, wherein (a) is an axial nonlinear gradient coil, and (b) is a radial nonlinear gradient coil;

[0048] Figure 3 A comparison diagram of the simulation effect of the scheme of the embodiment of the present invention and the shimming coil compensating the magnetic field on the axis of the magnetic shielding barrel;

[0049] Figure 4 A cloud diagram showing the distribution of residual magnetic field components in a comparison diagram of the simulation effect of the scheme of an embodiment of the present invention and the shimming coil compensating the xoy plane magnetic field of the magnetic shielding barrel;

[0050] Figure 5 The shimming coil is a cloud diagram showing the distribution of magnetic field components after compensation by the shimming coil in the comparison diagram of the simulation effect of compensating the xoy plane magnetic field of the magnetic shielding barrel with the scheme of the embodiment of the present invention;

[0051] Figure 6 This is a cloud diagram of the magnetic field component distribution after nonlinear gradient coil compensation in the comparison diagram between the solution of an embodiment of the present invention and the simulation effect of shimming coil compensating the xoy plane magnetic field of the magnetic shielding barrel. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0054] Embodiment 1

[0055] The present invention provides a single-ended open-type high-performance magnetic shielding system, which is composed of a passive magnetic shielding subsystem and an active magnetic compensation subsystem. The passive magnetic shielding subsystem is composed of single-ended open-type high magnetic permeability and high electrical conductivity magnetic shielding barrels alternately nested, such as Figure 1 As shown, this structure helps to simultaneously improve the shielding ability of the magnetic shielding barrel against full-band magnetic interference; the active magnetic compensation subsystem consists of a group of axial nonlinear gradient coils, two groups of radial nonlinear gradient coils with orthogonal magnetic fields and a magnetic compensation control circuit. The structural schematic diagram of the coil is shown in Figure 2 As shown; the magnetic compensation coil windings are all designed based on the reverse design of the nonlinear distributed magnetic field unique to the single-ended open-end magnetic shielding barrel, and can be used in conjunction with the magnetic compensation control circuit to achieve accurate compensation of magnetic interference in the working area of ​​the barrel.

[0056] Embodiment 2

[0057] The present invention also provides a single-ended open-type high-performance magnetic shielding system design method, which is applied to the above-mentioned single-ended open-type high-performance magnetic shielding system, and includes the following steps:

[0058] S1: Determine the dimensional constraints of each layer of the single-ended open-end magnetic shielding barrel, such as the maximum and minimum values ​​of the diameter and length. Under the condition of meeting the dimensional requirements of the test and support devices, construct a three-dimensional model of alternating nesting of high magnetic permeability and high electrical conductivity single-ended open-end alloy barrels in the finite element software. Preliminary parametric scanning is used to analyze the influence of various dimensions, such as length, radius, radial spacing, axial spacing and other parameters on the magnetic shielding effectiveness. Then, the Nelder-Mead local optimization method is used to obtain the structural parameters with the optimal magnetic shielding effectiveness under dimensional constraints.

[0059] S2: Determine the target working area, select multiple target field points on the central axis of the target area, test and analyze the magnetic field strength of the target field points, and normalize the magnetic field strength to obtain the expression of the target magnetic field;

[0060] S3: Design corresponding axial and radial nonlinear gradient coils. Based on the Fourier-Bessel expansion of the magnetic vector potential in the cylindrical coordinate system, combined with the boundary value equations at the interface of high magnetic permeability materials and the mirror method, obtain the analytical equations of the magnetic field components generated by the axial and radial nonlinear gradient coils in the internal space of the magnetic shielding barrel under the coupling of high magnetic permeability and high electrical conductivity materials. Then use the particle swarm optimization algorithm to globally optimize and obtain the coil parameters that accurately compensate for the magnetic field in the magnetic shielding barrel.

[0061] in, Figure 1 This is a schematic diagram of the structure of a passive magnetic shielding barrel. The axial direction is the z-axis, and the radial directions are the x and y directions. The size of the inner metal layer of the passive magnetic shielding barrel must meet the entry and exit requirements of the test equipment and supporting structure. The metal layers are filled with insulating materials. The outermost layer of aluminum alloy not only blocks the high-frequency magnetic field, but also plays a supporting role.

[0062] The process of constructing the alternating three-dimensional model of high magnetic permeability and high electrical conductivity in step S1 is as follows: the single-ended open shielding barrel is composed of five layers of single-ended open metal barrels coaxially nested with each other, from the inside to the outside, respectively: the 1st to 4th layers are Permalloy, and the 5th layer is aluminum alloy. The three-dimensional model of the alloy barrel of the above structure is constructed in the commercial software COMSOL Multiphysics, wherein the inner diameter and length of the innermost alloy barrel, the thickness of each layer of alloy barrel, and the radial and axial interlayer spacing between each layer of alloy barrel are all variables to be optimized.

[0063] The steps of analyzing the influence of various dimensional parameters of the passive magnetic shielding tube on the magnetic shielding effectiveness in step S1 are as follows:

[0064] a. Under the premise of meeting the physical constraints and the magnetometer probe array test requirements, set the range of variables to be optimized to ensure the practicality of the results, including the length of the magnetic shielding tube, inner and outer radii, radial spacing, and axial spacing.

[0065] b. Using the parametric scanning interface in COMSOL, with each parameter to be optimized as the scanning variable, several intermediate values ​​with equal step lengths are taken within the variation range to perform batch finite element simulation, and the magnetic shielding coefficient of the center point of the magnetometer array under different parameter combinations is recorded.

[0066] c. Based on the data results, determine the geometric parameters that have the greatest impact on the magnetic shielding performance, such as the contribution of the length and inner and outer radii of the magnetic shielding tube to the shielding effect; and select the best parameter combination as the initial value for further optimization and fine-tuning;

[0067] The process of using the Nelder-Mead local optimization method in step S1 to obtain the optimal structural parameters for magnetic shielding effectiveness under size constraints is as follows:

[0068] a. Select the goal of minimizing the magnetic field strength inside the magnetic shielding tube and set a suitable objective function. For a multi-layer structure, the magnetic permeability and electrical conductivity of each layer may be different. Therefore, it is necessary to calculate the magnetic field of each layer separately, and then integrate the results of each layer to obtain the magnetic field distribution inside the entire magnetic shielding tube;

[0069] b. Based on the optimal results and requirements of COMSOL parametric scanning, set the initial values ​​of the geometric parameters of the magnetic shielding tube and the feasible range of each parameter to ensure that the parameter values ​​in the optimization process meet the physical and actual manufacturing constraints;

[0070] c. Execute the Nelder-Mead optimization algorithm and select the vertex with the minimum objective function value as the optimal solution;

[0071] The steps of determining the target area and the magnetic field strength of the target point in step S2 are as follows:

[0072] The rectangular plane where the SERF magnetometer probe array test process is located is defined as the working area, and a line segment passing through the center of the working area and parallel to the axis of the cylindrical shielding barrel is discretized into NUM discrete target points with equal spacing. The strength and direction of the magnetic field at each target field point is measured using a magnetometer, or the magnetic flux density value and direction of each test point in the finite element simulation results are used.

[0073] The expression of the target magnetic field obtained by normalizing the magnetic field components in step S2 is as follows:

[0074]

[0075] in, represents the target magnetic field along the x, y and z directions at the jth (j=1, 2, ...Num) target point, respectively, B x,y,z,j is the actual measured magnetic field along the x, y and z directions at the jth (j = 1, 2, ... Num) target point, respectively, B x,y,z,1 are the actual measured magnetic fields along the x, y and z directions at the first target point.

[0076] Wherein, in step S3, the analytical equation of the magnetic field component generated by the nonlinear gradient coil in the internal space under the coupling effect of the high magnetic permeability and high electrical conductivity materials of the magnetic shielding barrel is:

[0077]

[0078] in,

[0079]

[0080] α b =(μ r μ 0 -μ 0 )I′ m (ka)K m (kb)

[0081] β b =μ r μ 0 I m (kb)K′ m (kb)-μ 0 I′ m (kb)K m (kb)

[0082] γ b =(μ r μ 0 -μ0 )I m (kb)K′ m (kb)

[0083] α c =(μ r μ 0 -μ 0 )I′ m (ka)K m (kc)

[0084] β c =(μ r μ 0 -μ 0 )I′ m (kb)K m (kc)

[0085] γ c =μ r μ 0 I′ m (kc)K m (kc)-μ 0 I m (kc)K′ m (kc)

[0086] Among them, μ r is the relative magnetic permeability of the material, μ 0 is the vacuum magnetic permeability, b is the inner radius of the innermost shielding material of the magnetic shielding barrel; c is the outer radius of the innermost shielding material of the magnetic shielding barrel; where R m It is the correction function of magnetic flux density under the boundary condition of high magnetic permeability cylindrical surface. b , β b , γ b , α c , β c and γ c It is an intermediate variable and has no practical significance; It is the Fourier transform expression of the current density of the coil under the coupling effect of the end face of the shielding layer, which is related to the coil configuration, where p is the number of mirror images in the process of calculating the coupling effect between the end face of the shielding layer and the coil by the mirror method. ρ (ρ,φ,z),B φ (ρ,φ,z) and B z (ρ, φ, z) can be converted into the magnetic field generated by the coil at the target point in the Cartesian coordinate system by the coordinate system conversion formula Axial nonlinear gradient coil It can be expressed as:

[0087]

[0088] The axial nonlinear coil is composed of Q current-carrying rings, such as Figure 2 As shown in (a), the planes where the rings are located are parallel to the xoy plane, and the centers of the rings are located on the axis of the magnetic shielding barrel; the distance between the qth (q=1,2…Q) ring and the bottom of the magnetic shielding barrel is d q , the number of turns is n q , the current flowing through is I q .

[0089] Radial nonlinear gradient coils It can be expressed as:

[0090]

[0091] Among them, the radial nonlinear coil is composed of T pairs of saddle-shaped windings, such as Figure 2 As shown in (b), the distance between the inner and outer planes of the tth (t=1,2…T) pair of windings and the bottom of the barrel is l t1 and l t2 , the winding arc angle is The number of turns is n t , the current flowing through is I t .

[0092] Among them, in step S3, the particle swarm optimization algorithm optimization sets the optimization goal to minimize the relative deviation between the actual magnetic field and the target magnetic field, and the optimization variables include the coil position, the number of coil turns, the coil size, etc. The objective function expression and the constraint condition equation of the optimization variable are:

[0093]

[0094] std min ≤d q ≤d max

[0095] l min ≤l t1 ≤l max

[0096] l min ≤l t2 ≤l max

[0097]

[0098] n q ,n t ∈N +

[0099] I q ,I t ∈N +

[0100] Among them, d min and d max is the minimum and maximum value of the current-carrying ring position of the axial nonlinear gradient coil, l min and l max are the minimum and maximum values ​​of the saddle winding height of the radial nonlinear gradient coil.

[0101] Figure 3 A comparison chart of the simulation effect of the scheme provided by the implementation patent of the present invention and the shim coil compensating the magnetic field on the axis of the magnetic shielding barrel. Figure 4 — Figure 6 The figure shows the simulation effect comparison of compensating the xoy plane magnetic field of the magnetic shielding barrel. As can be seen from the figure, the method proposed in the present invention can effectively reduce the magnetic field in the passive magnetic shielding barrel and improve the shielding ability of the single-ended open magnetic shielding system.

[0102] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A single-ended open-type high-performance magnetic shielding system, characterized in that: include: Passive magnetic shielding subsystem and active magnetic compensation subsystem; The passive magnetic shielding subsystem is composed of single-end open-type high magnetic permeability and high electrical conductivity magnetic shielding barrels alternately nested; The active magnetic compensation subsystem consists of a group of axial nonlinear gradient coils, two groups of radial nonlinear gradient coils with orthogonal magnetic fields, and a magnetic compensation control circuit. Among them, the three groups of nonlinear gradient coil windings are all designed based on the reverse nonlinear distributed magnetic field unique to the single-ended open-ended magnetic shielding barrel, and cooperate with the magnetic compensation control circuit to realize magnetic interference compensation in the working area of ​​the barrel.

2. A design method for a single-ended open-type high-performance magnetic shielding system, characterized in that: The single-ended open-type high-performance magnetic shielding system described in claim 1 is implemented by applying the following steps: S1: Determine the dimensional constraints of each layer of the single-ended open-end magnetic shielding barrel, such as the maximum and minimum values ​​of the diameter and length. Under the condition of meeting the dimensional requirements of the test and support devices, construct a three-dimensional model of alternating nesting of high magnetic permeability and high electrical conductivity single-ended open-end alloy barrels in the finite element software. Preliminary parametric scanning is used to analyze the influence of various dimensions, such as length, radius, radial spacing, axial spacing and other parameters on the magnetic shielding effectiveness. Then, the Nelder-Mead local optimization method is used to obtain the structural parameters with the optimal magnetic shielding effectiveness under dimensional constraints. S2: Determine the target working area, select multiple target field points on the central axis of the target area, test and analyze the magnetic field strength of the target field points, and normalize the magnetic field strength to obtain the expression of the target magnetic field; S3: Design corresponding axial and radial nonlinear gradient coils. Based on the Fourier-Bessel expansion of the magnetic vector potential in the cylindrical coordinate system, combined with the boundary value equations at the interface of high magnetic permeability materials and the mirror method, obtain the analytical equations of the magnetic field components generated by the axial and radial nonlinear gradient coils in the internal space of the magnetic shielding barrel under the coupling of high magnetic permeability and high electrical conductivity materials. Then use the particle swarm optimization algorithm to globally optimize and obtain the coil parameters that accurately compensate for the magnetic field in the magnetic shielding barrel.

3. The design method of a single-ended open-type high-performance magnetic shielding system according to claim 1 is characterized in that: The method of normalizing the magnetic field intensity to obtain the target magnetic field includes: in, represents the target magnetic field along the x, y and z directions at the jth target point, B x,y,z,j is the actual measured magnetic field along the x, y and z directions at the jth target point, B x,y,z,1 are the actually measured magnetic fields at the first target point along the x, y and z directions respectively, where j = 1, 2, …Num.

4. The design method of a single-ended open-type high-performance magnetic shielding system according to claim 1 is characterized in that: The method for obtaining the magnetic field components generated by the axial and radial nonlinear gradient coils in the inner space of the magnetic shielding barrel under the coupling effect of the high magnetic permeability and high electrical conductivity materials includes: in, a b =(μ r μ0-μ0)I′ m (ka)K m (kb) b b =μ r μ0I m (kb)K′ m (kb)-μ0I′ m (kb)K m (kb) c b =(μ r μ0-μ0)I m (kb)K′ m (kb) a c =(μ r μ0-μ0)I′ m (ka)K m (kc) b c =(μ r μ0-μ0)I′ m (kb)K m (kc) c c =μ r μ0I′ m (kc)K m (kc)-μ0I m (kc)K′ m (kc) Among them, μ r is the relative magnetic permeability of the material, ρ, φ and z represent the radial distance, azimuth and height of the target point in the cylindrical coordinate system; B ρ (ρ,φ,z),B φ (ρ,φ,z) and B z (ρ, φ, z) represents the magnetic field components of the target field point along the radial distance, azimuth and altitude in the cylindrical coordinate system, which are converted into the magnetic field generated by the coil at the target point in the Cartesian coordinate system by the coordinate system conversion formula μ0 is the vacuum magnetic permeability; a is the coil radius; b is the inner radius of the innermost shielding material of the magnetic shielding barrel; c is the outer radius of the innermost shielding material of the magnetic shielding barrel; I m , K m are the first and second modified Bessel functions, respectively, and I′ m and K′ m is the corresponding first-order derivative; k and m are integral parameters and have no practical meaning; i is an imaginary number and has no practical meaning; R m , α b , β b , γ b , α c , β c and γ c It is an intermediate variable and has no practical significance.

5. The design method of a single-ended open-type high-performance magnetic shielding system according to claim 4 is characterized in that: is the Fourier transform expression of the current density of the coil, which is related to the coil configuration. It is expressed as: The axial nonlinear coil is composed of Q current-carrying rings, the planes of each ring are parallel to the xoy plane, and the centers of the rings are located on the axis of the magnetic shielding barrel; the distance between the qth ring and the bottom of the magnetic shielding barrel is d q , the number of turns is n q , the current flowing through is I q , where q = 1, 2…Q; Radial nonlinear gradient coil It is expressed as: The radial nonlinear coil consists of T pairs of saddle-shaped windings, and the distance between the inner and outer planes of the tth pair of windings and the bottom of the barrel is l t1 and l t2 , the winding arc angle is The number of turns is n t , the current flowing through is I t , where t=1,2…T.

6. The design method of a single-ended open-type high-performance magnetic shielding system according to claim 1 is characterized in that: The method of using the particle swarm optimization algorithm to globally optimize and obtain the coil parameters for accurately compensating the magnetic field in the magnetic shielding barrel includes: s.t.d min ≤d q ≤d max l min ≤l t1 ≤l max l min ≤l t2 ≤l max n q ,n t ∈N + I q ,I t ∈N + Among them, d min and d max is the minimum and maximum value of the current-carrying ring position of the axial nonlinear gradient coil, l min and l max are the minimum and maximum values ​​of the saddle winding height of the radial nonlinear gradient coil.

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