Magnet system

Abstract

The utility model relates to the technical field of electron beam transmission, in particular to a magnet system. Comprising a first shaping unit and a second shaping unit, and the first shaping unit comprises first quadrupole iron tightly attached to an outlet of the accelerating tube and first octupole iron arranged at the focus position of the first quadrupole iron; the second shaping unit comprises second quadrupole iron and second octupole iron, the second quadrupole iron and the first octupole iron meet a first preset distance, the second octupole iron is arranged at the focus position of the second quadrupole iron, and the second shaping unit and the radiation field area meet a second preset distance; the inner diameter of the first octupole iron and the inner diameter of the second octupole iron are both larger than or equal to the maximum size of an electron beam group. Therefore, the quadrupole iron and the octupole iron are utilized to form the shaping unit which is homogenized in one direction, so that the problems that in the prior art, in the electron beam transmission process, a large amount of dose is lost, the size of a radiation field is limited, and the radiation field cannot be adjusted at will are solved; therefore, the electron beam bunch generated by the linear accelerator can be expanded and homogenized in a short distance.

Description

Technical Field

[0001] The utility model relates to the technical field of electron beam transmission, in particular to a magnet system. Background Art

[0002] High-energy electron beams are usually generated by accelerators. They have the characteristics of stable energy, small size, and easy use. They can be used for preclinical radiobiology experiments on small animals, clinical radiotherapy of superficial skin and intraoperatively, surface industrial irradiation, such as material surface modification, disinfection of bacteria, viruses, and pests on the surface of objects, irradiation sterilization of food, and safety inspection of cargo vehicles. They are of great significance to people's production and life. In applications such as electron beam radiotherapy and irradiation disinfection, the uniformity of the dose within the radiation field is crucial. Uneven doses will cause different parts of the irradiated object to receive different doses, affecting the final radiotherapy or disinfection effect. In severe cases, it may even harm the body and cause significant economic losses.

[0003] In related technologies, the dose homogenization scheme within the radiation field usually uses the scattering attenuation method, which utilizes two scattering sheets, which are usually made of materials with a smaller atomic number Z, such as aluminum, resin, etc. The first scattering sheet is a regular cylinder with a thickness that varies depending on the material. After a certain distance, the second scattering sheet is in the shape of a truncated cone with a larger radius on the top and a smaller radius on the bottom, so as to achieve the homogenization goal.

[0004] However, in this method, a large amount of dose will be lost after the electron beam passes through two scattering plates, resulting in a certain amount of waste; the size of the radiation field has certain limitations and cannot be adjusted at will. The fixed radiation field can only be adjusted by replacing different scattering plates, which urgently needs to be improved. Utility Model Content

[0005] The utility model provides a magnet system to solve the problems of large dose loss in the electron beam transmission process caused by the prior art solution, and the limitation of the field size and the inability to adjust it at will, so as to achieve the expansion and homogenization of the electron beam clusters generated by the linear accelerator within a relatively short distance.

[0006] To achieve the above-mentioned object, the first embodiment of the utility model provides a magnet system, including: a first shaping unit and a second shaping unit, wherein:

[0007] The first shaping unit includes a first quadrupole close to the accelerating tube outlet and a first octupole arranged at the focal position of the first quadrupole;

[0008] The second shaping unit includes a second quadrupole that meets a first preset distance with the first octopole and a second octopole that is arranged at a focal position of the second quadrupole, wherein the second shaping unit meets a second preset distance with the field area;

[0009] The inner diameter of the first octupol and the inner diameter of the second octupol are both greater than or equal to the maximum size of an electron bunch passing through, and the electron bunch is emitted from the accelerating tube outlet.

[0010] According to an embodiment of the present invention, the focus of the first quadrupole is greater than a first preset value.

[0011] According to an embodiment of the present invention, the inner diameter of the first octopole is larger than the inner diameter of the first quadrupole, and the inner diameter of the second octopole is larger than the inner diameter of the second quadrupole.

[0012] According to an embodiment of the present invention, the homogenization direction of the first shaping unit is perpendicular to the homogenization direction of the second shaping unit.

[0013] According to an embodiment of the present invention, the homogenization direction of the first shaping unit is a lateral direction, and the homogenization direction of the second shaping unit is a vertical direction.

[0014] According to an embodiment of the present invention, the lengths of the first quadrupole, the second quadrupole, the first octupole and the second octupole are all less than or equal to a second preset value.

[0015] According to an embodiment of the present invention, the remanence of the first quadrupole iron, the second quadrupole iron, the first octupole iron and the second octupole iron are all fixed values, which are determined by the materials of the corresponding magnets.

[0016] According to an embodiment of the present invention, the first quadrupole iron, the second quadrupole iron, the first octupole iron and the second octupole iron are all Halbach type magnets.

[0017] According to the magnet system proposed in the embodiment of the utility model, the first shaping unit includes a first quadrupole iron close to the outlet of the accelerating tube and a first octopole iron arranged at the focal position of the first quadrupole iron; the second shaping unit includes a second quadrupole iron that meets the first preset distance with the first octopole iron and a second octopole iron arranged at the focal position of the second quadrupole iron, wherein the second shaping unit meets the second preset distance with the field area, and the inner diameter of the first octopole iron and the inner diameter of the second octopole iron are both greater than or equal to the maximum size of the electron bunch passing through. Therefore, by using the quadrupole iron and the octopole iron to form a shaping unit for homogenizing one direction, the existing technical solution solves the problem that a large amount of dose loss will be caused during the transmission of the electron beam, and the field size is limited and cannot be adjusted at will, thereby achieving the expansion and homogenization of the electron bunch generated by the linear accelerator within a relatively short distance.

[0018] Additional aspects and advantages of the present invention will be given in part in the following description and in part will become apparent from the following description or will be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0020] Figure 1 A block diagram of a magnet system provided according to an embodiment of the utility model;

[0021] Figure 2 A schematic diagram of a design scheme for uniform radiation field dose in the related art;

[0022] Figure 3 is a schematic diagram of a Halbach type quadrupole iron according to an embodiment of the utility model;

[0023] Figure 4 A schematic diagram of magnetic field distribution of a quadrupole iron according to an embodiment of the utility model;

[0024] Figure 5 Schematic diagram of magnetic flux density distribution of a Halbach-type octupole iron according to an embodiment of the utility model;

[0025] Figure 6 It is a schematic diagram of the density distribution of the bunches in the transverse (x) direction under different octopole iron strengths according to one embodiment of the utility model;

[0026] Figure 7 It is a schematic diagram of the size distribution and phase space distribution of the bunches before entering the octopole iron according to one embodiment of the utility model;

[0027] Figure 8 It is a structural schematic diagram of a shaping unit according to an embodiment of the utility model;

[0028] Fig. 9 It is a schematic diagram of a square radiation field according to an embodiment of the utility model. DETAILED DESCRIPTION

[0029] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0030] The magnet system proposed according to the embodiment of the utility model is described below with reference to the drawings.

[0031] Figure 1 It is a block diagram of a magnet system according to an embodiment of the utility model.

[0032] Before introducing the magnet system proposed in the embodiment of the utility model, a solution for homogenizing the dose within the radiation field in the related art is briefly introduced.

[0033] In the related art, the homogenization scheme usually uses the scattering attenuation method, such as Figure 2 As shown in the figure, this method uses two scattering sheets, which are usually made of materials with a smaller atomic number Z, such as aluminum, resin, etc. After being emitted from the accelerator tube, the narrow beam of electrons passes through the first scattering sheet. The first scattering sheet is a regular cylinder, and the thickness has different values ​​according to the material. Its main purpose is to expand the divergence angle of the electron beam, that is, the beam expansion effect; after a certain distance, the second scattering sheet is in the shape of a truncated cone, with a larger radius on the top and a smaller radius on the bottom. The purpose of this is to attenuate more particles with a large center density. As the density decreases, the attenuation capacity decreases, thereby achieving the goal of homogenization.

[0034] However, although this scheme can simply achieve the goal of beam expansion and uniform dose within the field, it has certain disadvantages. First, the electron beam will lose a large amount of dose after passing through the two scattering plates, and this part of the dose is not used, resulting in a certain amount of waste; secondly, the size of the field has certain limitations and cannot be adjusted at will. It can only be adjusted by replacing different scattering plates to achieve a fixed field. Therefore, a method is needed to obtain a uniform target field without losing dose, and to conveniently and continuously adjust the field size.

[0035] Based on the above problems, an embodiment of the utility model proposes a magnet system, which utilizes quadrupole iron and octupole iron to form a shaping unit for homogenization in one direction, thereby solving the problems of large dose loss in the electron beam transmission process and the limitation of the field size, which cannot be adjusted at will in the prior art solution. The electron beam cluster generated by the linear accelerator can be expanded and homogenized within a short distance, and uniform dose irradiation can be achieved within a larger field at the target.

[0036] For example, Figure 1As shown, the magnet system 10 includes: a first shaping unit 100 and a second shaping unit 200. The first shaping unit 100 includes a first quadrupole 101 close to the accelerating tube outlet and a first octopole 102 arranged at the focal position of the first quadrupole 101; the second shaping unit 200 includes a second quadrupole 201 that meets a first preset distance with the first octopole 102 and a second octopole 202 that is arranged at the focal position of the second quadrupole 201, wherein the second shaping unit 200 meets a second preset distance with the field area; the inner diameter of the first octopole 102 and the inner diameter of the second octopole 202 are both greater than or equal to the maximum size of the electron bunch that passes through, and the electron bunch is emitted from the accelerating tube outlet.

[0037] It is understandable that charged particles moving in a magnetic field will be subject to a force perpendicular to the direction of motion, called the Lorentz force, and the magnitude of the force can be calculated by the following formula:

[0038] F = qv × B;

[0039] Among them, q is the charge of the particle, v is the speed of the particle (vector), and B is the magnetic field strength (vector).

[0040] Based on the movement laws of charged particles in a magnetic field, the corresponding magnetic elements, such as dipole iron, quadrupole iron, octapole iron, etc., can be designed in reverse according to the desired electron trajectory.

[0041] Taking the Halbach-type quadrupole iron as an example, the following introduces the magnetic field distribution characteristics of the quadrupole iron and its influence on the trajectory of electron motion.

[0042] like Figure 3 As shown in the figure, the light-colored area is the aluminum shell, the dark-colored area is the permanent magnet material, and the arrows indicate the magnetization direction of each permanent magnet. It can be seen that the magnets No. 1-4 constitute a magnetic pole period of the quadrupole iron, and finally the following can be obtained: Figure 4 The magnetic field distribution is shown.

[0043] Since permanent magnets are used, the current density is 0, and Maxwell's equation can be written as:

[0044]

[0045] Where B is the magnetic field of the quadrupole iron.

[0046] Consider the field along the longitudinal direction to be zero (i.e. B s =0) and does not change (i.e. ), ignoring the higher-order multipoles, we can obtain:

[0047]

[0048] Among them, G is the magnetic field gradient in the two transverse directions. It can be proved that the magnetic field gradually increases radially from the center, which can achieve the transverse expansion or focusing effect on the external bunch in one direction.

[0049] Similarly, the eight-level iron has eight magnetic poles, and the magnetic flux density distribution is as follows Figure 5 As shown. The octupole ferromagnetic field has a large magnetic field gradient change in the radial direction near the magnet. The particles outside the bunch will be subjected to a larger Lorentz force, and the direction of the force makes the external particles move inward. Usually, the electrons emitted from the cathode are Gaussian distributed in the transverse direction. The particles with sparse density outside are forced to shrink inward, while the high-density particles inside are not affected, achieving a uniform effect, such as Figure 6 Curve ① is the density distribution of particles without passing through octopole iron, and the other curves are the density distribution of particles after passing through octopole iron of different strengths. Therefore, the center can be regarded as a uniform distribution.

[0050] It should be noted that the uniformity effect of a single octopole is only for one dimension. For example, after the x-direction is uniformized, the y-direction is still Gaussian. The bunch must be flat before entering the octopole. The direction to be uniformized is the long side. The phase space distribution of the bunch is approximately an oblique straight line, such as Figure 7 As shown, Figure 7 (a) is a schematic diagram of the beam size distribution before entering the octopole. Figure 7 (b) Schematic diagram of the phase space distribution before entering the octupole iron.

[0051] To change the initial circular Gaussian distribution into the above distribution, only one quadrupole is needed. Therefore, one quadrupole and one octupole can form a shaping unit that is uniform in one direction. In terms of position, the octupole is placed at the focus of the quadrupole, such as Figure 8 As shown in Figure 2, the two shaping units can homogenize the entire beam bunch and finally obtain a square uniform radiation field, as shown in Figure 2. Fig. 9 shown.

[0052] That is to say, in order to expand and homogenize the electron beam bunch generated by the linear accelerator, the embodiment of the utility model can use quadrupole iron and octopole iron to set two shaping units, that is, the first shaping unit 100 is composed of a first quadrupole iron 101 and a first octopole iron 102. For the electron beam bunch generated by the accelerating tube, the initial distribution is a Gaussian distribution and has a large scattering angle, which means that as the transmission distance increases, the size of the electron beam bunch will also become larger and larger. In order to facilitate the design of the magnet and save space, the first quadrupole iron 101 can be positioned close to the accelerating tube outlet, which helps to reduce the inner diameter of the first magnet, and the first octopole iron 102 is set at the focal position of the first quadrupole iron 101. The second shaping unit 200 includes a second quadrupole iron 201 and a second octopole iron 202. The second quadrupole iron 201 and the first octopole iron 102 meet a first preset distance, and the second octopole iron 202 is set at the focal position of the second quadrupole iron 201, wherein the second shaping unit 200 meets a second preset distance with the field area.

[0053] Among them, the first preset distance and the second preset distance can be values ​​pre-set by researchers in the field, or values ​​obtained through a limited number of experiments, or values ​​after a limited number of computer simulations, and are not specifically limited here. That is, after the bunch is ejected from the outlet of the first octupole 102 of the first shaping unit 100, it needs to pass through a certain distance before becoming evenly distributed, and this distance is usually very short. Therefore, the first preset distance and the second preset distance can be freely set according to the range requirements of the target field area. Since the final field area is square and has two dimensions (i.e., the length and width of the square), the target length and target width can be obtained according to the range requirements of the target field area, and the target length and target width can be simulated and controlled in the simulation software through the first preset distance between the first shaping unit 100 and the second shaping unit 200 and the second preset distance between the second shaping unit 200 and the field area.

[0054] Therefore, when both the quadrupole iron and the octupole iron are permanent magnetic materials (i.e., the strength remains unchanged), the size of the final radiation field can be changed within a small range by adjusting the relative positions between the magnets. The operation is simpler and more convenient, and can be better applied to applications such as radiotherapy or disinfection.

[0055] The design parameters of the magnet in the shaping unit mainly include: magnet inner diameter, magnet outer diameter, magnet length and residual magnetism. When designing the above parameters, the embodiment of the utility model usually fixes the residual magnetism of the magnetic steel and changes the strength of the magnet by adjusting the magnet inner diameter and magnet length. The design process of the relevant parameters in the embodiment of the utility model is described in detail below.

[0056] In order to ensure that there is no beam loss, the inner diameter of all magnets should be larger than the maximum size of the electron bunch passing through at this time. Figure 7 As shown, the size of the electron bunch in one direction is almost 0, and the size in the other direction is much larger than 0. Therefore, when designing the inner diameter of the octopole iron in the embodiment of the utility model, the maximum size of the electron bunch can be referred to, and its inner diameter should be close to the maximum size of the electron bunch passing through at this time (the electron bunch is emitted from the outlet of the accelerating tube), so as to achieve the best shaping effect on the electron bunch. It should be noted that the inner diameter of the octopole iron should not be set too close to the maximum size of the electron bunch to prevent any slight deviation or vibration in actual operation from causing the beam to contact the inner wall of the magnet. Therefore, a certain safety margin can be reserved during design.

[0057] Therefore, in the embodiment of the present invention, the inner diameter of the first octopole 102 and the inner diameter of the second octopole 202 are both set to be greater than or equal to the maximum size of the electron bunch passing through, so as to ensure that the electron bunch can pass through the octopole smoothly without loss.

[0058] Optionally, in some embodiments, the focus of the first quadrupole iron 101 is greater than a first preset value.

[0059] The first preset value may be a value pre-set by researchers in the field, a value obtained through a limited number of experiments, or a value obtained through a limited number of computer simulations, which is not specifically limited here. Preferably, the first preset value is set to 3-4 cm in the embodiment of the utility model.

[0060] It is understandable that, for the quadrupole, the position of its focus can be changed by adjusting the inner diameter and length of the magnet to meet specific requirements. Since the first octopole 102 is set at the focus position of the first quadrupole 101, in order to avoid the two magnets being placed too close to generate a force, the focus of the first quadrupole 101 needs to be larger than the first preset value (such as 3-4cm).

[0061] Optionally, in some embodiments, the inner diameter of the first octupolar iron 102 is greater than the inner diameter of the first quadrupole iron 101 , and the inner diameter of the second octupolar iron 202 is greater than the inner diameter of the second quadrupole iron 201 .

[0062] It can be understood that for the electron bunches generated by the accelerating tube, the initial distribution is a Gaussian distribution and has a large scattering angle, which means that as the transmission distance increases, the size of the electron bunch will become larger and larger, and the quadrupole iron is located before the octopole iron. Therefore, the inner diameter of the first octopole iron 102 is larger than the inner diameter of the first quadrupole iron 101, and the inner diameter of the second octopole iron 202 is larger than the inner diameter of the second quadrupole iron 201, thereby ensuring the stability and integrity of the electron bunches during transmission.

[0063] Optionally, in some embodiments, the homogenization direction of the first shaping unit 100 is perpendicular to the homogenization direction of the second shaping unit 200 .

[0064] It should be noted that one shaping unit can only homogenize the electron beam in one direction. In order to finally obtain a square uniform radiation field, the homogenization direction of the first shaping unit 100 is perpendicular to the homogenization direction of the second shaping unit 200. By precisely configuring and adjusting the two shaping units, precise control of the shape of the electron beam can be achieved.

[0065] Optionally, in some embodiments, the homogenization direction of the first shaping unit 100 is a lateral direction, and the homogenization direction of the second shaping unit 200 is a vertical direction.

[0066] Specifically, the first shaping unit 100 can focus on homogenizing the electron bunch in the lateral direction (horizontal direction), while the second shaping unit 200 can focus on homogenizing the electron bunch in the vertical direction (vertical direction). Through such a configuration, the two shaping units work together to ensure that the electron bunch is uniformly shaped in both directions. In addition, the first shaping unit 100 can also focus on homogenizing the electron bunch in the vertical direction (vertical direction), while the second shaping unit 200 focuses on homogenizing the electron bunch in the lateral direction (horizontal direction), which is not specifically limited here.

[0067] It should be noted that, assuming that the first shaping unit 100 homogenizes the electron bunch in the lateral direction, and the second shaping unit 20 homogenizes the electron bunch in the vertical direction, after passing through the second shaping unit 200, although the vertical direction can show uniform distribution within a shorter distance, the lateral direction needs to pass through a longer distance than it to show uniform distribution. This is because the lateral direction is focused at the second shaping unit 200, and the focusing force will not be too large due to the limitation of the inner diameter. The bunch needs to pass through a distance of twice the focal length to form an obvious uniform distribution in the lateral direction.

[0068] Optionally, in some embodiments, the lengths of the first quadrupole iron 101 , the second quadrupole iron 102 , the first octopole iron 201 , and the second octopole iron 202 are all less than or equal to a second preset value.

[0069] The second preset value may be a value pre-set by researchers in the field, a value obtained through a limited number of experiments, or a value obtained through a limited number of computer simulations, which is not specifically limited here. Preferably, the second preset value is set to 2 cm in the embodiment of the utility model.

[0070] Specifically, the determination of the magnet length can be calculated by simulation software. From a theoretical perspective, the shorter the length, the better, generally not exceeding the second preset value (such as 2 cm). That is, a magnet system model including the first shaping unit 100 and the second shaping unit 200 is established in the simulation software, and the initial length is set for the quadrupole iron and the octupole iron. The motion trajectory of the electron bunch when passing through the quadrupole iron and the octupole iron is simulated in the model, and the motion trajectory changes of the particle in the non-uniform direction (usually horizontal or vertical direction) are observed. By adjusting the length of the quadrupole iron and the octupole iron, how the length change affects the motion trajectory of the particle is observed, thereby determining a suitable length value, which not only meets the length limit but also does not affect the motion trajectory of the particle in the non-uniform direction. It should be noted that the length of the octupole iron in each shaping unit should not be too long, otherwise it will affect the motion trajectory of the bunch in another direction.

[0071] Optionally, in some embodiments, the remanence of the first quadrupole iron 101 , the second quadrupole iron 102 , the first octopole iron 201 , and the second octopole iron 202 are all fixed values, which are determined by the materials of the corresponding magnets.

[0072] It can be understood that remanence refers to the magnetization intensity that a magnet can still maintain when there is no external magnetic field. The size of the remanence is generally related to the material of the magnet. The commonly used permanent magnet material is neodymium iron boron. For example, the maximum remanence of the N48H model neodymium iron boron is about 1.37T. Usually, the maximum remanence of this material is used as a fixed value. Therefore, after determining the permanent magnet material, the remanence is generally determined. The remanence is generally fixed at about 1.3T to 1.4T, which is the commonly used strength of permanent magnet materials. By selecting the appropriate permanent magnet material, it can be ensured that the shaping unit performs stably and efficiently in the magnet system.

[0073] Optionally, in some embodiments, the first quadrupole 101 , the second quadrupole 102 , the first octopole 201 , and the second octopole 202 are all Halbach type magnets.

[0074] It is understandable that for quadrupole and octopole, their main function is to adjust the lateral size and shape of the electron bunch. Compared with quadrupole and octopole of ordinary configuration, Halbach magnet has higher magnetic field strength. The special feature of Halbach magnet is that it can concentrate the magnetic field in one direction while generating almost no magnetic field in the other direction. This feature can reduce electromagnetic interference to other components and systems. By specially designing the Halbach magnet arrangement, the shape and strength of the magnetic field can be precisely controlled, thereby achieving precise control of the electron bunch.

[0075] According to the magnet system proposed in the embodiment of the utility model, the first shaping unit includes a first quadrupole iron close to the outlet of the accelerating tube and a first octupole iron arranged at the focal position of the first quadrupole iron; the second shaping unit includes a second quadrupole iron that meets the first preset distance with the first octupole iron and a second octupole iron that is arranged at the focal position of the second quadrupole iron, wherein the second shaping unit meets the second preset distance with the field area. Thus, by using the quadrupole iron and the octupole iron to form a shaping unit that is homogenized in one direction, the problem that the existing technical solution causes a large amount of dose loss during the electron beam transmission process, and the field size is limited and cannot be adjusted at will is solved, and the electron bunch generated by the linear accelerator is expanded and homogenized within a relatively short distance, and uniform irradiation is achieved in a larger field at the target. In addition, the utility model is applicable to electrons of any energy. For electrons of different energies, the parameter design of the magnet is not fixed, and personalized design can be performed in the simulation software.

[0076] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present utility model, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0077] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the utility model. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0078] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations of the present invention. Ordinary technicians in the field can change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A magnet system, characterized in that: include: A first shaping unit and a second shaping unit, wherein The first shaping unit includes a first quadrupole close to the accelerating tube outlet and a first octupole arranged at the focal position of the first quadrupole; The second shaping unit includes a second quadrupole that meets a first preset distance with the first octopole and a second octopole that is arranged at a focal position of the second quadrupole, wherein the second shaping unit meets a second preset distance with the field area; The inner diameter of the first octupol and the inner diameter of the second octupol are both greater than or equal to the maximum size of an electron bunch passing through, and the electron bunch is emitted from the accelerating tube outlet.

2. The magnet system according to claim 1, characterized in that The focus of the first quadrupole iron is greater than a first preset value.

3. The magnet system according to claim 2, characterized in that The inner diameter of the first octuple iron is larger than the inner diameter of the first quadrupole iron, and the inner diameter of the second octuple iron is larger than the inner diameter of the second quadrupole iron.

4. The magnet system according to claim 1, characterized in that The homogenization direction of the first shaping unit is perpendicular to the homogenization direction of the second shaping unit.

5. The magnet system according to claim 4, characterized in that The homogenization direction of the first shaping unit is a transverse direction, and the homogenization direction of the second shaping unit is a vertical direction.

6. The magnet system according to claim 1, characterized in that The lengths of the first quadrupole, the second quadrupole, the first octupole, and the second octupole are all less than or equal to a second preset value.

7. The magnet system according to claim 1, characterized in that The remanence of the first quadrupole iron, the second quadrupole iron, the first octupole iron and the second octupole iron are all fixed values, which are determined by the materials of the corresponding magnets.

8. The magnet system according to any one of claims 1 to 7, characterized in that The first quadrupole iron, the second quadrupole iron, the first octupole iron and the second octupole iron are all Halbach type magnets.

Images