Permanent magnet four-pole and eight-pole combined beam diffusion device and diffusion method thereof

Abstract

The application discloses a permanent magnet four-pole and eight-pole combined beam diffusion device and a diffusion method thereof, and relates to the technical field of electron accelerator beam shaping. The device comprises a first permanent magnet four-pole unit, a second permanent magnet four-pole unit, a third permanent magnet four-pole unit, a first permanent magnet eight-pole unit and a second permanent magnet eight-pole unit arranged in sequence along the beam propagation direction. The static magnetic field is generated by using permanent magnets, and no excitation power supply and coil water cooling is configured during operation, thereby effectively reducing the system complexity, land area and supporting cost. The quadrupole linear field generated by the three coaxial adjacent permanent magnet four-pole units with the same structure can continuously focus and defocus the electron beam in two orthogonal transverse planes, thereby realizing linear adjustment of the beam envelope and phase space correlation. The two permanent magnet eight-pole units with different structures and arranged adjacently generate two stages of asymmetric eight-pole nonlinear fields in sequence, and the deflection force of the off-axis particles nonlinearly changes with the transverse displacement.

Description

Technical Field

[0001] This invention relates to the field of electron accelerator beam shaping technology, and in particular to a permanent magnet quad-pole octole combined beam diffusion device and its diffusion method. Background Technology

[0002] Electron accelerators have wide applications in irradiation processing, medical sterilization, and material modification. Beam diffusion and shaping devices are key components at the accelerator's end, expanding the electron beam, which has a small cross-section, into a uniform beam spot covering the target irradiation area, thereby improving irradiation efficiency and processing uniformity. Currently, for electron irradiation accelerators with energies of 10 meV and above, the end-beam diffusion typically employs a scanning magnet system. This type of system drives the electron beam to reciprocate at high speed in the horizontal direction using an alternating magnetic field, coordinating with the transport motion of the irradiated object to form an irradiation surface covering the target area. A scanning magnet system generally includes a scanning magnet body, a high-power excitation power supply, a scanning waveform generation and synchronization control circuit, and coil water cooling equipment. Furthermore, to improve dose uniformity in the scanning edge region, some systems also require waveform compensation circuits or asymmetric scanning strategies. In permanent magnet multipole beam shaping, Halbach-type multipole magnets composed of fan-shaped magnetic blocks are mostly used, resulting in a complex number and structure of magnetic blocks, and specific reports on diffusion uniformity have not been available.

[0003] However, the aforementioned existing technical solutions have at least the following shortcomings: First, the scanning magnet system is complex, requiring a large number of peripheral devices and occupying a large area. Furthermore, harmonic interference from the excitation power supply may affect the stability of the accelerator control system. The water-cooling system suffers from safety hazards such as pipe aging and joint leaks, increasing the workload and cost of daily operation and maintenance. Second, the alternating magnetic field generated by the scanning magnet exhibits velocity nonlinearity at the beginning and end of the scanning cycle, causing the beam spot's residence time at the scanning edge region to differ from that in the central region, reducing the uniformity of the irradiation dose. Even with compensation measures, this is difficult to completely eliminate. Third, the scanning magnet system continuously consumes electrical energy and generates a large amount of heat during operation, resulting in high energy consumption, which does not meet the development requirements of miniaturization and low energy consumption for accelerator devices. Fourth, in the permanent magnet multipole scheme, the existing technology involves complex combinations of magnetic blocks, requiring a large number of blocks. It is also difficult to simultaneously achieve linear adjustment of the beam envelope and two-dimensional nonlinear uniform expansion of the beam spot within a short axial distance, and the beam spot uniformity and shape controllability need further improvement. Summary of the Invention

[0004] The permanent magnet quad-pole and octole combined beam diffusion device and its diffusion method provided by the present invention can solve the problems of complex structure, high supporting cost and high energy consumption of existing electron accelerator end beam scanning system, as well as the difficulty in processing existing permanent magnet multipole schemes and the difficulty in achieving coordinated optimization of beam envelope linear adjustment and beam spot two-dimensional uniform expansion in a compact space.

[0005] A permanent magnet quadrupole and octupole combined beam diffusion device is applied to an electron irradiation accelerator. It includes a first permanent magnet quadrupole unit, a second permanent magnet quadrupole unit, a third permanent magnet quadrupole unit, a first permanent magnet octupole unit, and a second permanent magnet octupole unit arranged sequentially along the beam propagation direction. Each unit includes a soft magnetic yoke, neodymium iron boron (NdFeB) permanent magnet blocks embedded inside the soft magnetic yoke, and a central through-hole for the vacuum beam box to pass through and for the electron beam to pass through. The permanent magnet quadrupole unit contains four NdFeB permanent magnet blocks, and the permanent magnet octupole unit contains eight NdFeB permanent magnet blocks. The central through-holes of each unit are coaxially arranged. The first, second, and third permanent magnet quadrupole units have identical structures and are arranged adjacent to each other. The first and second permanent magnet octupole units have different structures and are arranged adjacent to each other. An axial gap exists between the third permanent magnet quadrupole unit and the first permanent magnet octupole unit.

[0006] The present invention provides a permanent magnet quadrupole-octupole combined beam diffusion device, which, compared with the prior art, has the following beneficial effects, but is not limited to: This permanent magnet quadrupole and octupole combined beam diffuser utilizes permanent magnets to generate a static magnetic field, eliminating the need for excitation power supplies and coil water cooling during operation, effectively reducing system complexity, land area, and associated costs. The quadrupole linear field generated by three coaxial, adjacent, and structurally identical permanent magnet quadrupole units can continuously focus and defocus the electron beam in two orthogonal transverse planes, achieving linear adjustment of the beam envelope and phase space correlation. The axial gap between the third permanent magnet quadrupole unit and the first permanent magnet octupole unit provides drift space for the electron beam adjusted by the quadrupole units, allowing the electron beam to enter the octupole unit with a predetermined transverse dimension and divergence angle, ensuring that the octupole nonlinear field effectively acts on off-axis particles. To avoid interference from leakage magnetic field at the four extreme ends on the magnetic field inside the octagonal through-hole, two adjacent permanent magnet octagonal units with different structures sequentially generate two levels of asymmetric octagonal nonlinear fields. These fields apply a deflection force to off-axis particles that varies nonlinearly with lateral displacement, causing particles at the beam edge to expand outward and compensating for the particle surface density in the edge region. Within a compact axial space, this works in coordination with the upstream four-pole linear field to achieve two-dimensional uniform expansion and distribution shaping of the beam spot. This effectively solves the problems of complex scanning system structure, high supporting costs, high energy consumption, and the difficulty in processing existing permanent magnet multipole schemes, as well as the difficulty in achieving coordinated optimization of beam envelope linear adjustment and two-dimensional uniform beam spot expansion within a compact space.

[0007] Furthermore, the soft magnetic yoke is made of electromagnetic pure iron, and the remanence of the neodymium iron boron permanent magnet is 1.4T-1.5T. A portion of the neodymium iron boron permanent magnet is embedded in the soft magnetic yoke, and the other portion faces the central through hole.

[0008] Furthermore, the neodymium iron boron permanent magnet block is disposed in the limiting groove of the soft magnetic yoke, and the neodymium iron boron permanent magnet block is detachably connected to the limiting groove by a non-magnetic metal pressure plate.

[0009] Furthermore, the soft magnetic yoke is a closed rectangular frame structure, and the neodymium iron boron permanent magnet is a long strip block structure. Each of the neodymium iron boron permanent magnets is arranged at intervals along the inner sidewall of the soft magnetic yoke and is embedded in the limiting groove in the radial direction of the soft magnetic yoke.

[0010] Furthermore, the lateral dimensions of the first permanent magnet quadrupole unit, the second permanent magnet quadrupole unit, and the third permanent magnet quadrupole unit are all 230mm×210mm-250mm×230mm, and the axial thickness is all 45mm-65mm.

[0011] Furthermore, the first permanent magnet octet unit has a lateral dimension of 250mm×200mm-270mm×220mm and an axial thickness of 30mm-50mm, while the second permanent magnet octet unit has a lateral dimension of 250mm×230mm-270mm×250mm and an axial thickness of 50mm-70mm.

[0012] Furthermore, the axial distance between the third permanent magnet quadrupole unit and the first permanent magnet octupole unit is 10mm-30mm.

[0013] Furthermore, the four neodymium iron boron permanent magnet blocks of the first permanent magnet quadrupole unit, the second permanent magnet quadrupole unit, and the third permanent magnet quadrupole unit are arranged in a quadrupole symmetrical manner with N and S poles, and the eight neodymium iron boron permanent magnet blocks of the first permanent magnet octupole unit and the second permanent magnet octupole unit are arranged in an octupole symmetrical manner with N and S poles.

[0014] A permanent magnet quadrupole-octupole combined beam diffusion method, based on the aforementioned permanent magnet quadrupole-octupole combined beam diffusion device, includes the following steps: S1, the electron beam sequentially passes through a first permanent magnet quadrupole unit, a second permanent magnet quadrupole unit, and a third permanent magnet quadrupole unit arranged adjacent to each other along the beam axis. The quadrupole linear fields generated by the three quadrupole units continuously focus and defocus the electron beam in the same transverse plane to adjust the transverse envelope and phase space correlation of the electron beam; S2, the electron beam after passing through the third permanent magnet quadrupole unit drifts through an axial gap and enters the first permanent magnet octupole unit. The octupole nonlinear field generated by the first permanent magnet octupole unit applies a deflection force to off-axis particles that varies nonlinearly with transverse displacement, causing the particles at the beam edge to expand outward; S3, the electron beam after passing through the first permanent magnet octupole unit enters the second permanent magnet octupole unit. The octupole nonlinear field generated by the second permanent magnet octupole unit compensates for and redistributes the particle surface density in the beam edge region, shaping the beam into a rectangular transverse distribution and forming a rectangular beam spot on a predetermined downstream observation surface.

[0015] Furthermore, in S3, the particle coordinates on the predetermined downstream observation surface are exported, and a rectangular target area with a length of 25cm-35cm and a width of 5cm-7cm is statistically analyzed. The statistical area is divided into equal-width strips along the length of the target area, and the number of particles D in each strip is used as the distribution characterization quantity. Attached Figure Description

[0016] Figure 1 This is a front view of a permanent magnet quadrupole-octupole combined beam diffusion device according to an embodiment of the present invention; Figure 2 This is a top view of a permanent magnet quadrupole-octupole combined beam diffusion device according to an embodiment of the present invention; Figure 3 This is a side view of a permanent magnet quadrupole-octupole combined beam diffusion device according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the axial arrangement structure of a permanent magnet quad-pole octole combined beam diffusion device according to an embodiment of the present invention; Figure 5 for Figure 3 Schematic diagram of the structure of a medium-soft magnetic yoke; Figure 6 for Figure 1 Schematic diagram of the structure of the first permanent magnet quadrupole unit; Figure 7 for Figure 1 Schematic diagram of the structure of the first permanent magnet octet unit; Figure 8 for Figure 1 Schematic diagram of the structure of the second permanent magnet octet unit; Figure 9 This is a particle distribution map of the target observation surface obtained from CST particle tracking; Figure 10 A particle position distribution map within a 30cm×6cm target area obtained by MATLAB statistics; Figure 11 The results show the number of particles and the non-uniformity along the length of the target region. Figure 12 This is a flowchart of a permanent magnet quad-pole octole combined beam diffusion method according to an embodiment of the present invention.

[0017] Explanation of reference numerals in the attached figures: 1. First permanent magnet quadrupole unit; 2. Second permanent magnet quadrupole unit; 3. Third permanent magnet quadrupole unit; 4. First permanent magnet octupole unit; 5. Second permanent magnet octupole unit; 6. Soft magnetic yoke; 7. Neodymium iron boron permanent magnet block; 8. Central through hole; 9. Beam axis; 61. Limiting groove; 62. Non-magnetic metal pressure plate. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, specific embodiments of this application are described clearly and completely below with reference to the accompanying drawings. It should be understood that the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments described in this application without creative effort will fall within the scope of protection of this application.

[0019] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in the specification of this application is for the purpose of describing specific embodiments only and is not intended to limit this application; the terms "comprising," "including," "having," "containing," "comprise," etc., in the specification, claims, and accompanying drawings of this application are open-ended terms, indicating that a method comprises one or more steps, or an apparatus comprises one or more elements, but do not exclude the inclusion of other steps or elements. The terms "first," "second," etc., in the specification, claims, or accompanying drawings of this application are used to distinguish different objects, not to describe a specific order or primary / secondary relationship. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0020] In the description of this application, it should be understood that the terms "upper", "lower", "left", "right", "front", "rear", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0021] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "attachment" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0022] In this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, in this application, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0023] like Figure 1-4 As shown in the figure, an embodiment of the present invention provides a permanent magnet quadrupole and octupole combined beam diffusion device, which is applied to an electron irradiation accelerator. It includes a first permanent magnet quadrupole unit 1, a second permanent magnet quadrupole unit 2, a third permanent magnet quadrupole unit 3, a first permanent magnet octupole unit 4, and a second permanent magnet octupole unit 5 arranged sequentially along the beam propagation direction. Each unit includes a soft magnetic yoke 6, neodymium iron boron permanent magnet blocks 7 embedded in the soft magnetic yoke 6, and a central through hole 8 for the vacuum beam box to pass through and for the electron beam to pass through. Four neodymium iron boron permanent magnet blocks 7 are arranged in the permanent magnet quadrupole unit, and eight neodymium iron boron permanent magnet blocks 7 are arranged in the permanent magnet octupole unit. The central through holes 8 of each unit are coaxially arranged. The first permanent magnet quadrupole unit 1, the second permanent magnet quadrupole unit 2, and the third permanent magnet quadrupole unit 3 have the same structure and are arranged adjacent to each other. The first permanent magnet octupole unit 4 and the second permanent magnet octupole unit 5 have different structures and are arranged adjacent to each other. There is an axial gap between the third permanent magnet quadrupole unit 3 and the first permanent magnet octupole unit 4.

[0024] In this embodiment, by using permanent magnets to generate a static magnetic field, no magnet excitation power supply or coil water cooling is required during operation, effectively reducing system complexity, land area, and supporting costs. Simultaneously, the quadrupole linear field generated by three coaxial adjacent and structurally identical permanent magnet quadrupole units can continuously focus and defocus the electron beam in two orthogonal transverse planes, achieving linear adjustment of the beam envelope and phase space correlation. The axial gap between the third permanent magnet quadrupole unit 3 and the first permanent magnet octupole unit 4 provides drift space for the electron beam adjusted by the three permanent magnet quadrupole units, allowing the electron beam to enter the first permanent magnet octupole unit 4 and the second permanent magnet octupole unit 5 with a predetermined transverse dimension and divergence angle, thereby ensuring the octupole nonlinear field generated by the permanent magnet octupole unit. It can effectively act on off-axis particles while avoiding interference from the end leakage magnetic field of the permanent magnet quadrupole unit to the magnetic field inside the through hole of the permanent magnet octupole unit. Two permanent magnet octupole units with different structures and arranged adjacently generate two levels of asymmetric octupole nonlinear fields in sequence, which apply a deflection force to the off-axis particles that varies nonlinearly with the lateral displacement, causing the particles at the edge of the beam to expand outward and compensating for the particle surface density in the edge region. Thus, in a compact axial space, it works in coordination with the quadrupole linear field generated by the upstream permanent magnet quadrupole unit to complete the two-dimensional uniform expansion and distribution shaping of the beam spot. It effectively solves the problems of complex scanning system structure, high supporting cost, high energy consumption, and difficulty in achieving linear adjustment and uniform expansion synergistic optimization in a compact space in the existing technology.

[0025] Furthermore, the soft magnetic yoke 6 is made of electromagnetic pure iron, and the remanence of the neodymium iron boron permanent magnet block 7 is 1.4T-1.5T. Part of the neodymium iron boron permanent magnet block 7 is embedded in the soft magnetic yoke 6, and the other part faces the central through hole 8.

[0026] In this embodiment, the soft magnetic yoke 6 is made of electromagnetic pure iron, which allows the magnetic flux generated by the permanent magnet block to form a closed magnetic circuit along the yoke, effectively reducing magnetic leakage and improving magnetic field utilization efficiency. Simultaneously, the electromagnetic pure iron has good saturation magnetic induction and low coercivity, facilitating the control of the magnetic circuit's on / off state during the loading and unloading of the permanent magnet block. The remanence of the neodymium iron boron permanent magnet block 7 is 1.4T-1.5T, ensuring that a sufficiently strong quadrupole and octupole magnetic field can be generated within the central through-hole 8 to meet the beam current adjustment requirements, while avoiding excessively high remanence that could increase stray multipole components and affect... The uniformity of the electron beam spot is improved. A portion of the neodymium iron boron permanent magnet 7 is embedded in the soft magnetic yoke 6, while the other portion faces the central through hole 8. The embedded portion ensures a stable connection and good magnetic contact between the permanent magnet and the soft magnetic yoke 6, while the exposed portion facing the central through hole 8 allows the magnetic flux generated by the permanent magnet to directly act on the electron beam region within the central through hole 8. This effectively shortens the magnetic circuit length and reduces the magnetic resistance, thereby providing a stable high-intensity multipole magnetic field in a permanent magnet manner without the need for an excitation power supply and coil water cooling. This further reduces the operating cost and maintenance difficulty of the device.

[0027] Preferably, the soft magnetic yoke 6 is made of DT4 electrical pure iron, the remanence Br of the neodymium iron boron permanent magnet block 7 is 1.45T, and the relative uncertainty of the remanence measurement is 0.2%.

[0028] like Figure 3 and Figure 5 As shown, the neodymium iron boron permanent magnet block 7 is set in the limiting groove 61 of the soft magnetic yoke 6, and the neodymium iron boron permanent magnet block 7 is detachably connected to the limiting groove 61 through the non-magnetic metal pressure plate 62.

[0029] In this embodiment, the neodymium iron boron permanent magnet 7 is disposed within the limiting groove 61 of the soft magnetic yoke 6. The limiting groove 61 precisely defines the radial position of the neodymium iron boron permanent magnet 7 through its shoulder, ensuring that the installation position of each neodymium iron boron permanent magnet 7 inside the soft magnetic yoke 6 is accurate and consistent. This is beneficial for ensuring the symmetry and stability of the magnetic field distribution within the central through hole 8. The neodymium iron boron permanent magnet 7 is detachably connected to the limiting groove 61 by a non-magnetic metal pressure plate 62. The non-magnetic metal pressure plate 62 effectively fixes the neodymium iron boron permanent magnet 7 without interfering with the multipole magnetic field within the central through hole 8. The purity of the magnetic field is guaranteed. The detachable connection method makes the installation and disassembly of the neodymium iron boron permanent magnet block 7 convenient. During the assembly stage, the permanent magnet blocks can be easily regrouped or their positions finely adjusted according to the magnetic field measurement results to optimize the magnetic field parameters. During the later maintenance stage, the permanent magnet blocks with degraded performance can be replaced individually without scrapping the entire magnet unit, which effectively reduces manufacturing and maintenance costs. At the same time, the non-magnetic metal pressure plate 62 and the corresponding fastening structure are all located outside the central through hole 8, which avoids the fixed structure from blocking the electron beam path and ensures that the electron beam passes through the through hole without obstruction.

[0030] By selecting non-magnetic metal pressure plates 62 of different thicknesses, the exposed size of each NdFeB permanent magnet 7 relative to the central through hole 8 can be finely adjusted. Since the change in exposed size directly changes the air gap magnetic reluctance in the central through hole 8 region, an increase in exposed size reduces air gap magnetic reluctance and increases magnetic flux density in the through hole, while a decrease in exposed size increases air gap magnetic reluctance and weakens magnetic flux density in the through hole. This allows for simple and high-precision non-magnetization fine-tuning of the magnetic field strength in the through hole without replacing or remagnetizing the permanent magnet 7. At the same time, since the remanence of a single NdFeB permanent magnet 7 has slight discrete differences, the exposed size of the corresponding permanent magnet 7 can be adjusted by arranging non-magnetic metal pressure plates 62 of different thicknesses at 180° symmetrical positions to compensate for individual magnetic performance differences. This effectively improves the symmetry and uniformity of the magnetic field distribution in the through hole, thereby further improving the beam spot uniformity.

[0031] like Figure 3 and Figure 5 As shown, the soft magnetic yoke 6 is a closed rectangular frame structure, and the neodymium iron boron permanent magnet block 7 is a long strip block structure. Each neodymium iron boron permanent magnet block 7 is arranged at intervals along the inner sidewall of the soft magnetic yoke 6 and is embedded into the limiting groove 61 in the radial direction of the soft magnetic yoke 6.

[0032] In this embodiment, the closed rectangular frame structure of the soft magnetic yoke 6 has advantages over traditional annular magnetic yokes in terms of simple processing, clear positioning reference, and ease of setting processing references and measurement coordinate systems on the outside of the yoke, which is beneficial to improving the manufacturing accuracy and assembly alignment accuracy of each magnet unit. The elongated block structure of the neodymium iron boron permanent magnet 7 has advantages over traditional fan-shaped or arc-shaped permanent magnets in terms of low processing difficulty, controllable cost, and good dimensional consistency, which is beneficial to reducing the scrap rate in the manufacturing process of the magnet and ensuring stable performance during mass production. Each neodymium iron boron permanent magnet 7 is arranged at intervals along the inner sidewall of the soft magnetic yoke 6 and is embedded in the limiting groove 61 in the radial direction, so that each elongated permanent magnet can be independently installed or removed from the outside without disassembling adjacent magnets, which greatly simplifies the assembly and disassembly of permanent magnets. During the assembly stage, the position of a single permanent magnet can be easily fine-tuned or regrouped according to the magnetic field measurement results. During the maintenance stage, a permanent magnet with degraded performance can be replaced individually without disassembling the entire magnet unit.

[0033] In the radial embedding method, the permanent magnet block is limited in radial position by the shoulder of the limiting groove 61 and in axial position by the non-magnetic metal pressure plate 62. The limiting in the two directions is independent of each other, which facilitates precise control separately. In addition, the spaced arrangement allows the magnetic circuit channel formed by the yoke material to be retained between adjacent permanent magnet blocks, which is conducive to the magnetic flux generated by each permanent magnet block forming a closed loop inside the soft magnetic yoke 6, reducing magnetic leakage and improving magnetic field utilization efficiency.

[0034] like Figure 6 , Figure 7 and Figure 8 As shown, the lateral dimensions of the first permanent magnet quadrupole unit 1, the second permanent magnet quadrupole unit 2, and the third permanent magnet quadrupole unit 3 are all 230mm×210mm-250mm×230mm, and the axial thickness is 45mm-65mm. The lateral dimensions of the first permanent magnet octupole unit 4 are 250mm×200mm-270mm×220mm, and the axial thickness is 30mm-50mm. The lateral dimensions of the second permanent magnet octupole unit 5 are 250mm×230mm-270mm×250mm, and the axial thickness is 50mm-70mm.

[0035] In this embodiment, the three permanent magnet quadrupole units have identical lateral shapes and axial thicknesses, allowing them to share the same set of machining fixtures and measuring tools during manufacturing. This effectively reduces manufacturing costs and ensures the consistency of the magnetic field characteristics of the three units. Furthermore, the three structurally identical quadrupole units are arranged compactly adjacent to each other, with a moderate axial thickness, which helps to provide sufficient quadrupole field integration length within a limited axial space to effectively adjust the beam envelope. The two permanent magnet octupole units employ different lateral shapes and axial thicknesses, resulting in different strengths and action lengths of the octupole nonlinear fields generated within the through-hole. This creates a two-stage asymmetric octupole nonlinear effect. The first permanent magnet octupole unit 4 has a relatively lower axial thickness. The first permanent magnet quadrupole unit is thinner, focusing on applying nonlinear deflection forces to off-axis particles to achieve coarse diffusion. The second permanent magnet octupole unit is relatively thicker axially, focusing on fine compensation and redistribution of particle surface density in the edge region of the initially expanded beam. The two different nonlinear effects work together to achieve better beam uniformity and rectangularity in a compact axial space. The overall dimensions of the three permanent magnet quadrupole units and the two permanent magnet octupole units are all controlled within a lateral range of less than 260 mm, and the total axial length can be controlled within 320 mm, which is convenient for installation in the limited space at the end of the electron irradiation accelerator. At the same time, the kit with the vacuum beam box does not require major modification to the main vacuum system of the existing accelerator, which significantly reduces the difficulty of equipment modification and installation cost.

[0036] Preferably, the first permanent magnet quadrupole unit 1, the second permanent magnet quadrupole unit 2, and the third permanent magnet quadrupole unit 3 all have a lateral dimension of 240mm × 220mm and an axial thickness of 55mm. The upper and lower borders of their rectangular magnetic yokes are 40mm wide. The permanent magnet block is 20mm from the outer edge of the yoke. The exposed dimension facing the central through-hole 8 is 25mm, and the dimension embedded in the yoke is 20mm. The lateral width of the permanent magnet block is 35mm. Specific positions and polarities are detailed below. Figure 6 The first permanent magnet octet unit 4 has a lateral dimension of 260mm × 210mm and an axial thickness of 40mm. Its yoke frame width is 40mm. The permanent magnet blocks on the upper and lower sides are 40.5mm from their respective outer edges, with an exposed dimension of 20mm and an embedded dimension of 15mm towards the central through-hole. The permanent magnet blocks on the left and right sides are 35mm from their respective outer edges, with an exposed dimension of 15mm and an embedded dimension of 20mm towards the central through-hole 8. Specific positions and polarities are detailed in [reference needed]. Figure 7The second permanent magnet octet unit 5 has a lateral dimension of 260mm × 240mm and an axial thickness of 60mm. Its upper and lower yoke frame widths are 40mm, corresponding to a distance of 40mm from the outer edge of the permanent magnet block. The exposed dimension towards the central through-hole is 16.86mm, and the embedded dimension is 30mm. Its left and right yoke frame widths are 50mm, corresponding to a distance of 30mm from the outer edge of the permanent magnet block. The exposed dimension towards the central through-hole 8 is 25mm, and the embedded dimension is 25mm. For specific positions and polarities, see [details omitted]. Figure 8 .

[0037] like Figure 1 and Figure 4 As shown, the axial distance between the third permanent magnet quadrupole unit 3 and the first permanent magnet octupole unit 4 is 10mm-30mm.

[0038] In this embodiment, the axial spacing provides the necessary drift space for the electron beam after focusing and defocusing by the three permanent magnet quadrupole units. This allows the lateral envelope of the electron beam to naturally diverge to a predetermined size before entering the first permanent magnet octupole unit 4. Since the deflection force of the octupole nonlinear field generated by the permanent magnet octupole unit on off-axis particles changes nonlinearly with lateral displacement, if the lateral size of the electron beam is too small, the nonlinear deflection force on the edge particles will be insufficient, making effective diffusion difficult. If the lateral size is too large, some particles may be lost on the inner wall of the through-hole. The axial spacing of 10mm-30mm ensures that the electron beam enters with a suitable lateral size and divergence angle. The insertion of the first permanent magnet octet unit 4 ensures that the octet nonlinear field has a sufficiently strong deflection effect on edge particles to achieve beam spot expansion, while avoiding particle loss caused by excessive beam envelope. It also effectively prevents leakage magnetic field at the end of the permanent magnet quadrupole unit from directly entering the through-hole region of the permanent magnet octet unit and interfering with the purity of the octet nonlinear field. At the same time, during the assembly and debugging of the device, the relative position between the third permanent magnet quadrupole unit 3 and the first permanent magnet octet unit 4 can be finely adjusted within this range according to the actual beam conditions to adapt to electron beams with different energies or emittances, effectively improving the process adaptability and debugging flexibility of the device.

[0039] Preferably, the third permanent magnet quadrupole unit 3 and the first permanent magnet octupole unit 4 are spaced 17.5 mm apart, and the total axial length of the unit group is 282.5 mm.

[0040] like Figure 6 , Figure 7 and Figure 8 As shown, the four neodymium iron boron permanent magnet blocks 7 of the first permanent magnet quadrupole unit 1, the second permanent magnet quadrupole unit 2 and the third permanent magnet quadrupole unit 3 are arranged in a quadrupole symmetrical manner with N and S poles, and the eight neodymium iron boron permanent magnet blocks 7 of the first permanent magnet octupole unit 4 and the second permanent magnet octupole unit 5 are arranged in an octupole symmetrical manner with N and S poles.

[0041] In this embodiment, the four neodymium iron boron permanent magnet blocks 7 of the first permanent magnet quadrupole unit 1, the second permanent magnet quadrupole unit 2, and the third permanent magnet quadrupole unit 3 are arranged in a quadrupole symmetrical manner with their N and S poles aligned. This allows each permanent magnet quadrupole unit to generate a linear magnetic field with the quadrupole as the main component within the central through-hole 8. The alternating N and S poles of adjacent magnetic blocks ensure the angular distribution accuracy of the quadrupole field, avoiding additional stray components such as dipoles or hexapoles caused by asymmetrical pole arrangement. This ensures the accuracy and symmetry of the quadrupole linear field's focusing and defocusing effect on the electron beam, providing a good foundation for subsequent octupole nonlinear shaping. The symmetrical beam envelope is based on the arrangement of eight neodymium iron boron permanent magnet blocks 7 in the first permanent magnet octagonal unit 4 and the second permanent magnet octagonal unit 5 in an octagonal symmetrical manner with the N and S poles oriented in the direction of the octagonal poles. This allows the two permanent magnet octagonal units to generate a nonlinear magnetic field with the octagonal poles as the main component within the central through hole 8. Furthermore, the symmetrical arrangement of the N and S poles of each magnetic block along the circumference ensures the angular purity of the octagonal field. The two-stage octagonal nonlinear field and the upstream quadrupole linear field work together in coordination under the condition of no stray magnetic field interference to achieve precise deflection of particles at the edge of the beam and uniform compensation of surface density. Finally, a rectangular beam spot with high uniformity is obtained at the predetermined downstream observation surface.

[0042] Preferred, such as Figure 6 As shown, in the first permanent magnet quadrupole unit 1, the second permanent magnet quadrupole unit 2, and the third permanent magnet quadrupole unit 3, two neodymium iron boron (NdFeB) permanent magnet blocks 7 are disposed on top of their soft magnetic yokes 6, and the other two are disposed on the bottom of their soft magnetic yokes 6. The NdFeB permanent magnet block 7 on the top right, facing the central through hole 8, is the S pole; the NdFeB permanent magnet block 7 on the top left, facing the central through hole 8, is the N pole; the NdFeB permanent magnet block 7 on the bottom right, facing the central through hole 8, is the N pole; and the NdFeB permanent magnet block 7 on the bottom left, facing the central through hole 8, is the S pole. The distance between the NdFeB permanent magnet block 7 on the top right and the NdFeB permanent magnet block 7 on the top left is δ1, and the distance between the NdFeB permanent magnet block 7 on the top right and the NdFeB permanent magnet block 7 on the bottom right is δ2. Figure 7 As shown, in the first permanent magnet octagonal unit 4, two neodymium iron boron permanent magnet blocks 7 are arranged around the perimeter of its soft magnetic yoke 6. The neodymium iron boron permanent magnet block 7 on the top left side of the soft magnetic yoke 6, facing the central through hole 8, is the S pole. Clockwise, the magnetic poles of the neodymium iron boron permanent magnet blocks 7 facing the central through hole 8 are alternately arranged as S and N poles. The distance between adjacent neodymium iron boron permanent magnet blocks 7 on the top and bottom surfaces is δ1. The distance between two neodymium iron boron permanent magnet blocks 7 on the top and bottom surfaces at the same horizontal plane is δ2. The distance between adjacent neodymium iron boron permanent magnet blocks 7 on adjacent surfaces is δ3. The distance between adjacent neodymium iron boron permanent magnet blocks 7 on the side surfaces is δ4. The distance between two neodymium iron boron permanent magnet blocks 7 on the two sides at the same horizontal plane is δ5. Figure 8As shown, in the second permanent magnet octagonal unit 5, two neodymium iron boron permanent magnet blocks 7 are arranged around the soft magnetic yoke 6. The neodymium iron boron permanent magnet block 7 on the top left side of the soft magnetic yoke 6 faces the central through hole 8 as the S pole. In a clockwise direction, the magnetic poles of the neodymium iron boron permanent magnet blocks 7 facing the central through hole 8 are alternately arranged as S pole and N pole. The distance between adjacent neodymium iron boron permanent magnet blocks 7 on the top and bottom surfaces is δ1. The distance between two neodymium iron boron permanent magnet blocks 7 on the top and bottom surfaces at the same horizontal plane is δ2. The distance between adjacent neodymium iron boron permanent magnet blocks 7 on adjacent surfaces is δ3. The distance between adjacent neodymium iron boron permanent magnet blocks 7 on the side surfaces is δ4. The distance between two neodymium iron boron permanent magnet blocks 7 on the two sides at the same horizontal plane is δ5.

[0043] Specifically, the magnetomotive force provided by the neodymium iron boron permanent magnet block 7 is closed through the DT4 yoke, the air region of the through hole, and the leakage magnetic path. The multipole magnetic field inside the through hole is jointly determined by the working point of the permanent magnet material, the magnetic reluctance of the yoke, the magnetic reluctance of the air, and the structural leakage magnetic field. Figure 6 , Figure 7 and Figure 8 In this context, δ1 to δ5 represent the equivalent magnetic gap or relative position parameters between permanent magnet blocks. Changing the δ parameter will simultaneously change the air reluctance of the main magnetic circuit, the operating point of the permanent magnet block, and the angular distribution of the magnetic field within the magnet through-hole, thereby adjusting the integral quadrupole gradient, the integral octupole component, and the stray multipole component.

[0044] In the quadrupole unit, δ1 and δ2 are mainly used to adjust the effective aperture in the horizontal and vertical directions and the quadrupole gradient ratio. In the octupole unit, δ1 to δ5 together determine the relative positions of each permanent magnet block, used to adjust the octupole component and the nonlinear effects in the horizontal and vertical directions. The δ parameters are determined by the permanent magnet block size, embedding depth, exposed size, and installation position, and are optimized during the magnet design and assembly stages. The magnetic field calculation uses the demagnetization curve of NdFeB permanent magnet material, the actual BH curve of DT4 material, and a three-dimensional end field model.

[0045] A three-dimensional static magnetic field model was established using CST Studio Suite, and particle tracking was performed based on the phase space distribution of the 10meV electron beam at the magnet entrance. The particle coordinates on the predetermined downstream observation surface were exported to MATLAB, and statistical analysis was conducted on a rectangular target region with a length of 30cm and a width of 6cm. Figure 9 The results show the CST particle distribution. Figure 10 The distribution of particle positions within the target area is given, and the total number of particles within the target area is 10686.

[0046] The statistical region is divided into equal-width strips along the length of the target region. The particle number non-uniformity is calculated using the particle number non-uniformity within each strip, with the particle number D within each strip as the distribution characteristic, according to the following formula: ρ= (Dmax Dmin) / (Dmax +Dmin); In the formula, Dmax and Dmin represent the maximum and minimum number of particles in each statistical band, respectively. Figure 10 The result is ρ=0.0956, which means that the particle number non-uniformity along the length of the target region is 9.56%.

[0047] The total axial length of the three permanent magnet quadrupole units and the two permanent magnet octupole units is 282.5 mm. The units are coaxially and compactly arranged, which can be installed in the limited space at the end of the 10 meV electron irradiation accelerator. Each magnet unit uses a fixed permanent magnetic field, which eliminates the need for magnet excitation power supply and coil water cooling during steady-state operation. This reduces the excitation power supply, scanning waveform and synchronization control links, and lowers the power supply, cooling and maintenance costs. Under given 10 meV electron beam inlet conditions and observation surface position, particle tracking yields a rectangular distribution area of ​​approximately 30 cm × 6 cm, with a particle number non-uniformity of 9.56% along the length direction.

[0048] A permanent magnet quadrupole-octupole combined beam diffusion method, based on the aforementioned permanent magnet quadrupole-octupole combined beam diffusion device, includes the following steps: S1, the electron beam is sequentially passed along the beam axis 9 through a first permanent magnet quadrupole unit 1, a second permanent magnet quadrupole unit 2, and a third permanent magnet quadrupole unit 3, which are structurally identical and adjacent to each other. The quadrupole linear fields generated by the three quadrupole units continuously focus and defocus the electron beam in the same transverse plane to adjust the transverse envelope and phase space correlation of the electron beam; S2, the electron beam after being acted upon by the third permanent magnet quadrupole unit 3... After drifting through the axial gap, the electron beam enters the first permanent magnet octagonal unit 4. The octagonal nonlinear field generated by the first permanent magnet octagonal unit 4 applies a deflection force to the off-axis particles that varies nonlinearly with the lateral displacement, causing the particles at the edge of the beam to expand outward. S3, the electron beam after passing through the first permanent magnet octagonal unit 4 enters the second permanent magnet octagonal unit 5. The octagonal nonlinear field generated by the second permanent magnet octagonal unit 5 compensates for and redistributes the particle surface density in the edge region of the beam, shaping the beam into a rectangular lateral distribution and forming a rectangular beam spot on the predetermined downstream observation surface.

[0049] In this embodiment, three identical and adjacent permanent magnet quadrupole units continuously focus and defocus the electron beam in the same lateral plane to adjust the lateral envelope and phase space correlation of the beam. After being acted upon by the third permanent magnet quadrupole unit, the electron beam drifts within the axial gap to obtain an appropriate lateral size before sequentially entering two permanent magnet octupole units with different structures. The octupole nonlinear field generated by the first permanent magnet octupole unit causes the particles at the edge of the beam to expand outward, achieving coarse diffusion. The octupole nonlinear field generated by the second permanent magnet octupole unit compensates for and redistributes the particle surface density in the edge region, achieving fine shaping. The two-stage asymmetric octupole nonlinear field and the upstream quadrupole linear field work together without any external excitation power supply or real-time feedback control to complete the entire process of beam envelope adjustment, beam spot expansion, and surface density shaping in a compact axial space. Finally, a uniform rectangular beam spot is formed on the predetermined downstream observation surface, effectively simplifying the diffusion operation and reducing the system complexity.

[0050] Specifically, before installing the magnets, the vacuum beam box is cleaned according to the requirements of the vacuum components, and its dimensions and airtightness are checked. The beam axis is determined using the external mechanical reference of the vacuum beam box. Before the downstream scanning box is connected and the titanium window assembly is installed, the first, second, and third permanent magnet quadrupole units are installed one by one from the open end of the vacuum beam box according to their final axial positions, so that the vacuum beam box passes through the central through-hole 8 of the five magnet units. Each magnet unit is installed on a common support reference independent of the vacuum beam box. The external machining reference of the magnetic yoke, the external reference of the vacuum beam box, and the beam design axis are used for joint alignment to make the magnetic center of the five magnet units coincide with the axis of the vacuum beam box. After adjusting the lateral position, pitch angle, yaw angle, and roll angle, each unit is locked, and it is confirmed that there is a non-contact gap between the vacuum beam box and the central through-hole of the magnet. Then, the downstream end of the vacuum beam box is connected to the scanning box, the titanium window assembly at the end of the scanning box is installed, and the airtightness is checked again. The electron beam passes sequentially through three permanent magnet quadrupole units, the first permanent magnet octupole unit, and the second permanent magnet octupole unit within the vacuum beam box. It then enters the scanning box and is led out through a titanium window to the area to be irradiated.

[0051] During assembly, non-magnetic positioning fixtures are used to control the lateral position, embedding depth, and polarity direction of the permanent magnet blocks. They are installed in pairs with central symmetry, and the N and S pole orientations are checked after each pair is installed. The permanent magnet blocks are inserted into the stepped limiting grooves of the yoke, with the radial position defined by the groove shoulders. Axial and radial mechanical limiting is implemented by non-magnetic metal pressure plates and fasteners located at the axial ends or outer sides of the yoke. Non-magnetic metal washers are placed between the pressure plates and the permanent magnet blocks. Fasteners use all-metal lock nuts, locking washers, or mechanical locking wires to prevent loosening. The pre-tightening force is limited to avoid damaging the permanent magnet blocks, and all fastening components are made of non-magnetic materials. The fixing structure is located outside the central through hole. Four permanent magnet blocks are installed in a four-pole unit, and eight permanent magnet blocks are installed in an eight-pole unit. After assembly, a measurement coordinate system is established using the external machining reference of the yoke and the central through hole to check the position, polarity, embedding depth, and axial thickness of the permanent magnet blocks.

[0052] After a single magnet unit is assembled, a three-dimensional Hall probe is used to perform a transverse grid scan within the central through-hole, and the end field is measured along the beam direction; alternatively, a rotating coil matched to the through-hole size can be used to measure the integrated multipole components. Based on the measurement results, the integral gradient of the quadrupole unit, the integral octupole component of the octupole unit, the magnetic center, and the stray multipole components are determined. Units exceeding the design tolerance are corrected by reassembling permanent magnet blocks, adjusting the non-magnetic confinement structure, or reassembling.

[0053] Furthermore, in S3, the particle coordinates on the predetermined downstream observation surface are exported, and the rectangular target area with a length of 25cm-35cm and a width of 5cm-7cm is statistically analyzed. The statistical area is divided into equal-width strips along the length of the target area, and the number of particles D in each strip is used as the distribution characterization quantity.

[0054] In this embodiment, by statistically analyzing the particles on the predetermined downstream observation surface, a rectangular target area with a length of 25cm-35cm and a width of 5cm-7cm is divided into strips of equal width along the length direction. The number of particles D in each strip is used as the distribution characterization quantity, and the particle number non-uniformity is calculated according to the formula ρ=(Dmax-Dmin) / (Dmax+Dmin). Quantitative calculation based on two-dimensional particle distribution data is more scientific and accurate than the qualitative judgment method of visually observing the beam spot shape. It can accurately quantify the uniformity of particle distribution in the length direction of the beam spot. At the same time, the formula is simple and computationally efficient, and evaluation results can be quickly obtained after particle tracking simulation or actual beam measurement. This provides reliable data support for the effectiveness verification of diffusion methods, the optimization and adjustment of device parameters, and the dose uniformity assessment of irradiation processing technology.

[0055] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A permanent magnet quadrupole-octupole combined beam diffusion device, applied to an electron irradiation accelerator, characterized in that: It includes a first permanent magnet quadrupole unit (1), a second permanent magnet quadrupole unit (2), a third permanent magnet quadrupole unit (3), a first permanent magnet octupole unit (4), and a second permanent magnet octupole unit (5) arranged sequentially along the beam propagation direction; Each unit includes a soft magnetic yoke (6), a neodymium iron boron permanent magnet block (7) embedded inside the soft magnetic yoke (6), and a central through hole (8) for the vacuum beam box to pass through and for the electron beam to pass through. The permanent magnet quadrupole unit is provided with four neodymium iron boron permanent magnet blocks (7), and the permanent magnet octupole unit is provided with eight neodymium iron boron permanent magnet blocks (7). The central through holes (8) of each unit are coaxially arranged. The first permanent magnet quadrupole unit (1), the second permanent magnet quadrupole unit (2) and the third permanent magnet quadrupole unit (3) have the same structure and are arranged adjacent to each other. The first permanent magnet octupole unit (4) and the second permanent magnet octupole unit (5) have different structures and are arranged adjacent to each other. There is an axial gap between the third permanent magnet quadrupole unit (3) and the first permanent magnet octupole unit (4).

2. The permanent magnet quadrupole-octupole combined beam diffusion device as described in claim 1, characterized in that, The soft magnetic yoke (6) is made of electromagnetic pure iron, and the remanence of the neodymium iron boron permanent magnet block (7) is 1.4T-1.5T. A part of the neodymium iron boron permanent magnet block (7) is embedded in the soft magnetic yoke (6), and the other part faces the central through hole (8).

3. The permanent magnet quadrupole-octupole combined beam diffusion device as described in claim 1, characterized in that, The neodymium iron boron permanent magnet block (7) is set in the limiting groove (61) of the soft magnetic yoke (6), and the neodymium iron boron permanent magnet block (7) is detachably connected to the limiting groove (61) by a non-magnetic metal pressure plate (62).

4. The permanent magnet quadrupole-octupole combined beam diffusion device as described in claim 3, characterized in that, The soft magnetic yoke (6) is a closed rectangular frame structure, and the neodymium iron boron permanent magnet block (7) is a long strip block structure. Each of the neodymium iron boron permanent magnet blocks (7) is arranged at intervals along the inner sidewall of the soft magnetic yoke (6) and is embedded in the limiting groove (61) in the radial direction of the soft magnetic yoke (6).

5. The permanent magnet quadrupole-octupole combined beam diffusion device as described in claim 1, characterized in that, The first permanent magnet quadrupole unit (1), the second permanent magnet quadrupole unit (2) and the third permanent magnet quadrupole unit (3) all have a lateral external dimension of 230mm×210mm-250mm×230mm and an axial thickness of 45mm-65mm.

6. The permanent magnet quadrupole-octupole combined beam diffusion device as described in claim 1, characterized in that, The first permanent magnet octagonal unit (4) has a lateral external dimension of 250mm×200mm-270mm×220mm and an axial thickness of 30mm-50mm. The second permanent magnet octagonal unit (5) has a lateral external dimension of 250mm×230mm-270mm×250mm and an axial thickness of 50mm-70mm.

7. The permanent magnet quadrupole-octupole combined beam diffusion device as described in claim 1, characterized in that, The axial distance between the third permanent magnet quadrupole unit (3) and the first permanent magnet octupole unit (4) is 10mm-30mm.

8. The permanent magnet quadrupole-octupole combined beam diffusion device as described in claim 1, characterized in that, The four neodymium iron boron permanent magnet blocks (7) of the first permanent magnet quadrupole unit (1), the second permanent magnet quadrupole unit (2) and the third permanent magnet quadrupole unit (3) are arranged in the N and S pole directions in a quadrupole symmetrical manner, and the eight neodymium iron boron permanent magnet blocks (7) of the first permanent magnet octupole unit (4) and the second permanent magnet octupole unit (5) are arranged in the N and S pole directions in an octupole symmetrical manner.

9. A permanent magnet quadrupole / octupole combined beam diffusion method, characterized in that, The permanent magnet quadrupole-octupole combined beam diffusion device applied to any one of claims 1-8 includes the following steps: S1. The electron beam passes sequentially through the first permanent magnet quadrupole unit (1), the second permanent magnet quadrupole unit (2) and the third permanent magnet quadrupole unit (3) with the same structure and adjacent arrangement along the beam axis (9). The quadrupole linear field generated by the three quadrupole units continuously focuses and defocuses the electron beam in the same transverse plane to adjust the transverse envelope and phase space correlation of the electron beam. S2. After being acted upon by the third permanent magnet quadrupole unit (3), the electron beam drifts through the axial gap and enters the first permanent magnet octupole unit (4). The octupole nonlinear field generated by the first permanent magnet octupole unit (4) applies a deflection force to the off-axis particles that varies nonlinearly with the lateral displacement, causing the particles at the edge of the beam to expand outward. S3. After being acted upon by the first permanent magnet octagonal unit (4), the electron beam enters the second permanent magnet octagonal unit (5). The octagonal nonlinear field generated by the second permanent magnet octagonal unit (5) compensates for and redistributes the particle surface density in the edge region of the beam, shaping the beam into a rectangular lateral distribution and forming a rectangular beam spot on the predetermined downstream observation surface.

10. The permanent magnet quadrupole-octupole combined beam diffusion method as described in claim 9, characterized in that, In S3, the particle coordinates on the predetermined downstream observation surface are exported, and a rectangular target area with a length of 25cm-35cm and a width of 5cm-7cm is statistically analyzed. The statistical area is divided into equal-width strips along the length of the target area, and the number of particles D in each strip is used as the distribution characterization quantity.

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