Ultra-short linear beam homogenization transmission line

Abstract

The invention discloses an ultrashort linear beam homogenization transmission line. A linear high-current homogenization beam line is sequentially provided with an accelerator outlet, a beam matching mechanism, four-eight-pole composite iron 1, four-eight-pole composite iron 2, a beam homogenization effect observation mechanism and a terminal in the beam direction. The four-and-eight-pole composite iron 1 and the four-and-eight-pole composite iron 2 are used for respectively and simultaneously generating a quadrupole magnetic field and an eight-pole magnetic field, so that the effects of quadrupole iron and eight-pole iron can be simultaneously realized by only installing one transmission element; the four-and-eight-pole composite iron 1 and the four-and-eight-pole composite iron 2 are based on a four-and-eight-pole composite magnet system. According to the invention, the problem that the effect of octopole iron cannot be fully exerted because a place with the largest envelope in the prior art is usually occupied by a quadrupole magnetic field is solved; the problem that the field intensity requirement of the eight-pole magnet is very high and is difficult to realize in engineering due to the fact that the beam envelope at the position of the existing eight-pole iron is relatively small is solved. And the problems of large occupied area and high cost caused by overlong beam line length are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of accelerator transmission lines, and particularly relates to an ultra-short linear beam homogenization transmission line. Background Art

[0002] The secondary particles generated after the accelerator extracts the beam and hits the target are important radiation sources required for many nuclear technology research and applications. For example, in nuclear technology applications such as neutron imaging and boron neutron capture therapy based on high-intensity proton accelerators, the accelerator generates a high-energy and high-intensity proton beam. After passing through the transmission line, it hits the neutron target to generate a high-flux neutron.

[0003] The distribution of the beam extracted by the accelerator is generally approximately Gaussian, with the highest particle density at the center of the bunch. At the same time, it also causes excessive local power and temperature on the neutron target, resulting in damage to the neutron target.

[0004] In order to solve the problem of uneven particle distribution, octupole magnets are tried to be used on the beam transmission line to homogenize the Gaussian-distributed beam and reduce the peak power density on the neutron target.

[0005] One of the difficulties in using octupole magnets to homogenize the Gaussian-distributed beam is that: when the same octupole iron is installed at different positions on the beam transmission line, the role it plays will be different. Only when the octupole iron is installed at a position with a larger beam envelope can it provide the strongest homogenization effect. However, in the prior art, the place with the largest envelope is usually occupied by the quadrupole magnetic field: as Figure 6 shown, the transmission line of the prior art is arranged in the order of quadrupole iron 1, octupole iron 1, quadrupole iron 2, and octupole iron 2. Since the octupole iron has to focus first before homogenization (the role of focusing is to allow the beam to pass through the circular hole in the center of the octupole iron without hitting the octupole iron), in order to ensure focusing, the positions of octupole iron 1 and octupole iron 2 must first be selected at the waist position of the X-direction envelope and the waist position of the Y-direction envelope. However, since the Y-direction envelope of quadrupole iron 1 at the waist position of the X-direction envelope is small, and the X-direction envelope of quadrupole iron 2 at the waist position of the Y-direction envelope is small, at octupole iron 1, although the waist position of octupole iron 1 in the X direction is ensured, the Y-direction envelope at this place is small. Similarly, at octupole iron 2, although the waist position of octupole iron 2 in the Y direction is ensured, the X-direction envelope at this place is small. In short, according to the traditional method, the beam envelope has significantly decreased after the particles pass through quadrupole iron 1 and quadrupole iron 2, so the roles of octupole iron 1 and octupole iron 2 cannot be fully exerted.

[0006] The second difficulty in using octupole magnets to homogenize a Gaussian beam lies in that: due to the small beam envelopes at the positions of octupole magnet 1 and octupole magnet 2, the field strength K value of the octupole magnet is very large. Such a high magnetic field strength is not only difficult to achieve in engineering, but also too costly and hard to implement.

[0007] The third difficulty in using octupole magnets to homogenize a Gaussian beam is that: if octupole magnets are to be used to achieve a good beam homogenization effect, there are relatively strict requirements for the positions where the octupole magnets are placed and their phases. To meet these requirements, the length of the transmission line often needs to be more than ten meters. The diameter of the cyclotron does not exceed 2 meters. Dragging a beam line of more than ten meters is equivalent to making the accelerator smaller, which has no meaning. The longer the beam line, the more expensive the surrounding shielding civil engineering will be. In short, the disadvantages of a too long transmission line, such as large floor area and high cost, greatly limit the use of beam homogenization technology. Summary of the Invention

[0008] In view of the problems existing in the prior art, the present invention proposes an ultra-short linear beam homogenization transmission line. The first objective is to solve the problem that when discrete quadrupole magnets and octupole magnets are used in the prior art, since the place with the largest envelope is usually occupied by the quadrupole magnetic field, the role of the octupole magnet cannot be fully exerted; the second objective is to solve the problem that when discrete quadrupole magnets and octupole magnets are used, due to the small beam envelopes at the positions of octupole magnet 1 and octupole magnet 2, the requirement for the field strength of the octupole magnet is very high, and such a high magnetic field strength is too costly and hard to implement in engineering; the third objective is to solve the problem that when discrete quadrupole magnets and octupole magnets are used, the too long length of the transmission line results in more expensive surrounding shielding civil engineering, large floor area and high cost.

[0009] The present invention adopts the following technical solutions to solve its technical problems:

[0010] An ultra-short linear beam homogenization transmission line, characterized in that: the transmission line is a linear beam homogenization transmission line including two quadrupole-octupole composite irons; the linear beam homogenization transmission line is successively provided with: an accelerator outlet, a beam matching mechanism, quadrupole-octupole composite iron 1, quadrupole-octupole composite iron 2, a beam homogenization effect observation mechanism, and a terminal along the beam direction;

[0011] The quadrupole-octupole composite iron 1 and the quadrupole-octupole composite iron 2 are used to simultaneously generate a quadrupole magnetic field and an octupole magnetic field respectively, so that the functions of the quadrupole magnet and the octupole magnet can be realized simultaneously by installing only one transmission element; at the quadrupole-octupole composite iron 1, the beam envelope function in the Y direction or the X direction reaches a larger value, and at the quadrupole-octupole composite iron 2, the beam envelope function in the X direction or the Y direction reaches a larger value; the phase shift of the particles between the two quadrupole-octupole composite magnets and the target Are respectively close to integer multiples of 180 degrees (0, 1, 2, 3...); the transfer matrix between the two octupole magnetic fields is close to the identity matrix;

[0012] The quadrupole-octupole composite iron 1 and the quadrupole-octupole composite iron 2 are based on a quadrupole-octupole composite magnet system, which includes a composite magnet current control device, a composite magnet main power supply, and a quadrupole-octupole composite magnet; the composite magnet current control device is used to control the output of the quadrupole field coil current and the octupole field coil current of the composite magnet main power supply to the quadrupole-octupole composite magnet;

[0013] The beam matching mechanism is used to observe the initial state of the beam extracted from the accelerator, adjust the envelope size of the beam in the X or Y direction according to the initial state, and align the beam center with the mechanical center of the beam pipe;

[0014] The beam homogenization effect observation mechanism is used to observe the beam current intensity, the shape of the weak beam, and the cross-section of the strong beam after homogenization by the quadrupole-octupole composite iron 1 and the quadrupole-octupole composite iron 2.

[0015] Further, the beam matching mechanism includes a fluorescent target 1, a quadrupole magnet, and a guiding magnet; the fluorescent target 1 provides the initial state of the beam for the tester; the quadrupole magnet is used to form a large beam envelope in the Y direction or the X direction at the position of the quadrupole-octupole composite iron 1, and the guiding magnet is used to align the beam center with the mechanical center of the beam pipe.

[0016] Further, the beam homogenization effect observation mechanism includes a Faraday cup, a fluorescent target 2, and a double wire. The Faraday cup is used to measure the beam current intensity after homogenization, the fluorescent target 2 is used to observe the shape of the weak beam after homogenization; the double wire is used to observe the shape of the strong beam after homogenization.

[0017] Further, the phase shift of the particles between the two quadrupole-octupole composite magnets and the target Is close to an integer multiple of 180 degrees, which means close to but not equal to an integer multiple of 180 degrees: Let Be The remainder of The value is generally less than ±15 degrees.

[0018] Further, the transfer matrix between the two octupole magnetic fields is close to the identity matrix, that is, the phase shift between the front and rear quadrupole-octupole composite magnets is controlled within 30 degrees. This 30-degree range can greatly avoid the high-order nonlinear effects caused by the coupling of the two octupole magnets and can obtain a better homogenization effect.

[0019] Further, the expression of the magnet strength k at the quadrupole-octupole composite iron 1 and the quadrupole-octupole composite iron 2 is:

[0020]

[0021] Let: The starting point of the transport line is denoted as 0, the position of the first quadrupole-octupole magnet is denoted as 1, the position of the second quadrupole-octupole magnet is denoted as 2, and the position at the end point, which is the position of the target, is denoted as 3; ux02 in the above formula (1) represents the phase shift in the x direction between positions 0 and 2 of the particle, and ux23 represents the phase shift in the x direction between positions 2 and 3 of the particle; βx2 represents the envelope function in the x direction at position 2; uy01 in the above formula (2) represents the phase shift in the y direction between 0 and 1 of the particle; uy13 represents the phase shift in the y direction between 1 and 3 of the particle; βy1 represents the envelope function in the y direction at position 1.

[0022] Furthermore, the composite magnet current control device includes: a module for establishing a two-dimensional sampling point of coil current, a module for experimentally measuring and initially establishing a three-dimensional surface sample library of magnetic field gradients, a module for using interpolation to refine the three-dimensional surface sample library of magnetic field gradients, a module for solving the corresponding field gradient current curve for the input field gradient, a module for solving the intersection point of the quadrupole / octupole field gradient current curves, and a module for outputting the quadrupole / octupole coil current;

[0023] The module for establishing a two-dimensional sampling point of coil current is used to establish a two-dimensional data comparison table of the coil current of the quadrupole iron and the octupole iron;

[0024] The module for experimentally measuring and initially establishing a three-dimensional surface sample library of magnetic field gradients uses the current values in the two-dimensional data comparison table of coil current to conduct magnetic field experiments on the composite iron, so as to obtain the measured values of the quadrupole and octupole magnetic field gradients corresponding one-to-one to the two-dimensional current data comparison table, thereby obtaining a three-dimensional surface sample database of magnetic field gradients; the above three-dimensional surface sample database of magnetic field gradients includes a three-dimensional surface sample database of the quadrupole field magnetic field gradient and a three-dimensional surface sample database of the octupole field magnetic field gradient;

[0025] The module for using interpolation to refine the three-dimensional surface sample library of magnetic field gradients is used to perform two-dimensional interpolation on the three-dimensional surface sample database of the quadrupole field magnetic field gradient and the three-dimensional surface sample database of the octupole field magnetic field gradient using a cubic spline function to increase the grid point density;

[0026] The module for solving the corresponding field gradient current curve for the input field gradient is used to intersect the input quadrupole field gradient and octupole field gradient with the magnetic field gradient surface of the three-dimensional sample database, and two corresponding current curves are obtained after the intersection; specifically: select the quadrupole field magnetic field gradient plane, this plane intersects with the surface of the three-dimensional surface sample database of the quadrupole field magnetic field gradient to obtain a current curve that satisfies this quadrupole field gradient; select the octupole field magnetic field gradient plane, this plane intersects with the surface of the three-dimensional surface sample database of the octupole field magnetic field gradient to obtain a current curve that satisfies this octupole field gradient;

[0027] The module for solving the intersection point of the quadrupole / octupole field gradient current curve is used to obtain the intersection point of the current curve satisfying the quadrupole field gradient and the current curve satisfying the octupole field gradient, and use this intersection point as the solution of the composite iron excitation current;

[0028] The output quadrupole / octupole coil current module outputs the quadrupole field coil current and the octupole field coil current to the quadrupole / octupole composite iron according to the solution of the composite iron excitation current.

[0029] Furthermore, the quadrupole / octupole composite magnet has a total of eight pole heads. Each pole head is provided with two layers of current coils along the radial direction near the large radius. The inner layer coil is the octupole magnetic field excitation coil, and the outer layer coil is the quadrupole magnetic field excitation coil;

[0030] For the inner octupole magnetic field excitation coil, the current directions of two adjacent pole heads are opposite. That is, the octupole field coils are divided into two groups: pole heads 1, 3, 5, 7 and pole heads 2, 4, 6, 8. The excitation currents of the two groups are equal in magnitude but opposite in direction, thereby generating an octupole magnetic field;

[0031] For the outer quadrupole magnetic field excitation coil, the coils on two adjacent pole heads are in a group, divided into four groups, namely pole heads 1, 2; pole heads 3, 4; pole heads 5, 6; pole heads 7, 8; among them, the excitation currents of the two groups of quadrupole magnetic field coils of pole heads 1, 2 and symmetrically arranged pole heads 5, 6 are equal in magnitude and the same in direction, and the excitation currents of the two groups of coils of pole heads 3, 4 and symmetrically arranged pole heads 7, 8 are equal in magnitude and the same in direction. The current directions of the coils of pole heads 1, 2 and pole heads 3, 4 are opposite, and the directions of the coils of pole heads 5, 6 and pole heads 7, 8 are opposite, thereby generating a quadrupole magnetic field.

[0032] Advantages and effects of the present invention

[0033] 1. The linear beam uniformization transmission line of the present invention includes two quadrupole / octupole composite irons, and adopts the method of jointly generating a large envelope by the quadrupole / octupole composite iron and the quadrupole iron, which solves the problem that in the prior art, when discrete quadrupole irons and octupole irons are used, since the place with the largest envelope is usually occupied by the quadrupole magnetic field, the role of the octupole iron cannot be fully exerted.

[0034] 2. The linear beam uniformization transmission line of the present invention includes two quadrupole / octupole composite irons. Since the quadrupole / octupole composite iron is located at the large envelope, the requirement for the K value of the magnetic field strength of the quadrupole / octupole composite iron is reduced, which solves the problem that in the prior art, when discrete quadrupole irons and octupole irons are used, since the beam envelopes at the positions of octupole iron 1 and octupole iron 2 are small, the requirement for the field strength of the octupole magnet is very high, and the too high magnetic field strength is too costly and difficult to achieve in engineering.

[0035] 3. The present invention includes a linear beam homogenization transmission line composed of two four-octupole composite irons, which simultaneously generates a quadrupole magnetic field and an octupole magnetic field within the aperture of the four-octupole composite iron, and simultaneously realizes the functions of focusing and homogenizing the beam envelope. The installation of the composite iron on the beam transmission line can reduce the number of transmission elements and shorten the length of the transmission line, solving the problems of the overly expensive civil construction for shielding around, large floor area, and high cost caused by the excessively long transmission line when using discrete quadrupole irons and octupoles.

[0036] 4. The present invention includes a linear beam homogenization transmission line composed of two four-octupole composite irons, which limits the phase shift of particles between the two four-octupole composite magnets and the target to be respectively close to integer multiples of 180 degrees (0, 1, 2, 3...), but not equal to integer multiples of 180 degrees (0, 1, 2, 3...), such that the required magnetic field intensity values k1 and k2 at the first four-octupole composite magnet and the second four-octupole composite magnet are smaller, which is beneficial to reducing the manufacturing difficulty and cost of the magnet while ensuring the homogenization effect.

[0037] 5. The present invention includes a linear beam homogenization transmission line composed of two four-octupole composite irons, which limits the transfer matrix between the two octupole magnetic fields to be close to the unit matrix, that is, the phase shift between the front and rear four-octupole composite magnets is controlled within a range of 30 degrees. This 30-degree range can greatly avoid the high-order nonlinear effects caused by the coupling of the two octupole magnets and can obtain a better homogenization effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1a It is a schematic diagram of the linear beam homogenization transmission line of the present invention;

[0039] Figure 1b It is a schematic diagram of the linear beam homogenization transmission line - beam matching mechanism of the present invention;

[0040] Figure 1c It is a schematic diagram of the linear beam homogenization transmission line - beam homogenization effect observation mechanism of the present invention;

[0041] Figure 1d It is a schematic diagram for comparing the particle distribution states before and after the homogenization of the transmission line of the present invention;

[0042] Figure 2 It is a schematic diagram of the maximum envelope positions of the four-octupole composite irons 1 and 2 of the present invention respectively;

[0043] Figure 3 It is a schematic diagram of the representative meanings of the four points 0, 1, 2, and 3 involved in formula (1) and formula (2) of the present invention.

[0044] Figure 4Schematic diagram of the phase difference between two four-eight-pole composite irons of the present invention to the target point (target) and the phase difference between the two four-eight-pole composite irons

[0045] Figure 5a Schematic diagram of the current control device for the four-eight-pole composite magnet of the present invention

[0046] Figure 5b New four / eight-pole composite iron model of the present invention

[0047] Figure 5c Coil arrangement and current direction of the four-eight-pole composite magnet of the present invention

[0048] Figure 5d Schematic diagram of the current values of two groups of coils during magnetic field measurement of the present invention

[0049] Figure 5e Schematic diagram of the four-eight-pole magnetic field gradient corresponding to different currents measured in the experiment of the present invention

[0050] Figure 5f Schematic diagram of using cubic spline function for two-dimensional interpolation to encrypt grid point density of the present invention

[0051] Figure 5g Schematic diagram of the intersection of the selected magnetic field gradient plane and the magnetic field gradient surface of the three-dimensional sample database of the present invention

[0052] Figure 5h Schematic diagram of the current curve corresponding to the selected four / eight-pole field of the present invention

[0053] Figure 5i Schematic diagram of the solution of the excitation current of the composite iron of the present invention

[0054] Figure 6 Schematic diagram of the octupole iron in the small envelope position in the prior art

[0055] Figure 7 Schematic diagram of an embodiment of an ultra-short linear beam homogenization transmission line of the present invention Detailed implementation mode

[0056] Design principle of the present invention

[0057] 1. Innovation points of the present invention

[0058] One of the innovation points is: inventing a four-eight-pole composite iron. Using the four-eight-pole composite iron shown in Figure 5a , 5b , 5c to replace the quadrupole iron and octupole iron of discrete components shown in Figure 6 . One of the effects: After combination, a quadrupole magnetic field and an octupole magnetic field are generated inside the aperture of the four-eight-pole composite iron, and its effect is as shown in Figure 2As shown in the figure: The quadrupole magnet and the octupole magnet share the positions of the large envelopes in the Y direction and the X direction. The reason why they can share the positions of the large envelopes in the Y direction or the X direction is that the quadrupole-octupole composite magnet 1 and the quadrupole magnet in front of it generate a large envelope in the Y direction; the quadrupole-octupole composite magnet 2 and the quadrupole magnet of the quadrupole-octupole composite magnet 1 in front of it generate a large envelope in the X direction. Since the octupole magnets in the quadrupole-octupole composite magnet 1 and the quadrupole-octupole composite magnet 2 are arranged at the positions of the large envelopes, a better beam current homogenization effect can be achieved, that is, the functions of focusing and homogenizing the beam envelope can be realized simultaneously. Compared with Figure 6 the existing technology, in the existing technology, the quadrupole magnet and the octupole magnet are arranged separately. The octupole magnet itself cannot change the envelope like the quadrupole magnet, that is, it cannot generate a large envelope with the quadrupole magnet in front. Therefore, the envelopes in the Y direction and the X direction of the positions of the separately arranged octupole magnets are relatively small, so the homogenization effect is not good. The second effect: Installing the composite magnet on the beam transmission line can reduce the number of transmission elements, shorten the length of the transmission line, and reduce the project cost of the beam transport line; the reduction of the number of transmission elements not only saves the quadrupole magnets, but also saves multiple components that need to be added to the transmission line to achieve the same homogenization effect.

[0059] The second innovation point lies in: inventing a four / octupole composite magnet current control system and a four / octupole composite magnet excitation current design method. First, the design difficulty of the four / octupole composite magnet excitation current lies in: there is a coupling relationship between the multipole magnetic fields generated by the two sets of coils of the four / octupole composite magnet. This coupling relationship means that changing the current magnitude of the quadrupole field or octupole field coil will simultaneously change the original quadrupole magnetic field gradient and octupole magnetic field gradient of the magnet. Therefore, the one-to-one adjustment strategy for the magnetic field gradient of a single type of magnet current will no longer apply. In the composite magnet, if the current magnitude of the quadrupole field coil of the magnet is adjusted first and the quadrupole magnetic field gradient is adjusted to meet the usage requirements, and then the current magnitude of the octupole field coil is adjusted, once the current of the octupole field coil starts to change, it will change the previously adjusted quadrupole magnetic field gradient, making it no longer meet the usage requirements. Second, a four / octupole composite magnet current control system and a four / octupole composite magnet excitation current design method are innovatively designed. This system and method solve the problem of the coupling relationship between the multipole magnetic fields generated by the two sets of coils of the four / octupole composite magnet, and find the current intersection point that simultaneously satisfies the quadrupole field gradient and the octupole field gradient.

[0060] This system is as Figure 5a shown, and includes: a two-dimensional sampling point module based on coil current, an initial magnetic field gradient three-dimensional surface sample library module based on experimental measurement, a refined magnetic field gradient three-dimensional surface sample library module based on interpolation algorithm, a current curve module for solving the corresponding field gradient based on field gradient input, a current curve intersection point module based on field gradient solution, and an output quadrupole / octupole coil current module.

[0061] The design method of the four / eight-pole composite iron excitation current is as follows:

[0062] Step 1: As Figure 5d shown, establish a two-dimensional data comparison table of the coil currents of the four-pole iron and the eight-pole iron; this two-dimensional data comparison table of the coil currents is based on the two-dimensional data comparison table of the coil currents of the four / eight-pole composite iron; the above two-dimensional data comparison table of the coil currents is as Figure 5d shown, the abscissa is the four-level coil current, and the ordinate is the four-level coil current.

[0063] Step 2: As Figure 5e shown, conduct experimental measurements to obtain the measured values of the four- and eight-pole magnetic field gradients corresponding one by one to the two-dimensional current data comparison table, thereby obtaining a three-dimensional surface sample database of the magnetic field gradients; the above three-dimensional surface sample database of the magnetic field gradients includes a three-dimensional surface sample database of the four-pole field magnetic field gradient and a three-dimensional surface sample database of the eight-pole field magnetic field gradient;

[0064] The above three-dimensional surface sample database Figure 5e is shown, the four-level coil current is used as the X-axis coordinate, the eight-level coil current is the Y-axis coordinate, and the magnetic field gradient is the Z-axis coordinate.

[0065] Step 3: As Figure 5f shown, use the cubic spline function to perform two-dimensional interpolation on the three-dimensional surface sample database of the four-pole field magnetic field gradient and the three-dimensional surface sample database of the eight-pole field magnetic field gradient to increase the density of the grid points;

[0066] The effect of the above increased density of the grid points is as Figure 5f shown, the grid densities on the X-axis, Y-axis, and Z-axis are all increased.

[0067] Step 4: As Figure 5g shown, select the four-pole field magnetic field gradient plane, which intersects with the surface of the three-dimensional surface sample database of the four-pole field magnetic field gradient to obtain a current curve that satisfies the four-level field gradient; select the eight-pole field magnetic field gradient plane, which intersects with the surface of the three-dimensional surface sample database of the eight-pole field magnetic field gradient to obtain a current curve that satisfies the eight-level field gradient;

[0068] Step 5: As Figure 5h shown, obtain the current curve that satisfies the four-level field gradient and the current curve that satisfies the eight-level field gradient in the two-dimensional data grid plane of the coil current, and finally find the intersection point of the above two current curves;

[0069] Step 6: As Figure 5i shown, use this intersection point as the solution of the composite iron excitation current.

[0070] The third innovation lies in finding a balance among the homogenization of the transmission line, avoiding the non-linear effects caused by coupling, and shortening the transmission line. As Figure 2 shown, the homogenization effect of the ultra-short straight beam homogenization transmission line is obvious. There are three aspects of innovation, and none of them can be missing. Only by supporting each other can the ideal homogenization effect be achieved:

[0071] The first aspect: Taking ensuring homogenization as the premise. Specifically, two four-octupole composite irons are used on the transmission line. The quadrupole iron in front of the four-octupole composite iron 1 and its own quadrupole iron generate a large envelope in the Y direction at the four-octupole composite iron 1. The quadrupole iron of the four-octupole composite iron 1 in front of the four-octupole composite iron 2 and the quadrupole iron of the four-octupole composite iron itself generate a large envelope in the X direction at the four-octupole composite iron 2;

[0072] The second aspect: Solving the problem of optimizing the homogenization effect: Specifically as Figure 4 shown, it is achieved by the phase shift between the four-octupole composite magnet 1 and the target being close to an integer multiple of 180 degrees (0, 1, 2, 3...), and the phase shift between the four-octupole composite magnet 2 and the target being close to an integer multiple of 0 degrees. When it is close to the 180 degrees and close to the 0 degrees, the homogenization effect is the best;

[0073] The third aspect: On the premise of ensuring homogenization, it is also necessary to avoid the non-linear effects caused by coupling. The non-linear effect is: When the two four-octupole composite irons are used together, if not properly processed, a non-linear effect coupling effect will be generated. When the non-linear effect coupling occurs, the amplitude of the particles increases, resulting in particle loss, and the particle loss reduces the homogenization effect. Therefore, in order to avoid the non-linear effect coupling effect, the present invention also makes the transfer matrix between the two octupole magnetic fields of the two four-octupole composite irons close to the unit matrix. The close-to-unit matrix, as Figure 3 、 Figure 4As shown, the phase difference between the first four-eight-pole composite iron (point 1) and the second four-eight-pole composite iron (point 2) is close to 0 degrees and less than 30 degrees. At this time, the magnetic field strength of the required octupole magnet is relatively small, and due to the relatively small non-linear effect formed by coupling, the degree of homogenization is better. The difference between the present invention and the prior art is that: the transfer matrix between the two octupole magnetic fields in the prior art "equals the unit matrix" rather than "is close to the unit matrix", while the transfer matrix between the two octupole magnetic fields in the present invention "is close to the unit matrix" rather than "equals the unit matrix", that is, the phase difference between the first four-eight-pole composite iron (point 1) and the second four-eight-pole composite iron (point 2) is close to 0 degrees rather than equal to 0 degrees, and less than 30 degrees rather than equal to 30 degrees. The significance of the present invention using "close to" rather than "equal to" is to shorten the transmission line. If the method of "equal to the unit matrix" is adopted, many components need to be added to the transmission line, and with more components, the transmission line will be very long. The present invention adopts the method of "enough is enough" in the problem of "avoiding the non-linear effect formed by coupling": when the phase difference between the first four-eight-pole composite iron (point 1) and the second four-eight-pole composite iron (point 2) is close to 0 degrees and less than 30 degrees, it can not only effectively shorten the length of the transmission line, but also meet the requirement of "avoiding the non-linear effect formed by coupling", and a balance point is found among the homogenization of the transmission line, avoiding the non-linear effect formed by coupling, and shortening the transmission line.

[0074] The fourth innovation point lies in: inventing an ultra-short linear beam homogenization transmission line. As shown in Figure 1, two four-eight-pole composite irons are adopted on the ultra-short linear beam homogenization transmission line. The four-eight-pole composite iron 1 and the quadrupole iron in front of it jointly generate a large envelope in the Y direction. The generation of the large envelope is due to the defocusing of the quadrupole iron in front, and the beam gradually diverges in the Y direction. The quadrupole field of the subsequent four-eight-pole composite iron 1 focuses, and the beam gradually converges in the Y direction, so as to generate a large envelope in the Y direction at the four-eight-pole composite iron 1; the four-eight-pole composite iron 2 and the four-eight-pole composite iron 1 in front of it jointly generate a large envelope in the X direction. The generation of the large envelope is due to the focusing of the quadrupole iron in front and the defocusing of the quadrupole field of the four-eight-pole composite iron 1, and the beam gradually diverges in the X direction. The quadrupole field of the subsequent four-eight-pole composite iron 2 focuses, so as to generate a large envelope in the X direction at the four-eight-pole composite iron 2; the ultra-short linear beam homogenization transmission line uses two four-eight-pole composite irons, which can achieve a good beam homogenization effect, that is: it can simultaneously achieve the functions of focusing and homogenizing the beam envelope. The four-eight-pole composite iron and the large envelope are two complementary aspects. When the envelope is large, the k value of the magnetic field strength of the four-eight-pole composite iron is relatively small. When the k value is relatively small, the requirement for the magnetic field strength of the octupole magnet is relatively low. When the requirement for the magnetic field strength is relatively low, it is easy to be realized in engineering. When it is easy to be realized in engineering, the four-eight-pole composite iron is easy to be popularized and used. If the requirement for the magnetic field strength of the four-eight-pole composite iron for beam homogenization is very high, it is not easy to be realized in engineering, and if it is not easy to be realized, it is impossible to talk about using the four-eight-pole composite iron.

[0075] Based on the above invention principles, the present invention designs an ultra-short linear beam homogenization transmission line, as shown in Figure 1a , 1b , 1c, and 1d. Its characteristics are as follows: The transmission line is a linear beam homogenization transmission line containing two four-octupole composite irons; the linear beam homogenization transmission line is successively provided with an accelerator beam outlet, a beam matching mechanism, a four-octupole composite iron 1, a four-octupole composite iron 2, a beam homogenization effect observation mechanism, and a terminal along the beam direction;

[0076] As shown in Figure 2 , the four-octupole composite iron 1 and the four-octupole composite iron 2 are used to simultaneously generate a quadrupole magnetic field and an octupole magnetic field respectively, so that the functions of a quadrupole iron and an octupole iron can be realized simultaneously by installing only one transmission element; at the four-octupole composite iron 1, the beam envelope function in the Y direction or the X direction reaches a maximum value, and at the four-octupole composite iron 2, the beam envelope function in the X direction or the Y direction reaches a maximum value; the phase shift of the particles between the two four-octupole composite magnets and the target is respectively close to an integer multiple (0, 1, 2, 3...) of 180 degrees; the transfer matrix between the two octupole magnetic fields is close to the unit matrix;

[0077] Supplementary Note 1:

[0078] The above-mentioned "phase shift of the particles between the two four-octupole composite magnets and the target is respectively close to an integer multiple (0, 1, 2, 3...) of 180 degrees" but not equal to 180 degrees. The principle is shown in the following formula (1) and formula (2):

[0079]

[0080] In formula (1), since Csc[ux23] in the numerator = 1 / sin[ux23], when ux23 approaches 180, sin[ux23] tends to 0 and Csc[ux23] tends to infinity; similarly, Csc[uy13] in the numerator of formula (2) = 1 / sin[uy13], when ux13 approaches 180, sin[ux13] tends to 0 and Csc[uy13] tends to infinity; therefore, the phase shift of the particles between the two four-octupole composite magnets and the target is respectively close to but not equal to an integer multiple (0, 1, 2, 3...) of 180 degrees.

[0081] As shown in Figure 5aAs shown, the quadrupole-octupole composite iron 1 and the quadrupole-octupole composite iron 2 are based on a quadrupole-octupole composite magnet system, which includes a composite magnet current control device, a composite magnet main power supply, and a quadrupole-octupole composite magnet; the composite magnet current control device is used to control the output of the quadrupole field coil current and the octupole field coil current of the composite magnet main power supply to the quadrupole-octupole composite magnet;

[0082] The beam matching mechanism is used to observe the initial state of the beam extracted from the accelerator, and adjust the envelope size of the beam in the X or Y direction according to the initial state, and align the beam center with the mechanical center of the beam pipe;

[0083] The beam homogenization effect observation mechanism is used to observe the beam current intensity, the shape of the weak beam, and the cross-section of the strong beam after homogenization by the quadrupole-octupole composite iron 1 and the quadrupole-octupole composite iron 2.

[0084] As Figure 1b shown, the beam matching mechanism includes a fluorescent target 1, a quadrupole magnet, and a steering magnet; the fluorescent target 1 provides the initial state of the beam for the tester; the quadrupole magnet is used to form a large beam envelope in the Y direction or the X direction at the position of the quadrupole-octupole composite iron 1, and the steering magnet is used to align the beam center with the mechanical center of the beam pipe.

[0085] As Figure 1c shown, the beam homogenization effect observation mechanism includes a Faraday cup, a fluorescent target 2, and a double wire. The Faraday cup is used to measure the beam current intensity after homogenization, the fluorescent target 2 is used to observe the shape of the weak beam after homogenization; the double wire is used to observe the shape of the strong beam after homogenization.

[0086] The phase shift of the particles between the two quadrupole-octupole composite magnets and the target close to an integer multiple of 180 degrees means close to but not equal to an integer multiple of 180 degrees: Let be the remainder of divided by 180 degrees, and the value of

[0087] Supplementary Note 2

[0088] The above-mentioned "the phase shift of the particles between the two quadrupole-octupole composite magnets and the target close to an integer multiple of 180 degrees" is explained as follows: Assume that the phase of the quadrupole-octupole composite iron 1 is 170 degrees, and the remainder of 170 degrees divided by 180 degrees is 10 degrees; assume that the phase of the quadrupole-octupole composite iron 2 is 168 degrees, and the remainder of 168 degrees divided by 180 degrees is 12 degrees. The above remainders 10 and 12 are both within the range of less than ±15 degrees and are allowed.

[0089] Furthermore, the transfer matrix between the two octupole magnetic fields is close to the identity matrix, that is, the phase shift between the front and rear four-octupole composite magnets is controlled within a range of 30 degrees. This 30-degree range can greatly avoid the high-order nonlinear effects caused by the coupling of the two octupole magnets and can achieve a better homogenization effect.

[0090] The expression of the magnet strength k at the four-octupole composite magnet 1 and the four-octupole composite magnet 2 is as follows:

[0091]

[0092] As Figure 3 shown, let: the starting point of the transport line be denoted as 0, the position of the first four-octupole magnet be denoted as 1, the position of the second four-octupole magnet be denoted as 2, and the position at the end point, that is, the position of the target, be denoted as 3; ux02 in the above formula (1) represents the phase shift in the x direction between positions 0 and 2, and ux23 represents the phase shift in the x direction between positions 2 and 3; βx2 represents the envelope function in the x direction at position 2; uy01 in the above formula (2) represents the phase shift in the y direction between 0 and 1; uy13 represents the phase shift in the y direction between 1 and 3; βy1 represents the envelope function in the y direction at position 1.

[0093] Supplementary Note 3

[0094] The derivation process of the above formula (1) and formula (2) is briefly introduced as follows: In the reference "Yosuke Yuri, Uniformization of the transverse beam profile by means of nonlinear focusing method[J]. Physical Review Special Topics-Accelerators and Beams, 2007. DOI: 10.1103 / physrevstab.10.104001.", the octupole magnetic field strength formula in a single direction is given. This formula only considers one octupole magnet and the subsequent transport section. To more accurately describe the relationship between the octupole magnetic field strength and the transport line design, we expand this formula and consider the influence of the matching section from the accelerator exit to the octupole iron on the beam homogenization. The parameters involved are ux02 (phase shift in the x direction between positions 0 and 2), ux23 (phase shift in the x direction between positions 2 and 3), uy01 (phase shift in the x direction between positions 0 and 2), and uy13 (phase shift in the x direction between positions 2 and 3). Using the same "high-order transport mapping" derivation method as in the reference, the expression formulas (1) and (2) of the magnet strength k at the first four-octupole composite magnet and the second four-octupole composite magnet are obtained.

[0095] As Figure 5a shown, the composite magnet current control device includes: a coil current two-dimensional sampling point establishing module, an experimental measurement and preliminary establishment of a magnetic field gradient three-dimensional surface sample library module, a magnetic field gradient three-dimensional surface sample library refinement using interpolation module, an input field gradient corresponding field gradient current curve solving module, a four / eight-pole field gradient current curve intersection point solving module, and a four-pole / eight-pole coil current output module;

[0096] As Figure 5d shown, the coil current two-dimensional sampling point establishing module is used to establish a two-dimensional data comparison table of the coil currents of the quadrupole iron and the octupole iron;

[0097] As Figure 5e shown, the experimental measurement and preliminary establishment of a magnetic field gradient three-dimensional surface sample library module uses the current values in the two-dimensional data comparison table of the coil currents to conduct magnetic field experiments on the composite iron, so as to obtain the four- and eight-pole magnetic field gradient measurement values corresponding one-to-one to the two-dimensional current data comparison table, thereby obtaining a three-dimensional surface sample database of the magnetic field gradient; the above three-dimensional surface sample database of the magnetic field gradient includes a three-dimensional surface sample database of the quadrupole field magnetic field gradient and a three-dimensional surface sample database of the octupole field magnetic field gradient;

[0098] As Figure 5f shown, the magnetic field gradient three-dimensional surface sample library refinement using interpolation module is used to perform two-dimensional interpolation on the three-dimensional surface sample database of the quadrupole field magnetic field gradient and the three-dimensional surface sample database of the octupole field magnetic field gradient using a cubic spline function to increase the grid point density;

[0099] As Figure 5g 、 5h shown, the input field gradient corresponding field gradient current curve solving module is used to intersect the input quadrupole field gradient and octupole field gradient with the magnetic field gradient surface of the three-dimensional sample database, and two corresponding current curves are obtained after the intersection; specifically: select the quadrupole field magnetic field gradient plane, this plane intersects with the surface of the three-dimensional surface sample database of the quadrupole field magnetic field gradient to obtain a current curve that satisfies this quadrupole field gradient; select the octupole field magnetic field gradient plane, this plane intersects with the surface of the three-dimensional surface sample database of the octupole field magnetic field gradient to obtain a current curve that satisfies this octupole field gradient;

[0100] As Figure 5i shown, the four / eight-pole field gradient current curve intersection point solving module is used to obtain the intersection point of the current curve that satisfies the quadrupole field gradient and the current curve that satisfies the octupole field gradient, and use this intersection point as the solution of the composite iron excitation current;

[0101] The four-pole / eight-pole coil current output module outputs the quadrupole field coil current and the octupole field coil current to the four-pole / eight-pole composite iron according to the solution of the composite iron excitation current.

[0102] Furthermore, the four-octupole composite magnet has a total of eight pole heads. Each pole head is provided with two layers of current coils along the radial direction near the large radius. The inner layer coil is an octupole magnetic field excitation coil, and the outer layer coil is a quadrupole magnetic field excitation coil;

[0103] For the octupole magnetic field excitation coil in the inner layer, the current directions of two adjacent pole heads are opposite. That is, the octupole field coils are divided into two groups: pole heads 1, 3, 5, 7 and pole heads 2, 4, 6, 8. The excitation currents of the two groups are equal in magnitude but opposite in direction, thereby generating an octupole magnetic field;

[0104] As Figure 5c shown, for the quadrupole magnetic field excitation coil in the outer layer, the coils on two adjacent pole heads are in a group, and are divided into four groups, namely pole heads 1, 2; pole heads 3, 4; pole heads 5, 6; pole heads 7, 8; among them, the excitation currents of the two groups of quadrupole magnetic field coils of pole heads 1, 2 and symmetrically arranged pole heads 5, 6 are equal in magnitude and the same in direction, and the excitation currents of the two groups of coils of pole heads 3, 4 and symmetrically arranged pole heads 7, 8 are equal in magnitude and the same in direction. The current directions of the coils of pole heads 1, 2 and pole heads 3, 4 are opposite, and the directions of the coils of pole heads 5, 6 and pole heads 7, 8 are opposite, thereby generating a quadrupole magnetic field.

[0105] Embodiment 1

[0106] As Figure 7 shown, the present invention designs an ultra-short linear beam homogenization transmission line. When the total length of the ultra-short linear beam homogenization transmission line is 6 meters, the initial layout positions of the components that can enable the transmission line to obtain a better homogenization effect are as follows: the initial position of the fluorescent target 1 is 100 mm, the initial position of the quadrupole magnet (Q0) is 900 mm; the initial position of the guiding magnet is 1300 mm; the initial position of the four-octupole composite iron (Q1) is 1900 mm, the initial position of the four-octupole composite iron (Q2) is 3050 mm, the initial position of the Faraday cup is 4200 mm; the initial position of the fluorescent target 2 is 5200 mm, the initial position of the double wire is 5600 mm; the initial position of the terminal is 6000 mm. Among them, the magnetic field component of the quadrupole iron (Q0) is 2.9 (T / m); the quadrupole magnetic field component of the four-octupole composite iron (Q1) is 3.05 (T / m), and the octupole magnetic field component is 2.5e3 (T / m 3 ), the quadrupole magnetic field component of the four-octupole composite iron (Q2) is 0.45 (T / m), and the octupole magnetic field component is 3.82e3 (T / m 3 ).

[0107] As Figure 1d shown, it is a comparison before and after the homogenization of the transmission line of the present invention, Figure 1dThe left figure shows a screenshot of the beam current that was Gaussian distributed before homogenization, that is, the beam current cross-section at the 100 mm position of the transmission line for the fluorescent target 1; Figure 1d The right figure shows the cross-section of the beam current after homogenization, that is, the beam current cross-section at the 5200 mm position of the transmission line for the fluorescent target 2. It can be seen from the figure that before homogenization, the distribution of particles on the beam current cross-section was dense in the middle and sparse around, and after homogenization, the distribution of particles on the beam current cross-section was uniform in both the middle and around.

[0108] It should be emphasized that the above specific embodiments are only explanations of the present invention and are not limitations thereof. Those skilled in the art can make modifications without creative contributions to the above embodiments according to needs after reading this specification, but as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

Claims

1. A ultra-short straight beam homogenization transmission line, characterized in that: The beam line is a linear beam homogenization transmission line containing two quadruple-octupole composite irons; the linear beam homogenization transmission line is sequentially provided with: an accelerator extraction port, a beam matching mechanism, quadruple-octupole composite iron 1, quadruple-octupole composite iron 2, a beam homogenization effect observation mechanism, and a terminal along the beam direction; The quadrupole-octupole composite iron 1 and the quadrupole-octupole composite iron 2 are used to generate a quadrupole magnetic field and an octupole magnetic field respectively and simultaneously, so that the functions of a quadrupole iron and an octupole iron can be realized simultaneously by installing only one transmission element; at the quadrupole-octupole composite iron 1, the beam envelope function in the Y direction or the X direction reaches a maximum value, and at the quadrupole-octupole composite iron 2, the beam envelope function in the X direction or the Y direction reaches a maximum value; the phase shift of the particles between the two quadrupole-octupole composite magnets and the target are respectively close to integer multiples of 180 degrees (0, 1, 2, 3...); the transfer matrix between the two octupole magnetic fields is close to the identity matrix; The quadruple-octupole composite iron 1 and the quadruple-octupole composite iron 2 are based on a quadruple-octupole composite magnet system, and the quadruple-octupole composite magnet system includes a composite magnet current control device, a composite magnet main power supply, and a quadruple-octupole composite magnet; the composite magnet current control device is used to control the output of the four-pole field coil current and the eight-pole field coil current of the composite magnet main power supply to the quadruple-octupole composite magnet; The beam matching mechanism is used to observe the initial state of the beam extracted from the accelerator, adjust the envelope size of the beam in the X or Y direction according to the initial state, and align the beam center with the mechanical center of the beam pipe; The beam homogenization effect observation mechanism is used to observe the beam intensity, weak beam shape, and strong beam cross-section of the beam homogenized by the quadruple-octupole composite iron 1 and the quadruple-octupole composite iron 2.

2. The ultra-short linear beam homogenization transmission line according to claim 1, wherein: The beam matching mechanism includes a fluorescent target 1, a quadrupole magnet, and a guiding magnet; the fluorescent target 1 provides the initial state of the beam for the tester; the quadrupole magnet is used to form a large beam envelope in the Y direction or X direction at the position of the quadruple-octupole composite iron 1, and the guiding magnet is used to align the beam center with the mechanical center of the beam pipe.

3. The ultra-short linear beam homogenization transmission line according to claim 1, wherein: The beam homogenization effect observation mechanism includes a Faraday cup, a fluorescent target 2, and double wires. The Faraday cup is used to measure the intensity of the homogenized beam, the fluorescent target 2 is used to observe the shape of the weak beam after homogenization, and the double wires are used to observe the shape of the strong beam after homogenization.

4. The ultra-short linear beam homogenization transmission line according to claim 1, wherein: The phase shift of the particles between the two quadruple-octupole combined magnets and the target is close to an integer multiple of 180 degrees, which means close to but not equal to an integer multiple of 180 degrees. Let be the remainder with respect to 180 degrees, and its value is generally less than ±15 degrees.

5. The ultra-short linear beam homogenization transmission line according to claim 1, wherein: The transfer matrix between the two octupole magnetic fields is close to the unit matrix, that is, the phase shift between the front and rear quadruple-octupole composite magnets is controlled within 30 degrees. This 30-degree range can greatly avoid the high-order nonlinear effects caused by the coupling of the two octupole magnets and can obtain a better homogenization effect.

6. The ultra-short linear beam homogenization transmission line according to claim 1, wherein: The expression of the magnet strength k at the quadruple-octupole composite iron 1 and the quadruple-octupole composite iron 2 is: Let: the starting point of the transmission line is denoted as 0, the position of the first quadruple-octupole magnet is denoted as 1, the position of the second quadruple-octupole magnet is denoted as 2, and the position at the end, that is, the position of the target, is denoted as 3; ux02 in the above formula (1) represents the phase shift of the particle in the x direction between positions 0 and 2, ux23 represents the phase shift of the particle in the x direction between positions 2 and 3; βx2 represents the envelope function in the x direction at position 2; uy01 in the above formula (2) represents the phase shift of the particle in the y direction between 0 and 1; uy13 represents the phase shift of the particle in the y direction between 1 and 3; βy1 represents the envelope function in the y direction at position 1.

7. The ultra-short linear beam homogenization transmission line according to claim 1, characterized in that: The composite magnet current control device includes: a module for establishing two-dimensional sampling points of coil current, a module for experimentally measuring and initially establishing a three-dimensional surface sample library of magnetic field gradients, a module for using interpolation to refine the three-dimensional surface sample library of magnetic field gradients, a module for solving the corresponding field gradient current curve by inputting the field gradient, a module for solving the intersection point of the four / octupole field gradient current curves, and a module for outputting the four-pole / octupole coil current; The coil current two-dimensional sampling point establishing module is used to establish a two-dimensional data comparison table of the coil currents of the quadrupole iron and the octupole iron; The experimental measurement and preliminary establishment of the magnetic field gradient three-dimensional surface sample library module uses the current values in the two-dimensional data comparison table of the coil currents to conduct magnetic field experiments on the composite iron, so as to obtain the quadrupole and octupole magnetic field gradient measurement values corresponding one-to-one with the two-dimensional current data comparison table, and thus obtain a three-dimensional surface sample database of the magnetic field gradient; the above three-dimensional surface sample database of the magnetic field gradient includes a three-dimensional surface sample database of the quadrupole field magnetic field gradient and a three-dimensional surface sample database of the octupole field magnetic field gradient; The module for using interpolation to refine the three-dimensional surface sample library of the magnetic field gradient is used to perform two-dimensional interpolation on the three-dimensional surface sample database of the quadrupole field magnetic field gradient and the three-dimensional surface sample database of the octupole field magnetic field gradient using the cubic spline function to increase the grid point density; The input field gradient solving corresponding field gradient current curve module is used to intersect the input quadrupole field gradient and octupole field gradient with the magnetic field gradient surface of the three-dimensional sample database, and two corresponding current curves are obtained after the intersection; specifically: select the quadrupole field magnetic field gradient plane, and this plane intersects with the surface of the three-dimensional surface sample database of the quadrupole field magnetic field gradient to obtain a current curve that satisfies this quadrupole field gradient; select the octupole field magnetic field gradient plane, and this plane intersects with the surface of the three-dimensional surface sample database of the octupole field magnetic field gradient to obtain a current curve that satisfies this octupole field gradient; The module for solving the intersection point of the quadrupole / octupole field gradient current curves is used to obtain the intersection point of the current curve that satisfies the quadrupole field gradient and the current curve that satisfies this octupole field gradient, and use this intersection point as the solution of the excitation current of the composite iron; The output quadrupole / octupole coil current module outputs the quadrupole field coil current and the octupole field coil current to the quadrupole / octupole composite iron according to the solution of the excitation current of the composite iron.

8. The ultra-short linear beam homogenization transmission line according to claim 1, characterized in that: This quadrupole-octupole composite magnet has a total of eight pole heads, and each pole head is provided with two inner and outer current coils along the radial direction near the large radius. The inner coil is an octupole magnetic field excitation coil, and the outer coil is a quadrupole magnetic field excitation coil; For the inner octupole magnetic field excitation coil, the current directions of two adjacent pole heads are opposite, that is, the octupole field coils are divided into two groups: pole heads 1, 3, 5, 7 and pole heads 2, 4, 6, 8. The excitation currents of the two are equal in magnitude but opposite in direction, thereby generating an octupole magnetic field; For the outer quadrupole magnetic field excitation coil, the coils on two adjacent pole heads are in a group, and are divided into four groups, namely pole heads 1, 2, pole heads 3, 4, pole heads 5, 6, and pole heads 7, 8; among them, pole heads 1, 2 and symmetrically arranged pole heads 5, 6, the excitation currents of these two groups of quadrupole magnetic field coils are equal in magnitude and the same in direction, pole heads 3, 4 and symmetrically arranged pole heads 7, 8, the excitation currents of these two groups of coils are equal in magnitude and the same in direction, the current directions of the coils of pole heads 1, 2 and pole heads 3, 4 are opposite, and the directions of the coils of pole heads 5, 6 and pole heads 7, 8 are opposite, thereby generating a quadrupole magnetic field.

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