CN115604902A - Self-extraction cyclotron - Google Patents

Abstract

A self-leading-out cyclotron comprises a pair of cover plates, a pair of magnet exciting coils positioned in the cover plates, and two groups of main magnets which are positioned on the inner sides of the pair of magnet exciting coils and used for forming main magnetic fields and are arranged face to face, wherein each group of the main magnets consists of two fan-shaped long magnetic poles and two short magnetic poles, the long magnetic poles and the short magnetic poles are distributed in a staggered mode, the two long magnetic poles and the two short magnetic poles are respectively overlapped in axial line, four fan-shaped magnets are distributed in a centripetal mode, the centers of circles of the four fan-shaped magnets are overlapped, a fan-shaped valley area is formed between every two adjacent magnetic poles, and the magnetic pole gaps of the long magnetic poles and the short magnetic poles are changed along with the change of the radius; and the tail end of each long magnetic pole is provided with an arc notch. The invention can stably accelerate H + ions, has low requirement on vacuum degree compared with a cyclotron of a common endogenous negative hydrogen ion source in the market, and the output current intensity of the self-extraction cyclotron can be generally stably up to more than 100uA to 1 mA.

Description

A self-extracting cyclotron

technical field

The invention belongs to the field of nuclear technology, relates to accelerator technology, in particular to a self-extracting cyclotron.

Background technique

At present, cyclotrons are widely used in scientific research and medical treatment, but the mainstream accelerators on the market generally adopt the method of internal source ion source and carbon film stripping extraction, and the extraction current intensity is generally 50uA~100uA, which cannot meet the needs of scientific research with higher current intensity and Biopharmaceutical research, such as boron neutron therapy (BNCT) and germanium gallium generators.

The self-extraction technology refers to the drastic change of the magnetic field gradient caused by carving grooves at the end positions of the magnetic poles, and the beam current is separated from the main magnetic field due to the increase of the radius of gyration. A typical 18MeV self-extraction cyclotron is mainly composed of core components such as H+ ion source, central area, main magnet, high-frequency system, self-extraction system and vacuum chamber. H+ ions enter the plane of the central area through the H+ ion source and perform a circular gyration motion. When passing through the high-frequency gap, the energy of the H+ ions increases and the radius of gyration increases. Generally, it accelerates to the final energy after more than 100 cycles, and is continuously extracted through the self-extraction system. Intensive H+ ion beam. Wherein, the main magnet can provide a guiding magnetic field for accelerating H+ ions.

The main magnet is the basic guarantee that H+ ions can be accelerated to the final energy. Compared with the common H- ion accelerators in the market, the self-extraction technology requires the beam to continue to move for a certain distance in the edge region of the magnetic pole. The strong fringe field effect of the main magnet will make the beam The radial size of the flow becomes larger, so the 18MeV self-extracting cyclotron requires reducing the influence of the fringe field on the bunch as much as possible, and adopts a biaxially symmetrical magnetic pole structure design, which is divided into long magnetic poles and short magnetic poles. Adjacent distribution. Since the beam current is drawn in the direction of the short-pole yoke, the yoke needs to be designed with corresponding openings. Both effects will aggravate the second harmonic intensity inside the main magnetic field.

Since the particle energy of the cyclotron increases steadily, the phase of the particles passing through the gap of the accelerating cavity is basically constant, which requires that the magnetic field constraining the cyclotron motion of the particles gradually increases with the radius of the magnetic pole, and can also match the increase in particle energy. . Common cyclotrons on the market fine-tune the magnetic field distribution through magnetic field shimming. The common shimming method is generally to pad the magnetic field with a strip formed of a magnetically permeable material with a high saturation magnetic field on the side of the affixed magnetic pole. One end of the height of the inlay is equal to the surface of the magnetic pole, and the other side is farther away from the cover plate. Therefore, the magnetic field generated by the magnetic pole at the same angle on the central plane is larger than that of the central plane of the inlaid bar. In order to balance the isochronous magnetic field distribution in the magnetic pole range, It is required that the angle of the sector poles should not be too large, so in the case of a large radius, the inserts need to provide more magnetic field shimming energy, that is to say, the inserts need a larger angle under a large radius. This will bring about a series of problems including increased tangential force and deformation of the insert, and insufficient acceleration clearance at large radii, which will eventually lead to increased beam loss and limited beam intensity.

Self-extraction technology requires H+ particles to be able to accelerate to the edge of the magnetic pole. If only the isochronous adjustment of the insert is used, the coverage angle of the insert will be too large, resulting in the above problems. Therefore, the tuning work of the self-extraction main magnet adopts a variable magnetic gap Tuning technology cooperates with inlay tuning, among which the variable magnetic gap tuning technology means that the magnetic gap decreases with the increase of the radius. Due to the reduction of the magnetic gap, the magnetic field in the central plane will also increase with the increase of the radius of the magnetic pole, and the rib tuning mainly makes fine adjustments for isochronism.

The variable magnetic gap tuning is also helpful for the axial focusing of the beam. At the same time, the combination of the variable magnetic gap tuning technology and the strip tuning can effectively control the shape of the magnetic pole surface and reserve enough space for the axial space of the particles. Since the above-mentioned second harmonic may increase the emittance of the beam during transmission, the magnetic gap changes of the long and short magnetic poles do not need to be consistent to adjust the second harmonic. Among them, the long magnetic pole groove is used for self-leading, and the short magnetic pole magnetic gap change is used for shimming the second harmonic.

The self-extracting system is usually composed of five parts: the first harmonic coil, the long magnetic pole slot, the gradient corrector, the beam collector and the extracting magnetic channel. Among them, the first harmonic coil is used to precess and extract the beam, push the beam to the appropriate phase space, so that as much beam as possible can be extracted within the appropriate extraction angle; the long magnetic pole groove is used to provide the magnetic field gradient The change of the radius of gyration of the particles suddenly increases and thus breaks away from the main magnetic field; due to the effect of the magnetic pole edge field, when the beam is drawn out through the slot position of the long magnetic pole, a gradient corrector needs to be placed to guide the extraction of the slot position. The initial focusing of the beam prevents the radial size of the beam from being too large due to the magnetic pole edge field effect and is lost inside the main magnetic field; finally the beam will pass through the extracted magnetic channel, and the extracted magnetic channel will perform secondary focusing on the extracted beam, which is convenient for the beam The beam target and the subsequent beam transmission can match the lateral and axial dimensions of the beam as much as possible; the beam collector is used to collect stray beams that cannot be accepted by the magnetic channel to prevent the beam from bombarding other position thus causing strong induced radioactivity.

Therefore, compared with the traditional H-ion cyclotron, the main problems faced by the self-extracting cyclotron are as follows:

1) Tuning mode of variable magnetic gap + inlay: Since the first harmonic coil will be close to the end of the magnetic pole when it is precessed, the magnetic gap here will become smaller, and the energy of the beam at this time will be smaller. Large, if the particles bombard the surface of the magnetic pole or the first harmonic coil, it will cause relatively strong induced radioactivity. On the other hand, the false Dee plate (accelerator D-type electrode) of the high-frequency resonator is generally attached to the surface of the magnetic pole. Due to the existence of the second harmonic, the change of the magnetic gap between the long and short magnetic poles is not consistent. Subharmonic, if the magnetic gap is too small, it will cause the beam to bombard the false Dee, which will cause the secondary electron multiplication effect and reduce the output power of the high frequency to the beam.

2) Gradient corrector: Since the self-extracted beam is located in the edge field of the magnetic pole, the beam is in a state of strong radial defocusing. If no interference is added, the radial size of the beam will quickly increase due to this effect. As a result, the beam cannot be extracted normally. The defocus state can be effectively controlled by the gradient corrector, so that the beam can continue to transmit with a smaller radial dimension. However, due to the narrow space inside the accelerator, it is not suitable to place four-level electromagnets, so only permanent magnets can be used. Due to the divergence of the permanent magnet's own magnetic field, it will cause a large range of first harmonics inside the main magnetic field, so the gradient corrector needs to be placed symmetrically about the center of the accelerator, which will strengthen the second harmonic effect in the main magnetic field, but too strong The second harmonic of the second harmonic will increase the emittance of the beam transmitted in the main magnetic field.

3) Extracting the magnetic channel: After passing through the gradient corrector, the beam will still run for a certain distance in the fringe field, and the direction of extraction will be in the direction of the short magnetic pole yoke, so it is necessary to design the corresponding openings for the yoke corresponding to the short magnetic pole , used to place the extraction magnetic channel, but this opening design will also lead to further strengthening of the second harmonic inside the main magnetic field, which is not conducive to beam transmission.

Therefore, the opening design size of the yoke is limited, so the permanent magnet design is still selected for the magnetic channel. Since the surface of the permanent magnet can generally polarize the magnetic induction intensity of about 1T at most, if the inner diameter of the magnetic channel is too small, it will cause most of the beam current. Cannot be focused, resulting in beam loss at the surface of the magnetic channel. If the inner diameter is too large, the beam current will be lost inside the magnetic channel due to insufficient gradient of the fourth-stage magnetic field.

Contents of the invention

In order to overcome the defects of the prior art, the invention discloses a self-extracting cyclotron.

The self-extracting cyclotron of the present invention includes a pair of cover plates and a pair of excitation coils located inside the cover plates, and two sets of main magnets placed face to face inside the pair of excitation coils for forming the main magnetic field, each set of The main magnet is composed of two long magnetic poles and two short magnetic poles in a fan shape. The long and short magnetic poles are distributed alternately. The axes of the two long magnetic poles and the short magnetic poles coincide respectively. A fan-shaped valley area is formed, and the pole gap between the long and short poles changes with the radius; the end of each long pole is provided with an arc-shaped long pole groove;

The two straight sides of the fan-shaped poles are pasted with strips; the fan-shaped valley area is divided into a first fan-shaped valley area and a second fan-shaped valley area, and a high-frequency DEE board is arranged in the first fan-shaped valley area. False DEE boards are arranged on the straight sides of the magnetic poles on both sides of the frequency DEE board.

Preferably, the cover plate is provided with a circular groove, and the excitation coil is located in the circular groove.

Preferably, the second fan-shaped valley area is provided with a gradient corrector, and the gradient corrector includes a pair of magnet groups arranged symmetrically with respect to the plane of the main magnetic field, and each magnet group includes a correction focusing magnet and a correction shielding magnet, wherein the correction focus magnet consists of The inner focus magnet and the outer focus magnet are placed face to face. The magnetic field direction of the inner focus magnet close to the inside of the magnetic field is opposite to the main magnetic field; the magnetic field direction of the outer focus magnet near the outside of the magnetic field is the same as the main magnetic field. The strength is consistent; the side of the inner focusing magnet close to the center of the main magnetic field is provided with a correction shielding magnet, the magnetic field direction of the correction shielding magnet is the same as that of the main magnetic field, and the gap between the inner focusing magnet and the outer focusing magnet is in line with the direction of the long magnetic pole The direction of the tangent at the end of the notch is consistent.

Preferably, a harmonic coil is installed towards the center of the main magnetic field in the groove of the long magnetic pole, and the harmonic coil includes two parallel arcs with different radii but the same central angle, and a pair of symmetrical arcs between the two parallel arcs arc connection.

Preferably, in the beam extraction direction outside the main magnet, an extraction magnetic channel is provided, and the extraction magnetic channel includes a hollow cylindrical radial focusing magnet and a radial defocusing magnet with opposite polarization directions, wherein the radial focusing magnet is in the front .

Preferably, the radial focus magnet and the radial defocus magnet are regular hexagonal hollow columnar structures formed by continuous splicing of 16 identical magnetic strips. In the radial focus magnet, the The magnetization direction is set according to the following rules:

Define the magnetization direction B0 of the initial magnetic stripe A0, the magnetization direction B1 of the next magnetic stripe A1 next to the initial magnetic stripe, and the magnetization direction B2 of the next magnetic stripe A2 next to the magnetic stripe A1,

Define the difference rule of magnetization direction: B1-B0=2K, B2-B1=K, K=45 degrees;

Subsequent magnetic strips are replaced by A2 with A0, and the magnetization direction of subsequent magnetic strips changes according to the difference rule;

The magnetization direction of each magnetic strip of the radial defocus magnet is opposite to that of the magnetic strip in the corresponding radial focus magnet.

Preferably, it also includes a beam collector located in the beam extraction direction, installed behind the gradient corrector and before the extraction magnetic channel.

Compared with the prior art, the self-extracting cyclotron described in the present invention has the following technical advantages:

1. The present invention can stably accelerate H+ ions. Compared with the cyclotrons with internal negative hydrogen ion sources that are common in the market, the requirements for vacuum degree are lower, and the extraction current intensity of the self-extracting cyclotron can generally reach 100uA to 1mA or more stably .

2. Compared with the extraction method of the electrostatic deflection plate that also uses the H+ ion source, the extraction efficiency of the self-extraction of the present invention can be stabilized at more than 80%. Since the position of the beam loss is basically controllable, it can reduce the impact of stray beams on the main body of the accelerator.

3. Adopting the isochronous tuning technology of variable magnetic gap plus inserts, which reduces the magnetic field energy that the inserts need to fill up at large radii, and can improve the situation of insufficient acceleration gaps at large radii. The variable magnetic gap can also provide axial The focusing force facilitates the directional delivery of the beam.

Description of drawings

Fig. 1 is a schematic diagram of a specific embodiment of the self-extracting cyclotron of the present invention;

Fig. 2 is the magnetic gap change curve of long and short magnetic poles in a specific embodiment of the present invention;

In Figure 2, the ordinate is the magnetic gap, and the abscissa is the radius of the magnetic pole, and the unit is millimeter;

Fig. 3 is a schematic diagram of a specific embodiment of the gradient corrector of the present invention;

Fig. 4 is a schematic diagram of the influence of the magnetic field trend caused by a specific embodiment of the gradient corrector of the present invention;

In Fig. 4, the ordinate is the magnetic field strength, and the unit is Tesla, and the abscissa is the magnetic field radius, and the unit is millimeter; A1, A2 respectively represent the magnetic field distribution curve when there is and no gradient corrector;

Fig. 5 is a schematic diagram of the direction in which the beam is drawn out after the beam passes through the gradient corrector viewed from the main magnetic surface in a specific embodiment;

In Fig. 5, the abscissa and the ordinate are respectively taking the main magnetic surface as the coordinate plane, and the geometric center of the main magnetic surface as the origin to establish the X-axis and Y-axis coordinates of the coordinate system, and the units are millimeters;

Fig. 6 is a schematic diagram of a specific embodiment of the extraction magnetic channel of the present invention;

Fig. 7 is a schematic diagram of the magnetization direction of the radial focus magnet in the extraction magnetic channel of the present invention;

The thick arrow in the central part of Fig. 7 represents the schematic diagram of the magnetic field of the inner channel of the radial focus magnet, and the small arrow on the ring magnet represents the schematic diagram of the magnetization direction of each magnetic strip;

Fig. 8 is a schematic diagram of the magnetic pole opening angle of the present invention;

The names of reference signs in the figure are: 1-long magnetic pole, 2-short magnetic pole, 3-gradient corrector, 4-leading magnetic channel, 5-harmonic coil, 6-inlay, 7-long magnetic pole groove, 8- False Dee plate, 31-outer focus magnet, 32-inner focus magnet, 33-correction shield magnet, 9-radial focus magnet, 10-radial defocus magnet, 11-beam collector, 12-second fan valley Zone, 13-HF Dee board.

detailed description

The specific embodiment of the present invention will be further described in detail below in conjunction with the accompanying drawings.

A cyclotron magnet for self-extraction, including accelerator main magnet, harmonic coil, gradient corrector, extraction magnetic channel, magnetic yoke, excitation coil and beam collector.

The object of the present invention is to provide a cyclotron magnet capable of accelerating H+ ions to obtain a proton beam with an energy of about 18 MeV, while not generating too strong induced radioactivity inside the accelerator, and to control the position of beam damage.

The self-extracting cyclotron of the present invention includes a pair of cover plates and a pair of excitation coils located inside the cover plates, and two sets of main magnets placed face to face inside the pair of excitation coils for forming the main magnetic field, each set of The main magnet is composed of two long magnetic poles 1 and two short magnetic poles 2 in the shape of a fan. The long and short magnetic poles are distributed alternately. The axes of the two long and short magnetic poles coincide respectively. A fan-shaped valley area is formed between them, and the pole gap between the long and short poles changes with the radius; the end of each long pole is provided with an arc-shaped long pole groove 7;

The two straight sides of the fan-shaped magnetic poles are pasted with strips 6 for isochronous fine-tuning; A high-frequency DEE board 13 is arranged inside, and a dummy DEE board 8 is arranged on the magnetic pole straight sides on both sides of the high-frequency DEE board. The high-frequency DEE plate and the false DEE plate are combined to form an accelerator gap for beam acceleration

Wherein, the main magnet has four sector magnets, which are respectively two long magnetic poles 1 and two short magnetic poles 2, the long and short magnetic poles are alternately distributed, the axes of the two long magnetic poles and the short magnetic poles coincide respectively, and the four sector magnets are distributed centripetally and The center of the circle coincides, the plane where the four fan-shaped magnets are located is defined as the central plane, the center of the circle is defined as the origin, the end of the long magnetic pole is grooved, the shape of the magnet is a straight-sided fan-shaped structure, and a fan-shaped valley is formed between two adjacent magnetic poles. The magnetic pole gaps of the long and short magnetic poles change with the radius, that is, they increase with the increase of the radius, but they do not increase linearly. Usually, with the increase of the radius, the increase value decreases continuously to facilitate careful focusing. A typical implementation is shown in Figure 2 .

In a specific embodiment, the average magnetic field near the central plane of the main magnetic field is about 1.2T, and the time for one revolution of the particles is about 55 ns. The long pole radius is 600mm and the short pole radius is 575mm. The inlays are located on both sides of the magnetic pole, and one end coincides with the surface of the magnetic pole. The opening angle θ of the magnetic pole is 50.6° as shown in Fig. 8 . The angles of the strips 6 of the long and short magnetic poles vary in the same way, but the variation of the magnetic gaps of the long and short magnetic poles with the radius is not consistent, as shown in FIG. 2 .

The invention adopts grooves at the long magnetic poles for extracting beam currents, the drastic change of the magnetic field at the groove positions increases the radius of gyration of the beams, the extraction efficiency is high, and H+ particles can be transmitted. The advantage of transporting H+ particles is that it can reduce the vacuum inside the accelerator, and at the same time have lower requirements on the ion source, that is, the ion source under the same conditions, the H+ ion source will be an order of magnitude larger than the H- ion.

However, the structure of long magnetic pole grooves brings magnetic pole fringe field effect, which causes the beam current to diverge due to the fringe field effect after passing through the groove position. Fringe field effect, but the long and short magnetic pole structure will produce a strong second harmonic effect inside the magnetic field. In order to avoid or reduce the second harmonic effect, the magnetic gap changes of the long and short magnetic poles are not the same. This different magnetic gap change can be Effectively reduce the second harmonic inside the main magnetic field.

When the beam accelerates to the edge of the magnet in the main magnetic field, it will enter the long magnetic pole groove 7, where the width of the groove and the magnetic gap are constant, the width is 30mm, the covering radius is 545mm~575mm, and the magnetic gap is 27.5mm. The magnetic field here changes sharply. When the beam passes through the groove, the radius of gyration increases due to the rapid decrease of the magnetic field, and then it leaves the main magnetic field and enters the gradient corrector 3 .

The gradient corrector 3 is used for preliminary focusing of the beam drawn from the groove position, and includes a vertically symmetrical structure composed of permanent magnets. In a specific implementation as shown in Figure 3, the gradient corrector is divided into two groups of up and down, a total of 6 pieces , each group of 3 pieces includes correction focus magnets and correction shielding magnets 33, wherein the correction focus magnets are composed of inner focus magnets 32 and outer focus magnets 31, and the magnetic field direction of the inner focus magnets 32 close to the inside of the magnetic field is opposite to the main magnetic field, for The deflection is close to the inner beam current; the magnetic field direction of the outer focusing magnet 31 close to the outside of the magnetic field is the same as that of the main magnetic field, and is used to deflect the outer beam current. In order to reduce the interference of the inner focusing magnet on the main magnetic field, a correction shielding magnet 33 is placed close to its inner surface. The magnetic field direction of the correction shielding magnet is the same as that of the main magnetic field, but the residual magnetic intensity is usually smaller than that of the focusing magnet. If the correction shielding magnet If the residual magnetic strength is too large, the peak value of the gradient correction magnet will become lower, and at the same time, the interference of the shielding magnet to the inside of the magnetic field will increase.

The structure of the entire gradient correction magnet is divided into shielding magnet-focusing magnet from the inside to the outside, and in order to avoid the interference of the first harmonic to the beam current, they are placed symmetrically about the center of the accelerator.

It can be seen from Fig. 4 that although the correction shielding magnet 33 can shield part of the influence of the inner focusing magnet on the main magnetic field, it will still generate a large range of first harmonics inside the magnetic field, and the accumulation of this effect may lead to the first harmonic A large amplitude is required to push the beam into a proper phase space, so the entire gradient corrector should be placed symmetrically about the center of the origin to avoid the first harmonic.

If there is no external force interference, the symmetrical long magnetic pole grooves will lead to the beam current. In order to increase the beam current intensity, a harmonic coil 5 is installed on the main magnetic field to push the particles, so that the beam current can only be drawn out from one side. . The harmonic coil 5 is installed in front of the long magnetic pole groove 7, about 15mm away from the groove 7, its height is 6mm, and its width is 10mm. The harmonic coil is connected with a current generating circuit, and the surface current of the harmonic coil 5 is from 0A/mm ² ~10A/mm ² adjustable, under the condition of suitable surface current, the phase space of the beam can be pushed to the appropriate position, not only can the beam be extracted from one side, but also the radial size of the extracted beam can be made smaller.

The harmonic coil is installed on the surface of the long magnetic pole, close to the position of the groove, and is used for precession and extraction of the beam current. The harmonic coil is in the shape of a "banana" that matches the moving direction of the beam, and the "banana-shaped" harmonic coil is convenient for calculation, that is, it includes two parallel arcs with different radii but the same central angle, and the two parallel arcs are separated by A pair of symmetrical arcs are connected, and the harmonic coils are symmetrically placed about the origin and the central plane. In a specific embodiment, the central angle of the parallel arcs is 32°, and the distance between the harmonic coils and the central plane is 10 mm, that is, the magnetic gap is equal to 10mm.

A preferred mode is that the short magnetic poles are also equipped with harmonic coils, and the structure of the harmonic coils located at the short magnetic poles and the radius of the magnetic poles at the position are consistent with the harmonic coils of the long magnetic poles. When the strength of the wave coil, the frequency of the high frequency, and the cavity pressure have all been determined, it is used to adjust the phase space of the beam extraction, so that the extraction efficiency can be improved.

The extraction magnetic channel 4 is used to extract and refocus the beam after the gradient corrector is focused, and its structure is composed of two parts, as shown in Figure 6, which are radial focusing magnets 9 and radial defocusing magnets 10, both of which are A hollow 16-prism structure with a hollow channel composed of 16 identical permanent magnets spliced continuously. Since the beam will continue to move in the fringe field for a period of time after leaving the gradient correction magnet, under the action of the fringe field, the beam shape is flat ellipse, and the radial dimension will be much larger than the axial dimension, so the beam will first enter the radial towards the focusing magnet. The radial focusing magnet 9 and the radial defocusing magnet 10 are respectively used to form a radial focusing quaternary magnetic field and a radial defocusing quaternary magnetic field.

Under the influence of the edge field of the magnetic pole, the beam is in a state of strong radial defocusing and strong axial focusing. Subsequent beam transmission, therefore, the extraction magnetic channel 4 is placed at the position of the opening yoke, the structure of the extraction magnetic channel and the polarization direction of the radial focusing magnet 9 of the magnetic field are shown in Figure 7, which mainly includes two main bodies: radial focusing Magnet 9 and radial defocusing magnet 10. The polarization direction of the radial defocusing magnet is opposite to that of the radial focusing magnet, but the surface magnetic induction of the magnet is about 1T.

The two magnets are composed of 16 magnets in a hexahedral structure, with an inner diameter of 7cm and an outer diameter of 12cm. The beam will first enter the radial focusing magnet 9 and then enter the radial defocusing magnet 10. , so as to avoid beam bombardment inside the magnetic channel as much as possible. In order to match the radial size of the extracted beam with the axial size, the length of the radial focus magnet 9 is not consistent with the length of the radial defocus magnet 10. The length of the radial defocus magnet 9 is about 30 cm, and the length of the radial focus magnet 9 is about 30 cm. The length of 10 is about 25cm.

的计算方式如下：η_ex= N_ex /N_out According to the accelerator of the present invention, the self-extraction efficiency η _ex of the self-extraction cyclotron is generally more than 80% by setting gradient correction magnets, harmonic coils, magnetic channels and long and short magnetic pole structures.

is calculated as follows: η _ex = N _ex /N _out

In the formula, N _ex is the extracted particles that can pass through the magnetic channel, and N _out is all the particles that can be accelerated to the edge of the accelerator.

Compared with the electrostatic deflection plate, self-extraction has a greater extraction advantage, but compared with the cyclotron of the negative hydrogen ion source extracted from the carbon film, there will still be a small amount of beam loss inside the accelerator. For the H+ ion source, the emission beam intensity is generally above 1mA. When the beam energy is large, if there is no interference, the remaining about 20% of the beam will bombard the inside of the accelerator, resulting in a strong induction. Radioactive, not easy to debug and maintain.

Since the beam current is pushed by the harmonic coil, its extraction angle has a good distribution law. By controlling the extracted beam current from the gradient corrector, the position of beam current loss can be effectively controlled. Set the beam dump 11 as shown in FIG. 1 , assuming that the self-extraction efficiency is 80%, then most of the remaining 20% stray beams can bombard the beam dump 11.

The magnetization direction of each magnetic strip of the radial focusing magnet and the radial defocusing magnet needs to be specially designed to achieve the technical purpose of radial focusing of the beam by the radial focusing magnet and axial focusing of the beam by the radial defocusing magnet. In the radial focus magnet, the magnetization direction of each magnetic strip clockwise along the section is set according to the following rules:

Define the magnetization direction B0 of the initial magnetic stripe A0, the magnetization direction B1 of the next magnetic stripe A1 next to the initial magnetic stripe, and the magnetization direction B2 of the next magnetic stripe A2 next to the magnetic stripe A1,

Define the difference rule of magnetization direction: B1-B0=2K, B2-B1=K, K=45 degrees;

Subsequent magnetic strips are replaced by A2 with A0, and the magnetization direction of subsequent magnetic strips changes according to the difference rule;

The magnetization direction of each magnetic strip of the radial defocus magnet is opposite to that of the magnetic strip in the corresponding radial focus magnet.

For example, define the angle between the magnetization direction B0 of the initial magnetic strip A0 and the central horizontal line in Figure 7 to be 135 degrees, and the magnetization direction of the next magnetic strip A1 sorted clockwise is B1=B0+90=225 degrees, and the next magnetization direction next to A1 The magnetization direction of strip A2 is B2=B1+45 degrees=270 degrees, and the magnetization direction of the magnetic strip A3 next to A2 is B3=B2+90=0; that is, the difference between the magnetization directions of adjacent magnetic strips is 90 degrees and 45 degrees staggered Finally, the magnetization direction angles of the 16 magnetic strips from A0 to A15 are: 135, 225, 270, 0, 45, 135, 180, 270, 315, 45, 90, 180, 225, 315, 0, 90 .

After adopting the above magnetization direction design, a four-level magnetic field will be formed on the central plane to constrain the radial and axial directions of the beam. The polarization direction of the radial defocusing magnet is opposite to that of the radial focusing magnet. Its distribution follows the direction of beam movement, avoiding beam loss inside the magnetic channel as much as possible, resulting in induced radioactivity.

The extraction magnetic channel is located at the yoke position corresponding to the short magnetic pole. Since the beam current enters the magnetic channel, under the action of the fringe field, the radial dimension will be much larger than the axial dimension, so the length of the radial focusing magnet will be slightly longer than the radial dimension. For defocusing magnets, the length ratio of the two is usually 1-1.2:1.

The magnetic yoke is used to constrain the main magnetic field. Its structure is not the axisymmetric facility of the common cyclotron. Since the beam is drawn out at the short magnetic pole position, it is necessary to design the corresponding opening of the magnetic yoke. The opening size is slightly larger than the outer diameter of the magnetic channel. , the opening direction follows the beam movement direction, and is symmetrical about the center of the origin, so as to avoid a strong first harmonic.

The excitation coil is used to excite the main magnetic field. Due to the large outer diameter of the magnetic channel, the excitation coil will slightly sink to the position of the cover plate, so it is necessary to carve grooves on the cover plate for placing the excitation coil.

The beam collector is used to collect stray beams to avoid strong induced radioactivity caused by the stray beams bombarding other positions of the main magnet, and the beam collector is placed after the gradient corrector.

Compared with the prior art, the self-extracting cyclotron described in the present invention has the following technical advantages:

1. The present invention can stably accelerate H+ ions. Compared with cyclotrons with internal negative hydrogen ion sources commonly found in the market, the requirements for vacuum degree are lower, and the extraction current from the cyclotron can generally reach 100uA to 1mA or more stably.

2. Compared with the extraction method of the electrostatic deflection plate that also uses the H+ ion source, the extraction efficiency of the self-extraction of the present invention can be stabilized at more than 80%. Since the position of the beam loss is basically controllable, it can reduce the impact of stray beams on the main body of the accelerator.

3. Adopting the isochronous tuning technology of variable magnetic gap plus inserts, which reduces the magnetic field energy that the inserts need to fill up at large radii, and can improve the situation of insufficient acceleration gaps at large radii. The variable magnetic gap can also provide axial The focusing force facilitates the directional delivery of the beam.

The foregoing are various preferred embodiments of the present invention. If the preferred implementations in each preferred embodiment are not obviously self-contradictory or based on a certain preferred implementation, each preferred implementation can be used in any superposition and combination. The above examples and the specific parameters in the examples are only for clearly expressing the inventor's invention verification process, and are not used to limit the scope of patent protection of the present invention. The scope of patent protection of the present invention is still subject to its claims. The equivalent structural changes made in the description of the present invention should be included in the protection scope of the present invention in the same way.

Claims (8)

1. A self-extracting cyclotron, comprising a pair of cover plates and a pair of excitation coils located in the cover plates, and two groups of main magnets placed face to face in a pair of excitation coils inboards for forming the main magnetic field, characterized in that, The main magnets in each group are composed of two sector-shaped long magnetic poles (1) and two short magnetic poles (2). The long and short magnetic poles are distributed alternately. The center of the circle coincides, a fan-shaped valley is formed between two adjacent magnetic poles, and the magnetic pole gap between the long and short magnetic poles changes with the radius; each long magnetic pole end is provided with an arc-shaped long magnetic pole groove (7); The two straight sides of the fan-shaped poles are pasted with strips (6); the fan-shaped valleys are divided into the first fan-shaped valleys and the second fan-shaped valleys, and the first fan-shaped valleys are provided with high-frequency DEE plate (13), and a false DEE plate (8) is arranged on the straight sides of the magnetic poles on both sides of the high-frequency DEE plate. 2. The self-extracting cyclotron according to claim 1, wherein the cover plate is provided with a circular groove, and the excitation coil is located in the circular groove. 3. The self-extracting cyclotron according to claim 1, characterized in that, the second fan-shaped valley is provided with a gradient corrector (3), and the gradient corrector includes a pair of magnet groups arranged symmetrically with respect to the main magnetic field plane, Each magnet group includes correction focus magnets and correction shielding magnets (33), wherein the correction focus magnets are composed of inner focus magnets (32) and outer focus magnets (31) placed face to face, and the inner focus magnet magnetic field (32) is close to the inside of the magnetic field The direction is opposite to the main magnetic field; the magnetic field direction of the outer focusing magnet close to the outside of the magnetic field is the same as that of the main magnetic field, and the residual magnetic strength of the inner focusing magnet and the outer focusing magnet are consistent; the side of the inner focusing magnet close to the center of the main magnetic field is provided with a correction shielding magnet ( 33), the direction of the magnetic field of the correction shielding magnet is the same as that of the main magnetic field, and the direction of the gap between the inner focusing magnet (32) and the outer focusing magnet (31) is consistent with the direction of the tangent at the end of the long magnetic pole slot. 4. The self-extracting cyclotron according to claim 1, characterized in that a harmonic coil (5) is installed at the center of the main magnetic field where the long magnetic pole slot is directed, and the harmonic coil includes two sections with different radii but centered Parallel arcs with the same angle, and two parallel arcs are connected by a pair of symmetrical arcs. 5. The self-extracting cyclotron as claimed in claim 1, characterized in that, in the beam extraction direction outside the main magnet, an extraction magnetic channel (4) is provided, and the extraction magnetic channel includes a hollow cylindrical column with opposite polarization directions. Radial focusing magnet (9) and radial defocusing magnet (10), wherein the radial focusing magnet is in front. 6. The self-extracting cyclotron as claimed in claim 3, characterized in that, the radial focusing magnet and the radial defocusing magnet are positive hexagonal hollow columnar structures formed by continuous splicing of 16 identical magnetic strips, the In the radial focus magnet, the magnetization direction of each magnetic strip in the clockwise direction along the section is set according to the following rules: Define the magnetization direction B0 of the initial magnetic stripe A0, the magnetization direction B1 of the next magnetic stripe A1 next to the initial magnetic stripe, and the magnetization direction B2 of the next magnetic stripe A2 next to the magnetic stripe A1, Define the difference rule of magnetization direction: B1-B0=2K, B2-B1=K, K=45 degrees; Subsequent magnetic stripes replace A0 with A2, and the magnetization direction of subsequent magnetic stripes changes according to the difference rule; The magnetization direction of each magnetic strip of the radial defocus magnet is opposite to that of the magnetic strip in the corresponding radial focus magnet. 7. The self-extracting cyclotron according to claim 1, further comprising a beam collector (11) located in the beam extraction direction, installed after the gradient corrector and before the extraction magnetic channel. 8 . The self-extracting cyclotron according to claim 4 , wherein a harmonic coil having the same position and shape as the harmonic coil on the long magnetic pole is installed on the short magnetic pole.

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