CN118591073A - A magnet system for an ECR ion source producing high-intensity He2+ ions - Google Patents

Abstract

The invention provides a magnet system of ECR ion source for generating high-current-intensity He ²⁺ ions, which utilizes the structural space of the original 2.45GHzECR ion source, increases a constraint magnetic field formed by alternately arranging radial magnetic fields and tangential magnetic fields, and is characterized in that: the confining area formed by the confining magnetic field is nearly equal to the resonance area of helium di-positive ions generated in the middle of the discharge cavity, the helium di-positive ions and electrons are confined in the resonance area in the middle of the discharge cavity through the confining magnetic field, and the proportion of the helium di-positive ions in the mixed beam of the resonance area is improved; the radial tangential confinement magnet is added into the low-frequency ECR ion source which is only suitable for generating single charge state, so that helium di-positive ions with high current intensity in medium charge state are generated, and the optimal combination point is found between the improvement of the yield of helium di-positive ions and the reduction of the cost.

Description

A magnet system for an ECR ion source producing high-intensity He2+ ions

Technical Field

The invention belongs to the technical field of cyclotron accelerators, and in particular relates to a magnet system of an ECR ion source for generating high-current He ²⁺ ions.

Background Art

Multiply charged ions (MCIs) are necessary to increase the energy of accelerator particle output. Compared with high-frequency ECR ion sources that require very strong axial and radial fields, it is more economical to manufacture low-frequency (e.g., 2.45 GHz) ECR sources.

The low-frequency 2.45 GHz ECR ion source has the advantages of compact structure, long life, stable source operation and low economic cost. Therefore, some researchers suggest using the 2.45 GHz ECR ion source to produce medium-charged ion beams. Whether using permanent magnets or electromagnetic coils, the traditional magnetic field configuration of the 2.45 GHz ECR ion source is roughly divided into a resonant magnetic field (875 Gs or 930 Gs), an off-resonance magnetic field and a magnetic mirror field. And relying solely on the axial resonant field, the 2.45 GHz ECR ion source has achieved remarkable results in producing singly charged positive ions.

However, for the 2.45GHz ECR ion source, the beam drawn out this time is actually a mixed beam. The mixed beam mainly includes He ²⁺ and He ⁺ plasma. The difference between producing He ²⁺ and He ⁺ is that the steps are one more than the steps of producing protons from hydrogen, because it needs to collide twice, which is a step-by-step collision ionization. First, helium positive is produced, and then it needs to collide again to produce helium dipositive.

The difficulty in producing He ²⁺ using a 2.45GHz ECR ion source lies in the fact that because it requires a second collision, it involves the processes of dissociation, recombination, and charge exchange. Precisely because of the processes of dissociation, recombination, and charge exchange, the proportion of helium dipositive produced is extremely small, and the proportion of He ²⁺ produced in the mixed beam is far from enough.

Summary of the invention

In view of the problems existing in the prior art, the present invention proposes a magnet system for an ECR ion source that produces high-intensity He ²⁺ ions, aiming to solve the problem that the proportion of He ²⁺ produced by the prior art is extremely small and far from enough.

In order to solve the technical problems, the present invention proposes the following technical solutions:

A magnet system for an ECR ion source that generates high-intensity He ²⁺ ions. The system utilizes the structural space of an original 2.45GHz ECR ion source and adds a confining magnetic field consisting of an alternating radial magnetic field and a tangential magnetic field. The system is characterized in that: the confining area formed by the confining magnetic field is approximately equal to the resonance area in the middle of a discharge cavity where He ²⁺ ions are generated. The confining magnetic field confines He ⁺ ions and electrons in the resonance area in the middle of the discharge cavity, thereby increasing the proportion of helium dications in a mixed beam in the resonance area.

Furthermore, the structural space of the original 2.45GHzECR ion source is as follows: arranged along the axial direction from one end to the other end: the injection magnetic ring, the intermediate magnetic ring, and the lead-out magnetic ring; arranged along the radial direction from the inside to the outside: the discharge chamber, the clip device, and the intermediate magnetic ring; the injection magnetic ring and the lead-out magnetic ring are on both sides of the discharge chamber; the clip device is outermost of the intermediate magnetic ring and innermost of the discharge chamber; the inner diameter and outer diameter of the injection magnetic ring, the intermediate magnetic ring, and the lead-out magnetic ring are all the same.

Furthermore, the confining magnetic field is composed of a plurality of radial magnetic bars uniformly arranged circumferentially on the spare groove of the clip device, the length and area of the radial magnetic bar rectangle, and the residual magnetism of the radial magnetic bar, as well as a plurality of tangential magnetic bars uniformly arranged circumferentially on the spare groove of the clip device, the length and area of the tangential magnetic bar rectangle, and the residual magnetism of the tangential magnetic bar.

Furthermore, the plurality of radial magnetic bars are six in number; the length of the radial magnetic bars is the length of the discharge chamber, and the cross-section thereof is 6×9 mm.

Furthermore, the number of the plurality of tangential magnetic bars is 6; the length of the tangential magnetic bar 6 is the length of the discharge chamber, and the cross-section thereof is 8×9 mm.

Furthermore, the six radial magnetic bars are of N38 brand with a residual magnetism of 1.25T; the magnetization direction of the radial magnetic bars is along the radial direction, and the polarities of adjacent radial magnetic bars are opposite; the maximum field strength of the multi-peak tangent magnetic field provided on the discharge chamber wall reaches about 1860Gs.

Furthermore, the six tangential magnetic bars are of N35 brand with a residual magnetism of 1.21T; the magnetization direction of the radial magnetic bars is along the circumferential direction, and the polarities of adjacent tangential magnetic bars are opposite; by increasing the tangential magnets, the maximum field strength on the discharge chamber wall reaches about 2580Gs.

Furthermore, the radial distance between the clip device and the injection magnetic ring, the intermediate magnetic ring, and the lead-out magnetic ring is 4.5 mm.

Advantages and effects of the present invention

1. The plasma usually produced by the 2.45GHz ECR ion source is in a high-density state. Since it is a low-frequency ECR source, the value of the resonant magnetic field is only around 875Gs or 930Gs. Considering the electron relativistic effect and the Doppler frequency shift that lead to the widening of the resonance zone, the three permanent magnet rings produce an off-resonance field or a magnetic mirror field with a relatively high magnetic mirror ratio. Using the highest possible magnetic mirror ratio can enhance the confinement effect of the magnetic mirror field and contribute to the production of highly charged helium ions.

2. Six permanent magnet rods are used to provide radial confinement fields that are superimposed on the original axial magnetic field to form a "minimum magnetic field structure", which greatly enhances the synthetic magnetic field in the middle part, thereby increasing the magnetic pressure in the middle part of the magnetic mirror to achieve the purpose of stabilizing the plasma. Since the value of the resonant magnetic field itself is not very large, according to the proportional relationship of previous research experience, the strength of the radial confinement magnetic field is approximately close to 2000Gs. By enhancing the magnetic confinement of the plasma, the helium plasma is confined in the middle of the discharge cavity to prevent it from moving to the edge and colliding with the cavity wall, thereby reducing the loss of the He ²⁺ ion beam, helping to increase the yield of He ²⁺ ions and increasing the proportion of He ²⁺ in the mixed beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a cross-sectional view of a magnet system of an ECR ion source for generating high-intensity He ²⁺ ions according to the present invention;

FIG1b is a side view of a magnet system of an ECR ion source for generating high-intensity He ²⁺ ions according to the present invention;

FIG. 2 is a schematic diagram of the radial confinement magnetic field of the magnet system of the ECR ion source for producing high-intensity He ²⁺ ions according to the present invention.

FIG3 is a schematic diagram of arranging radial magnets and tangential magnets on the spare groove of the clip device according to the present invention;

1: injection magnetic ring; 2: clip device; 2-1: spare slot; 3: intermediate magnetic ring; 4: lead-out magnetic ring; 5: radial magnetic rod; 6: tangential magnetic rod.

DETAILED DESCRIPTION

Design principle of the present invention

1. The innovation of the present invention is that a radial tangential confinement magnet is added to the low-frequency ECR ion source that originally produces a single-charge state, thereby producing high-current ^He2+ ions with a medium charge state (medium and high charge states refer to the number of positive charges carried by charged particles, and when the number of positive charges is 2 to 6, it is called a medium-charge state ion).

First, the difference between the present invention and the conventional method is that the conventional method uses medium-high frequency ECR to generate ions with medium and high charge states, while the present invention uses a low-frequency ECR ion source to generate He ²⁺ ions.

Second, the reason why medium-high frequency ECR is used in the conventional method to produce medium-high charge ions (the medium-high frequency ECR refers to an ECR ion source greater than 5 GHz): the reason why medium-high frequency ECR is used when producing medium-high charge ions is that multiple collisions are required to become ions with multiple positive charges. In order to ensure the stable existence of medium-high charge ions, a very strong confinement magnetic field is required. A very strong confinement magnetic field requires a very strong axial magnetic field, that is, a very strong resonant magnetic field. It is not like the 2.45GHz ECR ion source of the present embodiment, which has a magnetic field of only 1000 Gauss, but it may be around several thousand Gauss. First of all, its axial resonant magnetic field is very strong. If a radial confinement magnet is added, the strength of the confinement magnet can reach about a little more than Tesla. Such a very strong magnetic field is used to produce medium-high charge states. Conclusion: According to the traditional method, if you want to pursue medium-high charge states, you have to sacrifice the current intensity. If you want to produce such a strong current medium-high charge ions, you have to work hard on the magnetic field. According to the relevant formula, the magnetic field is related to the frequency of the microwaves fed in. A strong magnetic field requires a high frequency to be fed in. Therefore, the conventional method of generating ions with medium and high charge states uses a medium and high frequency ECR source.

Third, the present invention takes into account the interests of both (increasing the output of He ²⁺ and reducing economic costs) and finds the best combination point. Since it is not necessary to generate a very strong radial confinement magnetic field when changing from He ⁺ to He ²⁺ , it is completely possible to utilize the low resonance magnetic field required by the 2.45GHzECR ion source and the structural characteristics of the 2.45GHzECR ion source to establish a confinement magnetic field. Since the resonance magnetic field required by the 2.45GHzECR ion source is low, the radial confinement field matched therewith is also relatively low, which means that the space occupied by the confinement magnetic field established on the 2.45GHzECR ion source is small, which is the first feature that can be utilized by the low-frequency 2.45GHzECR ion source; since the clip device 2 of the 2.45GHzECR ion source reserves a spare slot 2-1 along the circumferential direction, and since the space occupied by the required radial confinement magnetic field is relatively small, it is not necessary to use a magnet array structure composed of multiple permanent magnet blocks as in the conventional method, and the space of the existing spare slot 2-1 can be utilized to establish the confinement magnetic field, which is the second feature that can be utilized by the low-frequency 2.45GHzECR ion source.

2. Design principle of the confining magnetic field: First, the difficulty in designing the confining magnetic field is that the confining range of the confining magnetic field must meet the requirements. It is not like the previous multi-peak field confining magnetic field. The previous multi-peak field confining magnetic field has no principled limit on the size of the confining field. The present invention must limit the range of the confining field to within the resonance region, and the confining field area is almost equal to the resonance region. When no radial constraint is added, the diameter of the resonance region accounts for about 80% of the diameter of the entire discharge chamber. The present invention limits the action area of the confining magnetic field to be almost consistent with the resonance region. It has been proved through experiments that when the control area of the confining magnetic field is consistent with the resonance region, the yield of He ²⁺ ions can be increased from tens of microamperes to hundreds of microamperes. Second, if the design requirements of the confining magnetic field are to be met, six aspects need to be coordinated: the number, length, and cross-sectional area of the radial magnetic rods 5, and the number, length, and cross-sectional area of the tangential magnetic rods 6; the combined effect of these six aspects is to control the range of the confining magnetic field to be consistent with the resonance region. The detailed design of the dimensions of the confining field of the present invention is shown in Example 1 of this application.

Based on the above principle, the present invention relates to a magnet system of an ECR ion source for generating high-current He ²⁺ ions, as shown in FIG1, FIG2, and FIG3. The system utilizes the structural space of the original 2.45GHz ECR ion source and adds a confining magnetic field composed of alternating radial magnetic fields and tangential magnetic fields. The system is characterized in that: the confining area formed by the confining magnetic field is approximately equal to the resonance area in the middle of the discharge cavity where He ²⁺ ions are generated. The confining magnetic field confines the He ⁺ ions and electrons in the resonance area in the middle of the discharge cavity, thereby increasing the proportion of He ²⁺ ions in the mixed beam in the resonance area.

Furthermore, the structural space of the original 2.45GHzECR ion source is: arranged along the axial direction from one end to the other end: injection magnetic ring 1, intermediate magnetic ring 3, lead-out magnetic ring 4; arranged along the radial direction from inside to outside: discharge chamber, clip device 2, intermediate magnetic ring 3; injection magnetic ring 1 and lead-out magnetic ring 4 are on both sides of the discharge chamber; the clip device 2 is covered with the intermediate magnetic ring 3 and the discharge chamber; the inner diameter and outer diameter of the injection magnetic ring 1, intermediate magnetic ring 3 and lead-out magnetic ring 4 are the same.

Furthermore, the confining magnetic field is composed of a plurality of radial magnetic bars 5 uniformly arranged circumferentially on the spare slot 2-1 of the clip device 2, the length and area of the rectangular parallelepiped of the radial magnetic bars 5, and the residual magnetism of the radial magnetic bars, as well as a plurality of tangential magnetic bars 6 uniformly arranged circumferentially on the spare slot 2-1 of the clip device 2, the length and area of the rectangular parallelepiped of the tangential magnetic bars 6, and the residual magnetism of the tangential magnetic bars 6.

Furthermore, the number of the plurality of radial magnetic bars 5 is six; the length of the radial magnetic bars 5 is the length of the discharge chamber, and the cross-section thereof is 6×9 mm.

Furthermore, the number of the plurality of tangential magnetic bars 6 is six; the length of the tangential magnetic bar 6 is the length of the discharge chamber, and the cross-section thereof is 8×9 mm.

Furthermore, the magnet grade of the six radial magnetic bars 5 is N38, and the residual magnetism is 1.25T; the magnetization direction of the radial magnetic bars is along the radial direction, and the polarities of adjacent radial magnetic bars 5 are opposite; the maximum field strength of the multi-peak tangent magnetic field provided on the discharge chamber wall reaches about 1860Gs.

Furthermore, the six tangential magnetic bars 6 are of N35 grade with a residual magnetism of 1.21T; the magnetization direction of the radial magnetic bars 5 is along the circumferential direction, and the polarities of adjacent tangential magnetic bars 6 are opposite; by increasing the tangential magnets, the maximum field strength on the discharge chamber wall reaches about 2580Gs.

Furthermore, the distance between the clip device 2 and the injection magnetic ring 1, the intermediate magnetic ring 3, and the lead-out magnetic ring 4 is 5 mm.

Additional notes:

As shown in FIG1 , the injection magnetic ring 1 and the lead-out magnetic ring 4 are respectively located on both sides of the discharge chamber in the left front view, and the inner diameter and outer diameter of the three magnetic rings are the same, which are 40 mm and 55 mm respectively. In the ECR source, since the six-pole permanent magnet is often placed in the axial coil and the iron yoke, in order not to sacrifice the axial magnetic field, the outer diameter of the six-pole permanent magnet must be limited. In addition, under the premise that the diameter of the discharge chamber is determined, the size of the permanent magnet bar is also determined. Taking advantage of the compact structure of the 2.45GHz ECR ion source and the low required resonance magnetic field, it is not necessary to generate a very strong radial confinement magnetic field, so there is no need to use a magnet array structure composed of multiple permanent magnet blocks. Only 6 small permanent magnet bars can meet the design requirements of the multi-peak cut field. The size of the permanent magnet bar is 6×9 mm, and the length is equal to the length of the discharge chamber. The permanent magnet bar is close to the cooling circulation component of the discharge chamber and is fixed by the clip device 2. The clip device 2 is sleeved in the middle magnetic ring 3. It can be seen from the top view on the right that it is located in the concentric circle of the middle magnetic ring 3.

As shown in Fig. 2, a top view of a radially constrained multi-peak field magnet 5 of a magnet system of an ECR ion source for generating high-current He ²⁺ ions of the present invention is shown: a permanent magnet bar is close to the discharge chamber. Where a is a radial magnet, with polarity pointing to the radial direction of the circumference, providing a multi-peak tangential field; b is a tangential magnet, with polarity pointing to the tangential direction of the circumference, providing a tangential magnetic field.

In the 2.45GHz ECR ion source with a full permanent magnet structure, since the field strength of the excitation magnetic field is relatively weak (up to more than one thousand gauss), the magnetic field of the confinement magnet used does not need to be too strong (more than two thousand gauss can meet the requirements). The radial magnetic confinement is used to limit the generated plasma to the center of the discharge chamber to avoid collisions between them and the wall and cause ion loss.

Example 1: Calculation of various dimensions of the confining magnetic field

1. The first step: determine the length, height, and radial distance between the restraining magnets (radial magnetic rods 5 and tangential magnetic rods 6) and the magnetic ring. According to the size of the discharge chamber and the inner diameter of the magnetic ring, the radial distance between the restraining magnets and the magnetic ring is 4.5 mm. The height of the cross section of the radial magnetic rod 5 is determined to be 9 mm, and the length of the magnetic rod is equal to the length of the discharge chamber.

2. The second step: determine the cross-sectional dimensions of the radial magnetic bar 5. When no tangential magnet is added, the cross-sectional height of the radial magnetic bar 5 is determined to be 9 mm, and the magnetization is 1.21 T, the maximum field strength of the inner wall of the discharge chamber corresponding to different cross-sectional widths of the radial magnetic bar 5 is shown in Table 1

Table 1

Cross-sectional dimensions of radial magnetic rods/mm Maximum electric field strength of the inner wall of the discharge chamber/Gs 4×9 1330 5×9 1590 6×9 1790 7×9 1970

The cross-sectional dimensions of the magnetic bar are selected to be 6×9 mm when the maximum field strength/Gs of the inner wall of the discharge chamber is 1790, and the magnetic field strength generated is closer to twice the resonance field strength (1750 Gs).

3. The third step: determine the magnetization amount of the radial magnetic rod 5. When the radial magnetic rod 5 is selected with a cross-sectional size of 6×9 mm, the maximum field strength of the inner wall of the discharge chamber corresponding to the radial magnetic rod 5 with different magnetization amounts is shown in Table 2

Table 2

Magnetization of radial magnetic bar/T Maximum electric field strength of the inner wall of the discharge chamber/Gs 1.21 1790 1.25 1860 1.28 2080 1.31 2140

It can be seen from Table 2 that the field strengths generated by the radial magnetic bars 5 with four different magnetization amounts are all greater than twice the resonant magnetic field. In order to facilitate the addition of radial magnetic bars 5 with different magnetization amounts to adjust the maximum field strength of the inner wall of the discharge chamber, a magnetization amount of 1.25T is selected as a compromise.

4. Step 4: Determine the cross-sectional dimensions of the tangential magnet. When the radial magnetic bar 5 is selected with a cross-sectional dimension of 6×9 mm and a magnetization of 1.25 T, and the magnetization of the tangential magnet is 1.21 T, the maximum field strength of the inner wall of the discharge chamber corresponding to the addition of tangential magnets with different cross-sectional widths is shown in Table 3.

Table 3

Considering that after adding 8×9mm tangential magnet, the maximum field strength of the inner wall of the discharge chamber reaches 2580Gs, the magnetic field strength is closer to three times the resonant magnetic field (2625Gs), and it is convenient to adjust the magnetic field strength. In summary, a tangential magnet with a cross-sectional size of 8×9mm is selected.

5. Explanation for the number of radial magnetic rods 5 and tangential magnetic rods 6: Previous studies have shown that the more magnetic poles there are in a multi-peak field, the stronger the magnetic confinement ability is. However, at the same time, the frequency of electrons escaping from the magnetic mirror loss cone is greater. Therefore, it is necessary to consider comprehensively and select the optimal number of magnetic poles. The quadrupole field with superimposed mirror fields has only a very thin loss line at both ends, which makes it difficult to extract ions. Therefore, in most modern ECR sources with synthetic magnetic field structures composed of superimposed axial and radial fields, the sextupole field configuration is the first choice. This time, the number of radial magnetic rods 5 selected is 6, and the number of tangential magnetic rods 6 to match it is also 6.

The above contents are merely examples and explanations of the concept of the present invention. The technicians in this technical field may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should all fall within the protection scope of the present invention.

Claims (8)

1. A magnet system for an ECR ion source that produces high-intensity He2 ⁺ ions. The system utilizes the structural space of the original 2.45GHz ECR ion source and adds a confining magnetic field composed of alternating radial magnetic fields and tangential magnetic fields. The system is characterized in that: the confining area formed by the confining magnetic field is nearly equal to the resonance area in the middle of the discharge chamber where helium dications are produced. The confining magnetic field confines helium+ particles and electrons in the resonance area in the middle of the discharge chamber, thereby increasing the proportion of helium dications in the mixed beam in the resonance area. 2. A magnet system for an ECR ion source that produces high-intensity He2 ⁺ ions according to claim 1, characterized in that: the structural space of the original 2.45GHz ECR ion source is: arranged along the axial direction from one end to the other end: an injection magnetic ring (1), an intermediate magnetic ring (3), and an extraction magnetic ring (4); arranged along the radial direction from the inside to the outside: a discharge chamber, a clamping device (2), and an intermediate magnetic ring (3); the injection magnetic ring (1) and the extraction magnetic ring (4) are on both sides of the discharge chamber; the clamping device (2) is surrounded by the intermediate magnetic ring (3) and the discharge chamber; the inner diameter and outer diameter of the injection magnetic ring (1), the intermediate magnetic ring (3), and the extraction magnetic ring (4) are the same. 3. According to claim 2, a magnet system for an ECR ion source that produces high-intensity He2 ⁺ ions is characterized in that: the confining magnetic field is composed of a plurality of radial magnetic bars uniformly arranged circumferentially on the spare groove of the clip device (2), the length and area of the radial magnetic bar rectangle, and the residual magnetism of the radial magnetic bar, and a plurality of tangential magnetic bars uniformly arranged circumferentially on the spare groove of the clip device (2), the length and area of the tangential magnetic bar rectangle, and the residual magnetism of the tangential magnetic bar. 4. A magnet system for an ECR ion source that produces high-intensity He ²⁺ ions according to claim 3, characterized in that the plurality of radial magnetic bars are 6 in number, the length of which is the length of the discharge chamber, and the cross-section of which is 6×9 mm. 5. A magnet system for an ECR ion source that produces high-intensity He ²⁺ ions according to claim 3, characterized in that the plurality of tangential magnetic rods are 6 in number; the length of the tangential magnetic rod is the length of the discharge chamber, and the cross-section thereof is 8×9 mm. 6. According to claim 4, a magnet system for an ECR ion source that produces high-intensity He2 ⁺ ions is characterized in that: the six radial magnetic bars are of N38 brand with a residual magnetism of 1.25T; the magnetization direction of the radial magnetic bars is along the radial direction, and the polarities of adjacent radial magnetic bars are opposite; and the multi-peak tangential magnetic field provided has a maximum field strength of about 1860Gs on the wall of the discharge chamber. 7. A magnet system for an ECR ion source that produces high-intensity He ²⁺ ions according to claim 5, characterized in that: the six tangential magnetic bars are of N35 grade with a residual magnetism of 1.21T; the magnetization direction of the radial magnetic bars is along the circumferential direction, and the polarities of adjacent tangential magnetic bars are opposite; by adding tangential magnets, the maximum field strength on the discharge chamber wall reaches about 2580Gs. 8. A magnet system for an ECR ion source that produces high-intensity He ²⁺ ions according to claim 2, characterized in that the distance between the clamp device (2) and the injection magnetic ring (1), the intermediate magnetic ring (3), and the lead-out magnetic ring (4) is 4.5 mm.