A kind of based on 2.45GHz ECR ion source dynamic discharge cavity extraction structure

Abstract

The application discloses a kind of based on 2.45GHz ECR ion source dynamic discharge cavity extraction structure, the extraction structure is composed of four cylindrical electrodes including two ground electrodes in inside and outside nesting;The two acceleration regions of the inside and outside nesting four cylindrical electrode groups constitute the extraction zone;The two acceleration regions are used to adjust the field line distribution of extraction zone;The deceleration region between suppression electrode 3 and second ground electrode 4 is used to suppress the electron accelerated to the source in reverse;First electrode 2 prevents the damage caused by excessive load on the power supply of suppression electrode.It is changed with the size change of the inner diameter of dynamic discharge cavity that the gap d between the four electrodes and the respective radius r change.The application changes the field line distribution of the original extraction structure by adding a ground electrode between the plasma electrode and the suppression electrode, so that the extracted ion passes through two acceleration processes.Due to the increase of the ground electrode between the plasma electrode 1 and the suppression electrode 3, the frequent occurrence of sparking phenomenon caused by excessive electric and magnetic field at the extraction port is avoided.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of cyclotron, in particular to a kind of based on 2.45GHz ECR ion source dynamic discharge cavity extraction structure. BACKGROUND

[0002] 2.45GHz ECR ion source dynamic discharge cavity refers to the condition that the size of discharge cavity is unchanged, increase the discharge cavity lining, the discharge cavity lining is equipped in the discharge cavity, the material of discharge cavity lining adopts the aluminum with high secondary electron emission coefficient, on the basis of the limitation law of the minimum size of discharge cavity, the diameter of discharge cavity is appropriately reduced to improve the extracted flow intensity;

[0003] The new problems brought by reducing the diameter of discharge cavity to improve the extracted flow intensity are: first, due to the increase of the flow intensity in the extraction area, the strong electric field and magnetic field at the extraction port lead to the easy occurrence of sparking in the extraction area;Second, due to the increase of the flow intensity in the extraction area, the continuous change of the position of the extraction port leads to the more difficult adjustment of the field line uniformity in the extraction area.

[0004] One of the reasons for the above-mentioned easy occurrence of sparking in the extraction area is shown in Figure 1 、 Figure 2 The existing 2.45GHz ECR ion source adopts a three-electrode extraction system based on two high-voltage power supplies, the third electrode is generally at ground potential, which is called ground electrode. The second electrode is called suppression electrode, which is at negative potential relative to the ground electrode. Positive ions are accelerated from the plasma electrode 1 to the suppression electrode 2, and then decelerated to ground potential. The main function of this mechanism is to suppress the electrons accelerated in the opposite direction into the source by the suppression electrode to protect the microwave window. However, this mechanism is also prone to sparking: as shown in Figure 2 During the process of increasing the high voltage, due to the increase of the flow intensity at the extraction port and the strong magnetic field at the extraction port, the sparking caused by the strong magnetic field is also very sensitive to the vacuum state. If the vacuum state at the extraction port is not good, the arc phenomenon is easy to occur between the plasma electrode and the suppression electrode.

[0005] The reasons for the above-mentioned more difficult adjustment of the field line uniformity in the extraction area are shown in Figure 1 、 Figure 2 Firstly, the existing 2.45GHz ECR ion source adopts a three-electrode structure, which forms an acceleration region and a deceleration region. It is difficult to adjust the field line uniformity under the condition of only one acceleration region. Secondly, the position of the three electrodes is fixed, and the acceleration gap d is not adjustable. The acceleration gap d is large at the deepest position of the plasma electrode 1 (such as the second peak shown in Figure 5 ), and the influence of electric field on the shape of emission surface is small, which leads to the problem of difficult adjustment of the field line uniformity in the extraction area. SUMMARY

[0006] The application is to solve the problems existing in the prior art, and proposes an extraction structure based on a 2.45GHz ECR ion source dynamic discharge cavity. The first purpose is to solve the problem that the extraction zone of the 2.45GHz ECR ion source dynamic discharge cavity is prone to sparking due to the increase of the current in the extraction zone. The second purpose is to solve the problem that it is more difficult to adjust the field line uniformity of the 2.45GHz ECR ion source with a three-electrode structure because there is only one acceleration region. The third purpose is to solve the problem that the acceleration gap d between the deepest position of the plasma electrode 1 of the 2.45GHz ECR ion source and the second electrode is very large, the electric field has little effect on the shape of the emission surface, and it is difficult to adjust the field line uniformity of the extraction zone.

[0007] The application proposes the following technical solutions to solve the technical problems:

[0008] An ECR ion source extraction structure based on a dynamic discharge cavity, characterized in that the extraction structure is composed of four nested cylindrical electrodes including two ground electrodes; the four nested cylindrical electrodes form two acceleration regions for adjusting the field line distribution of the extraction zone; the deceleration region between the suppression electrode and the second ground electrode is used to suppress the electrons accelerated in the reverse direction into the source; the first ground electrode is used to prevent damage caused by excessive load on the suppression electrode power supply. The gap d between the four cylindrical electrodes and the radius r of each cylindrical electrode changes with the size of the inner diameter of the dynamic discharge cavity.

[0009] Further, the extraction structure sequentially comprises a plasma electrode, a first ground electrode, a suppression electrode, and a second ground electrode; the two acceleration regions are a first acceleration region composed of the plasma electrode and the first ground electrode, and a second acceleration region composed of the first ground electrode and the suppression electrode; the two ground electrodes, the first ground electrode is used to divide the plasma electrode and the suppression electrode into two acceleration regions; the second ground electrode is used to decelerate the ions after acceleration through the two acceleration regions to the ground potential, thereby suppressing the electrons accelerated in the reverse direction into the source and protecting the microwave window.

[0010] Further, the angle between all electrodes and the axis of the discharge cavity is 45°.

[0011] Further, the extraction port of the plasma electrode is a circular hole with a diameter of 6-8mm.

[0012] Further, the hole diameters of the first ground electrode, the suppression electrode, and the second ground electrode are 8mm.

[0013] Further, the distance between the plasma electrode and the first ground electrode is 10mm; the distance between the first ground electrode and the suppression electrode is 3mm; the distance between the suppression electrode and the second ground electrode is 3mm.

[0014] Further, the material of the electrodes is copper or stainless steel.

[0015] Advantages and effects of the present application

[0016] 1、The present application adds a ground electrode between the plasma electrode and the suppression electrode, for the general four-electrode system, if the intermediate electrode is grounded, the intermediate electrode shields the strong focusing force of the acceleration zone in the general three-electrode system, the ions pass through two acceleration processes, and thus the focusing strength in the deceleration zone is weakened, and the aberration is also reduced to a certain extent. At the same time, the field line distribution of the original extraction structure is changed, and a relatively ideal electric potential equipotential line distribution is formed, which is crucial for beam extraction and transmission, reduces the occurrence of Penning discharge in the extraction zone, and is beneficial to the stable operation of the source.

[0017] 2、The present application changes the length of the plasma electrode into the discharge chamber, thereby changing the magnetic field size near the extraction port, and the flexible change of the extraction port position is beneficial to the research on the influence of the magnetic field size at different extraction port positions on the extracted beam. At the same time, the frequent occurrence of sparking phenomenon caused by too large electric field and magnetic field at the extraction port can be avoided. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 A schematic diagram of an embodiment of the three-electrode extraction structure of the previous 2.45Hz ECR ion source under an extraction high voltage of 30-50kV;

[0019] Figure 2 A diagram of the ion source power supply potential interrelation of the three-electrode extraction structure of the previous 2.45Hz ECR ion source under an extraction high voltage of 30-50kV;

[0020] Figure 3 A schematic diagram of the extraction structure of the ECR ion source based on the dynamic discharge cavity after improvement of the present application;

[0021] Figure 4 A diagram of the ion source power supply potential interrelation of the four-electrode extraction structure of the ECR ion source based on the dynamic discharge cavity after improvement of the present application;

[0022] Figure 5 A diagram of the axial magnetic field distribution of the ECR ion source permanent magnet;

[0023] Figure 6 A diagram of the electric field distribution in the original structure;

[0024] Figure 7 A diagram of the electric potential equipotential line distribution of the present application after changing the field line distribution of the original extraction structure. DETAILED DESCRIPTION

[0025] Design principle of the present application

[0026] 1、Innovation point of the present application

[0027] The innovation lies in overcoming traditional biases and giving the ground electrode a new purpose. (Traditional bias: the ground electrode is only used to form a reverse voltage drop with the suppressor electrode to suppress electrons accelerated in the reverse direction into the source and protect the microwave window.) By adding a first ground electrode between the plasma electrode 1 and the suppressor electrode 3, a dual function is achieved: it solves the problem of arcing at the lead-out point and also improves the difficulty in adjusting the uniformity of the field lines in the lead-out region. By cleverly utilizing the characteristic that the first ground electrode 2 is at a low potential relative to the plasma electrode 1, and at a high potential relative to the suppressor electrode 3 (which is at a negative potential), a two-stage acceleration field is achieved, thus solving the problem of difficulty in adjusting the uniformity of the field lines using only a single-stage acceleration field. Furthermore, since the first ground electrode 2 is placed between the plasma electrode 1 and the suppressor electrode 3, it also helps to prevent arcing from the high-voltage power supply and protects the suppressor electrode 3 from damage by high-voltage discharge.

[0028] 2. Design Objectives of this Invention

[0029] The design goal is to improve beam extraction efficiency by addressing the frequent sparking issue in the three-electrode extraction system while considering the aspect ratio (r / d) of the first electrode. The dynamic discharge cavity addresses the issue of increasing the extraction current intensity by increasing the plasma density within the cavity. The extraction current intensity is proportional to the product of the extraction efficiency and the ideal space charge confinement current. Achieving high extraction efficiency requires optimizing the extraction system to make the extraction current intensity infinitely close to the ideal space charge confinement current. The concept of space charge confinement current is the space charge limit that can be accommodated between two electrodes under sufficient plasma density conditions, given the extraction structure (electrode shape, aperture radius r, and accelerating gap d) and extraction voltage U. Therefore, improving extraction efficiency involves addressing the shapes of the four electrodes, the aperture radius r, and the gaps d between them.

[0030] 3. Design of the shape, aperture r, and gap d between the four electrodes

[0031] 1) Electrode shape design: Since the 90° cone-angle electrode system has been experimentally proven to be used for the extraction of ion beams from high-current ECR ion sources, the electrode shape selection process also adopts the same 45° shape as most ECR ion source extraction structure electrode shapes (electrode symmetrical structure, half of 90° is 45°).

[0032] 2) Design of the first acceleration zone gap d: The greater the distance between the plasma electrode 1 and the suppressor electrode 3, the smaller the influence of the electric field on the shape of the emitting surface. However, if the distance is too close, it is easy to exceed the withstand voltage limit and break down. Therefore, this invention chooses a compromise: on the one hand, it takes into account a good electric and magnetic field, and on the other hand, it also takes into account that it will not spark due to the close distance. The acceleration gap d between the plasma electrode 1 and the first ground electrode 2 is selected as 10mm.

[0033] 3) Design of the second acceleration zone gap and the deceleration gap d: In the research of previous studies, it was found that the size of the deceleration gap is generally set to 3mm. The acceleration gap between the first ground electrode 2 and the suppression electrode 3 is also set to 3mm. As a result, the electric field line distribution in the simulation figure is approximately symmetrical and uniform between the first ground electrode 2, the suppression electrode 3 and the second ground electrode 4.

[0034] 4) Design of the four electrode apertures: The shape of the first electrode (plasma electrode 1) determines the electric field distribution near the extraction aperture, significantly affecting the dispersion angle of the emitted ions. The shapes and potential distributions of the second and third electrodes also affect beam quality parameters, but the aperture sizes of the second and third electrodes have little impact on beam performance, and even negligible influence on beam emissivity during simulation. Therefore, appropriately increasing the extraction aperture of plasma electrode 1 is beneficial for beam extraction. Based on the 3 / 2 power law of space charge confinement current, the aperture diameter of plasma electrode 1 is adjusted to 6–8 mm. The first ground electrode 2, the suppressor electrode 3, and the second ground electrode 4 can retain the previous extraction structure, with the first ground electrode aperture radius of 4 mm, the suppressor electrode aperture radius of 4 mm, and the ground electrode aperture radius of 4 mm.

[0035] 4. Differences between this invention and existing technologies

[0036] 1) Existing 2.45GHz ECR ion source. ① Existing 2.45GHz ECR ion source 90° cone angle three-electrode extraction system, such as... Figure 1 As shown, the three-electrode extraction system is also called an acceleration / deceleration system. The third electrode is generally at ground potential and is called the ground electrode. The second ground electrode is called the suppression electrode and is at a negative potential relative to the ground electrode. Ions are first accelerated and then decelerated to ground potential. The main functions of this mechanism are: first, to suppress electrons that are accelerated in the opposite direction to the source by the suppression electrode, protecting the microwave window and facilitating the stable operation of the source; second, to help suppress the space charge effect and reduce the beam emittance; and third, to increase the extraction current intensity without changing the total extraction energy. ② The aperture radius of plasma electrode 1 is 3mm, the plasma voltage is set to 50kV, the suppression voltage is -3kV, and the acceleration gap d is initially determined by the empirical formula s1=r1 / d1≈0.8 and... (V1, V2 are plasma electrode, suppress electrode to draw voltage) is 8mm, and then considering that the field strength between the plasma electrode and the suppress electrode is too large to exceed the voltage limit between the electrodes, causing the arc phenomenon, the length of the suppress electrode head is adjusted to change the acceleration gap d to 12mm; the deceleration gap is 3mm, and the hole radius of the suppress electrode and the ground electrode is 4mm. 3) The existing ion source uses two high-voltage power supplies to provide the required high voltage for drawing, and the potential diagram is shown in Figure 2 . The plasma electrode and the discharge cavity are at the same potential, the drawing high-voltage power supply provides an adjustable positive voltage in the range of 30-50kV, and the suppress electrode power supply provides an adjustable negative voltage in the range of -2--3kV. If the vacuum state of the drawing port is not good, the arc phenomenon is easy to occur in the process of increasing the high voltage due to the large magnetic field at the drawing port position, the high-voltage power supply is repeatedly protected, and the stable operation of the ion source is affected.

[0037] 2) Improved 2.45GHz ECR ion source. First, prevent the suppress electrode power supply from being damaged due to excessive load. The improved ion source electrode drawing system is shown in Figure 3 . The improved ion source also uses two high-voltage power supplies to provide the required high voltage for drawing, and the potential diagram is shown in Figure 4 . Unlike the original drawing structure, a ground electrode is added between the plasma electrode and the suppress electrode. In this way, when the vacuum state of the drawing area is not good, the arc phenomenon occurs between the plasma electrode and the ground electrode, and the tens of kilovolts of high voltage is applied to the ground electrode instead of the suppress electrode, thereby preventing the suppress electrode power supply from being damaged due to excessive load. Second, the most core is that unlike the original drawing structure which has only one acceleration region, the improved drawing structure has two acceleration regions to adjust the field line distribution between the electrodes, so as to make the electric field line distribution on the beam transmission line as uniform as possible. Due to the uniform electric field line distribution on the beam transmission line, 1) the arc phenomenon is significantly improved; 2) the beam aberration is reduced to a certain extent, and the beam aberration is the deviation between ideal imaging and actual imaging of the beam.

[0038] The application will be further explained in combination with the drawings;

[0039] An ECR ion source drawing structure based on a dynamic discharge cavity is shown in Figures 3-4 . Its characteristics are as follows: the drawing structure is composed of four nested cylindrical electrodes including two ground electrodes; the four nested cylindrical electrodes form two acceleration regions for adjusting the field line distribution of the drawing region; the deceleration region between the suppress electrode 3 and the second ground electrode 4 is used to suppress the electrons accelerated to the source; the first ground electrode 2 prevents the suppress electrode power supply from being damaged due to excessive load. The gap d between the four cylindrical electrodes and the radius r of each change with the size of the inner diameter of the dynamic discharge cavity.

[0040] Further, the extraction structure is sequentially provided with: a plasma electrode 1, a first ground electrode 2, a suppression electrode 3, and a second ground electrode 4; the two acceleration regions are: a first acceleration region composed of the plasma electrode 1 and the first ground electrode 2, and a second acceleration region composed of the first ground electrode 2 and the suppression electrode 3; the two ground electrodes, the first ground electrode 2 is used to divide the space between the plasma electrode 1 and the suppression electrode 3 into two acceleration regions; the second ground electrode 4 is used to slow down the ions after acceleration through the two acceleration regions to the ground potential, thereby suppressing the reverse acceleration of electrons into the source and protecting the microwave window.

[0041] Further, the angle between all electrodes and the axis of the discharge cavity is 45°.

[0042] Further, the extraction port of the plasma electrode 1 is a circular hole with a diameter of 6-8 mm.

[0043] Further, the hole diameters of the first ground electrode 2, the suppression electrode 3, and the second ground electrode 4 are 8 mm.

[0044] Further, the distance between the plasma electrode 1 and the first ground electrode 2 is 10 mm; the distance between the first ground electrode 2 and the suppression electrode 3 is 3 mm; and the distance between the suppression electrode 3 and the second ground electrode 4 is 3 mm.

[0045] Further, the material of the electrodes is copper or stainless steel.

[0046] Supplementary Note 1

[0047] The hole radius or diameter of the improved four electrodes is as shown in Figure 3 The hole diameter of the plasma electrode 1 at mark 6 is 6 mm; and the hole diameters of the first ground electrode 2, the suppression electrode 3, and the second ground electrode 4 at mark 8 are 8 mm.

[0048] Supplementary Note 2

[0049] 1) The field line distribution before improvement is as shown in Figure 6 There are still residual field lines on the right side of the second ground electrode, which will affect the divergence of the beam (the beam is affected by the field line force) when passing through the area where the beam passes, and the beam will diverge to some extent.

[0050] 2) The field line distribution after improvement is as shown in Figure 7 The residual field lines on the right side of the second ground electrode completely disappear, Figure 7 Compared with Figure 6 , the field lines at the root of the first acceleration region are converging and uniform, which means that the field lines do not extend to the right of the area where the beam passes. And Figure 6The root of the first accelerating zone, the field lines diverge to the right until the residual field lines extend to the place where the beam passes through the extraction zone.

[0051] After Figure 6 and Figure 7 comparing, it can be seen that the present application improves the uniformity of the field lines by adding the first ground electrode 2 between the plasma electrode 1 and the suppression electrode 3, and the residual field lines completely disappear when the beam passes through the first accelerating zone, the second accelerating zone, the decelerating zone and reaches the second ground electrode, so that the situation that the residual field lines cause the beam to diverge is avoided.

[0052] The above is only an example and description of the concept of the present application. Those skilled in the art can make various modifications or supplements to the described specific embodiments or use similar ways to replace, as long as they do not deviate from the concept of the present application or exceed the scope defined by the present claims, which shall belong to the protection scope of the present application.

Claims

1. A dynamic extraction structure based on 2.45GHz ECR ion source dynamic discharge cavity, characterized in that: The extraction structure is composed of four nested cylindrical electrodes including two ground electrodes; the four nested cylindrical electrodes compose two accelerating areas of the extraction region; the two accelerating areas of the extraction region are used to adjust the field line distribution of the extraction region; the decelerating area between the suppressor electrode (3) and the second ground electrode (4) is used to suppress the electrons accelerated reversely into the source; the first ground electrode (2) is used to prevent the damage caused by the over-high load on the suppressor electrode power supply; the gap d and the respective diameter r between the four cylindrical electrodes vary with the size of the inner diameter of the dynamic discharge cavity; the angles between all the electrodes and the axis of the discharge cavity are all 45°; the extraction port of the plasma electrode (1) is a circular hole with a diameter of 6-8mm; the hole diameter of the first ground electrode (2), the suppressor electrode (3) and the second ground electrode (4) is 8mm; the distance between the plasma electrode (1) and the first ground electrode (2) is 10mm; the distance between the first ground electrode (2) and the suppressor electrode (3) is 3mm; the distance between the suppressor electrode (3) and the second ground electrode (4) is 3mm; the ion source uses two identical high-voltage power supplies to provide the high voltage required for extraction, and a ground electrode is added between the plasma electrode and the suppressor electrode; when the vacuum state of the extraction region is poor, the arc phenomenon occurs between the plasma electrode and the ground electrode, and the tens of kilovolts of high voltage extracted is added to the ground electrode instead of the suppressor electrode, thereby preventing the damage caused by the over-high load on the suppressor electrode power supply; The extraction structure is sequentially provided with: a plasma electrode (1), a first ground electrode (2), a suppressor electrode (3), and a second ground electrode (4); the two accelerating areas are: a first accelerating area composed of the plasma electrode (1) and the first ground electrode (2), and a second accelerating area composed of the first ground electrode (2) and the suppressor electrode (3); the two ground electrodes, the first ground electrode (2) is used to divide the plasma electrode (1) and the suppressor electrode (3) into two accelerating areas; the second ground electrode (4) is used to decelerate the ions after acceleration through the two accelerating areas to the ground potential, thereby suppressing the electrons accelerated reversely into the source and protecting the microwave window.

2. The extraction structure of a dynamic discharge cavity based on a 2.45 GHz ECR ion source according to claim 1, characterized in that: The material of the electrode is copper or stainless steel.

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