Online frequency compensation method for drift tube linear accelerator

Abstract

The invention provides a drift tube linear accelerator on-line frequency compensation method, which comprises the following steps: firstly, constructing an initial cavity three-dimensional simulation model, judging the flatness of an axial field through calculation, and adjusting the angle of an eccentric sheet if the flatness does not reach the standard; calculating the stability of the electric field, and adjusting the length of the coupling rod if the electric field does not reach the standard; the field flatness is judged again after stability tuning, and the angle of the eccentric piece is adjusted if the field flatness does not reach the standard; a tuning block is added according to an opening of the cavity, wherein the tuning block comprises a new pickup port tuner, and a vacuum pump mesh, a fixed tuner and a rod coupler are improved; after a tuning block is added, frequency, axial field flatness and stability calculation is carried out, if the three indexes all meet requirements, compensation is ended, and otherwise, it is considered that compensation cannot be carried out; according to the method, a frequency compensation scheme can be simulated and determined, so that the field flatness and stability meet the operation requirements, and a cold test system is prevented from being rebuilt in a tunnel for field adjustment.

Description

Technical Field

[0001] This invention relates to the field of high-frequency cavity frequency compensation technology, and specifically to an online frequency compensation method for a drift tube linear accelerator. Background Technology

[0002] In fields such as accelerator engineering, frequency control of room-temperature high-frequency cavities is crucial. The CSNS DTL operates at 324MHz, and only cavities falling within the tuner's tuning range can be truly applied in engineering practice. Precisely controlling the frequency changes during cavity operation to keep them within the expected range is essential for cavity development.

[0003] When testing the DTL cavity at low power, there are three main tuning objectives: (1) target resonant frequency, (2) axial average electric field flatness, and (3) axial electric field stability.

[0004] The target resonant frequency of the cavity is tuned to 323.9MHz during low-power testing. The low-power test is conducted under atmospheric conditions, while the operation is conducted under vacuum conditions. The cavity changes from an atmospheric state to a vacuum state, and the frequency increases by 0.1MHz. That is, the target resonant frequency is 323.9MHz during low-power testing, and the frequency will increase to the operating frequency of 324MHz during operation.

[0005] Axial mean electric field flatness E Amplitude error ≥ ±3% increases beam corona and can easily cause beam loss. Referring to international adjustment accuracy, it is set at ±1%. However, in actual field adjustment, due to the large number of elements, it is adjusted to ±2%. Based on actual operation experience of spallation, a field flatness of ±2% meets the actual operation requirements.

[0006] Axial electric field stability. A high-splitting-impedance and high-stability acceleration structure is obtained. This high stability manifests in its effective overcoming of beam load effects and its relatively stiff field distribution, so that changing the frequency of each element does not significantly alter the field distribution within the cavity. During operation, a 2°C temperature change causes an 8kHz change in resonant frequency, and the loaded beam produces a 2kHz frequency shift, resulting in a total shift of 10kHz. The maximum field deviation should be within 1%, therefore, a field stability of ±1% / 10kHz (±100% / MHz) is sufficient. During tuning, the stability coefficient formula for each element is as follows: ,in These are the field distribution values ​​of each unit after perturbation. It represents the field distribution values ​​of each unit before the perturbation. It is the average of the absolute values ​​of the frequency changes caused by the tuners at both ends.

[0007] Currently, the CSNS DTL operates at a duty cycle of 25 Hz / 745 μs, and the maximum tuning range of the movable tuner is ±60 kHz (Reference 1, Peihua Qu, Yao Yang, Ahong Li, Huachang Liu et al. Development of achoke-type movable tuner for CSNS DTL). The China Spallation Neutron Source α-isotope mass production project utilizes the high-power proton beam from the CSNS linear accelerator to irradiate a stacked metal thorium target. Through precipitation separation and multi-stage ion exchange / solid-phase extraction chromatography combined separation and purification methods, various medical α-isotopes are produced. 223 Ra、 225 Ac and 212 Pb / 212 The mass production of Bi is expected to be achieved. By 2031, the duty cycle of the linear accelerator at the China Spallation Neutron Source is expected to be increased to 50Hz / 900μs, with 25Hz beam used for target firing at the spallation neutron source and the other 25Hz beam used for target firing at the isotope target station.

[0008] Increasing the duty cycle raises high-frequency heating in the cavity and exceeds the tuning range of the existing movable tuner, necessitating a redesigned tuning scheme. Since the CSNS DTL is an online tunnel operating device, retuning and compensating for the frequency is difficult. Therefore, it is urgent to find a solution for online frequency compensation through simulation calculations. Summary of the Invention

[0009] To address the frequency control issue during the operation of the CSNS DTL cavity, this invention constructs a three-dimensional model of the cavity and uses simulation calculations to find a frequency compensation scheme that meets the operational requirements. After compensation, the axial field flatness is within ±2%, and the stability is within ±100% / MHz, providing an online frequency compensation method for drift tube linear accelerators.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for online frequency compensation in a drift tube linear accelerator includes the following steps: Step 1: Construct the initial three-dimensional simulation model of the cavity; Step 2: Calculate the frequency and axial average electric field flatness of the three-dimensional simulation model from Step 1; Step 3: Determine whether the axial electric field flatness in Step 2 is within ±2%; Step 4: If the axial electric field flatness in Step 3 is not within ±2%, then adjust the eccentric angle of the cavity 3D model in Step 2 and continue with Step 3; if the axial field flatness in Step 3 is within ±2%, then perform stability calculation on the 3D simulation model in Step 3. Step 5: Determine whether the stability in Step 4 is within ±100% / MHz; Step 6: If the stability in Step 5 does not reach ±100% / MHz, then adjust the length of the coupling rod in the cavity 3D model in Step 4; if the stability in Step 5 reaches within ±100% / MHz, then determine whether the axial field flatness in Step 5 is within ±2%. Step 7: If the axial field flatness in Step 6 is not within ±2%, then adjust the eccentric angle of the cavity three-dimensional model in Step 6 until Steps 4, 5 and 6 meet the requirements; if the flatness in Step 6 is within ±2%, then add tuners according to the existing openings of the cavity, mainly including pickup port tuner (11), vacuum pump grid tuner (7 and 8), fixed tuner (5) and rod coupler (9).

[0011] Step 8: Calculate the frequency, axial electric field flatness, and stability of the three-dimensional simulation model from Step 7.

[0012] Step 9: Determine whether the frequency increase in Step 8 meets the design requirements, whether the axial electric field flatness in Step 8 is within ±2%, and whether the stability in Step 8 is within ±100% / MHz.

[0013] Step 10: If the flatness and stability requirements in Step 9 are not met, continue searching for other combinations in Step 7. If no suitable combination is found, compensation is considered impossible. If a suitable combination is found, the compensation process ends.

[0014] In step 1, a three-dimensional simulation model of the cavity is constructed. This invention describes the CSNS DTL3 cavity, which is specifically: cavity tube (1), low-energy end semi-drift tube (2), high-energy end semi-drift tube (3), drift tube with support rod (4), fixed tuner (5), movable tuner (6), ion pump grid (7), molecular pump grid (8), coupling rod with eccentric plate (9) and ridge waveguide (10). The coupling rod with eccentric plate is divided into coupling rod (901) and eccentric plate (902). The coupling rod (901) includes coupling rod body (9011) and coupling rod collar (9012). The low-energy half-drift tube and the high-energy half-drift tube are located on both sides of the cavity (1), each containing one low-energy half-drift tube and one high-energy half-drift tube. The drift tube (4) with the support rod is vertically suspended above the cavity (1) by the support rod, and contains 29 drift tubes. The fixed tuner (5), the ion pump grid (7), and the molecular pump grid (8) are evenly distributed directly below the cavity (1), containing 12 fixed tuners and 6 ion pump grids (701, 701, 702 ... 3, 704, 706, 707, 709) and 3 molecular pump grids (802, 805, 808), the movable tuner (6) is located in the first quadrant of the cavity (1) at 45° and includes 2 movable tuners, the coupling rod (9) with eccentric plate is located on both sides of the cavity in the horizontal direction, each coupling rod is facing the drift tube and includes 29 coupling rods, the ridge waveguide (10) is located in the second quadrant of the cavity (1) at 45° and includes 1 ridge waveguide.

[0015] In step 2, the axial electric field flatness is calculated, specifically as follows: the formula for calculating the electric field flatness of each unit is... .in E design It is the design electric field of each unit. E i It is the field distribution value of each unit cell normalized to the design electric field.

[0016] Step 4 involves calculating the electric field stability, specifically as follows: Step 4.1: Without introducing any perturbations into the cavity, the axial average electric field distribution is calculated, and the field distribution values ​​for each element are... ; Step 4.2: The first fixed tuner is inserted into the cavity, causing an increase in frequency of... f ; Step 4.3: Pulling the last fixed tuner out of the cavity causes a frequency increase of - f ; Step 4.4: After introducing the perturbation, recalculate the axial electric field distribution value. The field distribution value of each element is... ; Step 4.5, the formula for calculating the electric field stability of each unit is as follows: ,in These are the electric field distribution values ​​of each unit after perturbation. It represents the electric field distribution of each element before the perturbation. It is the average of the absolute values ​​of the frequency changes caused by the tuners at both ends, i.e. f .

[0017] If the stability in step 5 does not reach ±100% / MHz in step 6, then the length of the coupling rod in the cavity 3D model in step 5 will be adjusted, specifically as follows: Step 6.1: If the slope of the TS curve is negative, it indicates that the PC1 mode and the TM010 mode in the cavity field are overcoupled at that point. Increase the length of the corresponding coupling rod to reduce the coupling. Step 6.2: If the slope of the TS curve is positive, it indicates that the PC1 mode and TM010 mode in the cavity field are undercoupled. Reduce the length of the corresponding coupling rod to increase the coupling.

[0018] If the axial electric field flatness in step 6 is not within ±2% in step 7, then the eccentric plate adjustment is performed on the three-dimensional model of the cavity in step 6, specifically as follows: Step 7.1: If the electric field of the unit is too high, rotate the corresponding eccentric piece toward the direction of the electric field to reduce the electric field. Step 7.2: If the electric field of the unit is too low, rotate the corresponding eccentric piece in the opposite direction of the electric field to increase the electric field.

[0019] The feature is that, in step 7, if the axial field flatness in step 6 reaches within ±2%, then a tuner is added based on the existing opening in the cavity, specifically: Step 7.3: Add a pickup tuner (11), a vacuum pump grid tuner (7 and 8), a fixed tuner (5), and a rod coupler (9) respectively. The pickup tuner is a new component. The vacuum pump grid tuner is an improvement on the existing vacuum pump grid. After the improvement, it becomes a vacuum pump grid tuner and is used as both a vacuum grid and a tuner. The fixed tuner is an improvement on the existing component. Specifically, the insertion depth of the fixed tuner is increased. The rod coupler is an improvement on the existing component. Specifically, the collar length of the rod coupler is increased.

[0020] Step 7.4: Combine multiple schemes among the four tuners.

[0021] The online frequency compensation method for drift tube linear accelerators of the present invention has the following significant advantages compared with the prior art: Simplified compensation process: In existing technologies, DTL cavities use a bead-pulling method to tune the low-power field distribution, adjusting the target resonant frequency, field flatness, and stability to the target values. This invention eliminates the need to construct a low-power field tuning system in the tunnel. By using simulation calculations to find a reliable frequency compensation scheme, it provides a reliable solution for precise frequency control during DTL cavity operation.

[0022] Meeting practical needs: This invention enables online frequency compensation, provides feasibility for improving duty cycle, meets the needs of practical engineering applications, and has high practical value. Attached Figure Description

[0023] Figure 1 This is a flowchart of the method of the present invention.

[0024] Figure 2 This is a schematic diagram of the 3D simulation model of the CSNS DTL3 cavity.

[0025] Figure 3 This is the frequency compensation scheme of the present invention.

[0026] Figure 4 This is a schematic diagram of the pickup tuner structure.

[0027] Figure 5 Figure (a) shows the ion pump grid tuner before improvement, and Figure (b) shows the ion pump grid tuner after improvement.

[0028] Figure 6 Figure (a) shows the molecular pump grid tuner before improvement, and Figure (b) shows the molecular pump grid tuner after improvement.

[0029] Figure 7 This is a schematic diagram of the small ball drive device in the comparative example.

[0030] Figure 8 This is a block diagram of a low-power field distribution measurement system in a comparative model – a ball-and-wire system. Detailed Implementation

[0031] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings; see reference Figure 1 A method for online frequency compensation in a drift tube linear accelerator includes the following steps: Step 1: Construct an initial 3D simulation model of the cavity; Step 2: Calculate the frequency and axial average electric field flatness of the three-dimensional simulation model from Step 1; Step 3: Determine whether the axial electric field flatness in Step 2 is within ±2%; Step 4: If the axial electric field flatness in Step 3 is not within ±2%, then adjust the eccentric angle of the cavity 3D model in Step 3 and continue with Step 3; if the axial electric field flatness in Step 3 is within ±2%, then perform electric field stability calculation on the 3D simulation model in Step 3. Step 5: Determine whether the electric field stability in Step 4 is within ±100% / MHz; Step 6: If the stability in Step 5 does not reach ±100% / MHz, adjust the length of the coupling rod in the cavity 3D model in Step 5; if the stability in Step 5 reaches within ±100% / MHz, determine whether the axial electric field flatness in Step 5 is within ±2%. Step 7: If the axial electric field flatness in Step 6 is not within ±2%, adjust the eccentric angle of the cavity 3D model in Step 6 until Steps 4, 5, and 6 all meet the requirements. If the flatness in Step 6 is within ±2%, add tuners according to the existing openings in the cavity. These mainly include a pickup tuner, a vacuum pump grid tuner, a fixed tuner, and a rod coupler. The pickup tuner is a new component. The vacuum pump grid tuner is an improvement on the existing vacuum pump grid, becoming a vacuum pump grid tuner that also serves as a vacuum grid and tuner. The fixed tuner is an improvement on an existing component, specifically by increasing the insertion depth of the fixed tuner. The rod coupler is an improvement on an existing component, specifically by increasing the length of the collar of the rod coupler.

[0032] Step 8: Calculate the frequency, axial electric field flatness, and stability of the three-dimensional simulation model from Step 7.

[0033] Step 9: Determine whether the frequency increase in Step 8 meets the design requirements, whether the axial electric field flatness in Step 8 is within ±2%, and whether the stability in Step 8 is within ±100% / MHz.

[0034] Step 10: If the flatness and stability requirements in Step 9 are not met, continue searching for other combinations in Step 7. If no suitable combination is found, compensation is considered impossible. If a suitable combination is found, the compensation process ends.

[0035] In step 1, a three-dimensional simulation model of the cavity is constructed. This invention describes the CSNS DTL3 cavity, which is specifically: cavity tube (1), low-energy end semi-drift tube (2), high-energy end semi-drift tube (3), drift tube with support rod (4), fixed tuner (5), movable tuner (6), ion pump grid (7), molecular pump grid (8), coupling rod with eccentric plate (9) and ridge waveguide (10). The coupling rod with eccentric plate is divided into coupling rod (901) and eccentric plate (902). The coupling rod (901) includes coupling rod body (9011) and coupling rod collar (9012). The low-energy half-drift tube and the high-energy half-drift tube are located on both sides of the cavity (1), each containing one low-energy half-drift tube and one high-energy half-drift tube. The drift tube (4) with the support rod is vertically suspended above the cavity (1) by the support rod, and contains 29 drift tubes. The fixed tuner (5), the ion pump grid (7), and the molecular pump grid (8) are evenly distributed directly below the cavity (1), containing 12 fixed tuners and 6 ion pump grids (701, 701, 702 ... 3, 704, 706, 707, 709) and 3 molecular pump grids (802, 805, 808), the movable tuner (6) is located in the first quadrant of the cavity (1) at 45° and includes 2 movable tuners, the coupling rod (9) with eccentric plate is located on both sides of the cavity in the horizontal direction, each coupling rod is facing the drift tube and includes 29 coupling rods, the ridge waveguide (10) is located in the second quadrant of the cavity (1) at 45° and includes 1 ridge waveguide.

[0036] In step 2, the axial electric field flatness is calculated, specifically as follows: the formula for calculating the electric field flatness of each unit is... .in E design It is the design electric field of each unit. E i It is the average electric field distribution value of each unit normalized to the design electric field.

[0037] Step 4 involves calculating the electric field stability, specifically as follows: Step 4.1: Without introducing any perturbations into the cavity, the axial average electric field distribution is calculated, and the field distribution values ​​for each element are... ; Step 4.2: The first fixed tuner is inserted into the cavity, causing an increase in frequency of... f ; Step 4.3: Pulling the last fixed tuner out of the cavity causes a frequency increase of - f ; Step 4.4: After introducing the perturbation, recalculate the axial field distribution values. The field distribution values ​​for each element are... ; Step 4.5, the formula for calculating the electric field stability of each unit is as follows: ,in These are the field distribution values ​​of each unit after perturbation. It represents the field distribution values ​​of each unit before the perturbation. It is the average of the absolute values ​​of the frequency changes caused by the tuners at both ends, i.e. f .

[0038] If the stability in step 5 does not reach ±100% / MHz in step 6, the length of the coupling rod in the cavity 3D model in step 5 will be adjusted, specifically as follows: Step 6.1: If the slope of the TS curve is negative, it indicates that the PC1 mode and the TM010 mode in the cavity field are overcoupled at that point. Increase the length of the corresponding coupling rod to reduce the coupling. Step 6.2: If the slope of the TS curve is positive, it indicates that the PC1 mode and TM010 mode in the cavity field are undercoupled. Reduce the length of the corresponding coupling rod to increase the coupling.

[0039] If the axial field flatness in step 6 is not within ±2% in step 7, then the eccentric plate adjustment is performed on the three-dimensional model of the cavity in step 6, specifically as follows: Step 7.1: If the electric field of the unit is too high, rotate the corresponding eccentric piece toward the direction of the electric field to reduce the electric field. Step 7.2: If the electric field of the unit is too low, rotate the corresponding eccentric piece in the opposite direction of the electric field to increase the electric field.

[0040] The feature is that, in step 7, if the axial electric field flatness in step 6 reaches within ±2%, then a tuner is added based on the existing opening in the cavity, specifically: Step 7.3: Add a pickup tuner (11), a vacuum pump grid tuner (7 and 8), a fixed tuner (5), and a rod coupler (9). The pickup tuner is a new component. The vacuum pump grid tuner is an improvement on the existing vacuum pump grid, which is then used as both a vacuum grid and a tuner. The fixed tuner is an improvement on an existing component, specifically by increasing the insertion depth of the fixed tuner. The rod coupler is an improvement on an existing component, specifically by increasing the length of the collar of the rod coupler.

[0041] Step 7.4: Combine multiple schemes among the four tuners.

[0042] The online frequency compensation method for drift tube linear accelerators provided by this invention yields a CSNS DTL3 50Hz operating frequency compensation scheme, as follows: Figure 3As shown, through four pickup tuners (11), specifically pickup tuner-1 (1101), pickup tuner-2 (1102), pickup tuner-4 (1104) and pickup tuner-5 (1105), two ion pump grid tuners (7) and two molecular pump grid tuners (8), specifically ion pump grid tuner-1 (701), ion pump grid tuner-9 (709), molecular pump grid tuner-2 (802) and molecular pump grid tuner-8 (808), a total compensation of 99.94kHz is achieved, of which the pickup tuners compensate 60.17kHz and the grid tuners compensate 39.77kHz, resulting in the best performance. Figure 4 This is a schematic diagram of the pickup tuner structure, where Figure (a) is a three-dimensional assembly diagram and Figure (b) is a cross-sectional structure diagram, specifically: tuning cylinder (1), flange (2), upper O-ring pressure plate (3), spiral water circuit assembly (4), water sealing plug (5), internal hexagonal flat end set screw (6), pressure plate assembly (7), water outlet pipe (8), M16 lock nut (9), and water inlet pipe (10). The tuning cylinder (1) realizes frequency increase, the flange (2) realizes vacuum sealing of the assembly and the cavity, the upper O-ring pressure plate realizes vacuum sealing of the spiral water circuit assembly and the cavity, the spiral water circuit assembly (4), water sealing plug (5), water inlet pipe (10), and water outlet pipe (8) are used for heating and cooling, and the internal hexagonal flat end set screw (6), pressure plate assembly (7), and M16 lock nut are used for positioning and locking the tuning cylinder. Figure 5 This is a schematic diagram of the ion pump grid tuner structure. Figure (a) shows the ion pump grid tuner before improvement, and Figure (b) shows the ion pump grid tuner after improvement. Specifically, it consists of: flange (1), bellows assembly (2), grid tuning cylinder (3), and water pipe (4). In Figure (a), the grid tuning cylinder (3) only has the function of grid, which is used for vacuuming. In Figure (b), the grid tuning cylinder (3) has both grid and tuner functions, which are used for vacuuming and increasing frequency. Flange (1) is used to connect the cavity and the vacuum pump. Bellows assembly (2) is used for length adjustment during the connection process. Water pipe (4) is used for heat generation and cooling. Figure 6 This is a schematic diagram of the molecular pump grid tuner structure. Figure (a) shows the molecular pump grid tuner before improvement, and Figure (b) shows the molecular pump grid tuner after improvement. Specifically, it consists of: flange (1), bellows assembly (2), grid tuning cylinder (3), and water pipe (4). In Figure (a), the grid tuning cylinder (3) only has the function of grid, which is used for vacuuming. In Figure (b), the grid tuning cylinder (3) has both grid and tuner functions, which are used for vacuuming and increasing frequency. Flange (1) is used to connect the cavity and the vacuum pump. Bellows assembly (2) is used for length adjustment during the connection process. Water pipe (4) is used for heat generation and cooling.

[0043] The online frequency compensation method for drift tube linear accelerators provided by this invention yields a CSNS DTL3 50Hz operating frequency compensation scheme, as follows: Figure 3 As shown in Table 1. Figure 3 The compensation results corresponding to the compensation scheme are shown in Table 1. It can be seen from the frequency compensation results of the present invention that the axial electric field distribution of the cavity after compensation is within ±2%, and the stability is within ±100% / MHz.

[0044] Table 1 Comparative Example To facilitate understanding, a comparative illustration is provided here. The low-power field distribution and stability tuning of the DTL cavity are primarily achieved by measuring the axial field distribution using the bead-pulling method, and by measuring the cavity frequency and spectrum using a vector network analyzer. Flatness and stability tuning are then performed based on the measured electric field distribution data. A schematic diagram of the bead-pulling method (small ball drive device) is shown below. Figure 7 As shown. Figure 8 This is the logic diagram of this measurement system. Its main components are a vector network analyzer and a computer. As the ball moves along the cavity's axis, the phase of the measurement signal coupled from inside the cavity is compared with the phase of the signal when there is no disturbance. The phase shift between these two signals... satisfy .in It refers to the frequency deviation caused by the small ball. Q L Let be the loaded quality factor of the cavity. If <5°, and The relationship remains essentially linear. This way, measurement is unnecessary. direct measurement The distribution of the electric field on the axis can then be obtained.

[0045] Figure 7 and Figure 8 This is a commonly used device for measuring low-power field distribution. However, this measurement method presents certain difficulties for equipment operating online in tunnels. The method requires the installation of guy wire supports on both sides of the cavity. The sides of the end plate of the cavity inside the tunnel are very compact, and there is not enough space to install guy wire supports.

Claims

1. A method for online frequency compensation in a drift tube linear accelerator, characterized in that, Including the following step: Step 1: Construct an initial 3D simulation model of the cavity; Step 2: Calculate the frequency and axial field flatness of the three-dimensional simulation model from Step 1; Step 3: Determine whether the axial field flatness in Step 2 is within ±2%; Step 4: If the axial field flatness in Step 3 is not within ±2%, adjust the eccentric angle of the cavity 3D model in Step 2 and continue with Step 3; if the axial field flatness in Step 3 is within ±2%, calculate the electric field stability of the 3D simulation model in Step 3. Step 5: Determine whether the stability in Step 4 is within ±100% / MHz; Step 6: If the stability in Step 5 does not reach ±100% / MHz, then adjust the length of the coupling rod in the cavity 3D model in Step 4; if the stability in Step 5 reaches within ±100% / MHz, then determine whether the axial field flatness in Step 5 is within ±2%. Step 7: If the axial field flatness in Step 6 is not within ±2%, then adjust the eccentric angle of the cavity 3D model in Step 6 until Steps 4, 5 and 6 meet the requirements; If the flatness in Step 6 is within ±2%, then add tuning blocks according to the existing openings of the cavity, mainly including pickup port tuner, vacuum pump grid tuner, fixed tuner and rod coupler. Step 8: Calculate the frequency, axial field flatness, and stability of the three-dimensional simulation model from Step 7. Step 9: Determine whether the frequency increase in Step 8 meets the design requirements, whether the axial field flatness in Step 8 is within ±2%, and whether the stability in Step 8 is within ±100% / MHz. Step 10: If the flatness and stability requirements in Step 9 are not met, continue searching for other combinations in Step 7. If no suitable combination is found, compensation is considered impossible. If a suitable combination is found, the compensation process ends.

2. The online frequency compensation method for a drift tube linear accelerator according to claim 1, characterized in that, In step 1, an initial three-dimensional simulation model of the cavity is constructed. This invention describes the CSNS DTL3 cavity, which is specifically: cavity tube (1), low-energy end semi-drift tube (2), high-energy end semi-drift tube (3), drift tube with support rod (4), fixed tuner (5), movable tuner (6), ion pump grid (7), molecular pump grid (8), coupling rod with eccentric plate (9) and ridge waveguide (10). The coupling rod with eccentric plate is divided into coupling rod (901) and eccentric plate (902). The coupling rod (901) includes coupling rod body (9011) and coupling rod collar (9012). The low-energy half-drift tube and the high-energy half-drift tube are located on both sides of the cavity (1), each containing one low-energy half-drift tube and one high-energy half-drift tube. The drift tube (4) with the support rod is vertically suspended above the cavity (1) by the support rod, and contains 29 drift tubes. The fixed tuner (5), the ion pump grid (7), and the molecular pump grid (8) are evenly distributed directly below the cavity (1), containing 12 fixed tuners and 6 ion pump grids (701, 701, 702 ... 3, 704, 706, 707, 709) and 3 molecular pump grids (802, 805, 808), the movable tuner (6) is located in the first quadrant of the cavity (1) at 45° and includes 2 movable tuners, the coupling rod (9) with eccentric plate is located on both sides of the cavity in the horizontal direction, each coupling rod is facing the drift tube and includes 29 coupling rods, the ridge waveguide (10) is located in the second quadrant of the cavity (1) at 45° and includes 1 ridge waveguide.

3. The online frequency compensation method for a drift tube linear accelerator according to claim 1, characterized in that, Step 2 involves calculating the axial average electric field flatness, specifically as follows: the formula for calculating the average electric field flatness of each unit is... .in E design It is the design electric field of each unit. E i It is the average electric field distribution value of each unit normalized to the design electric field.

4. The online frequency compensation method for a drift tube linear accelerator according to claim 1, characterized in that, Step 4 involves calculating the electric field stability, specifically as follows: Step 4.1: Without introducing any perturbations into the cavity, the axial average electric field distribution is calculated, and the field distribution values ​​for each element are... ; Step 4.2: The first fixed tuner is inserted into the cavity, causing an increase in frequency of... f ; Step 4.3: Pulling the last fixed tuner out of the cavity causes a frequency increase of - f ; Step 4.4: After introducing the perturbation, recalculate the axial average electric field distribution. The field distribution values ​​for each element are... ; Step 4.5, the formula for calculating the electric field stability of each unit is as follows: ,in These are the field distribution values ​​of each unit after perturbation. It represents the field distribution values ​​of each unit before the perturbation. It is the average of the absolute values ​​of the frequency changes caused by the tuners at both ends, i.e. f .

5. The online frequency compensation method for a drift tube linear accelerator according to claim 1, characterized in that, If the stability in step 5 does not reach ±100% / MHz in step 6, the length of the coupling rod in the cavity 3D model in step 5 will be adjusted, specifically as follows: Step 6.1: If the slope of the TS curve is negative, it indicates that the PC1 mode and the TM010 mode in the cavity field are overcoupled at that point. Increase the length of the corresponding coupling rod to reduce the coupling. Step 6.2: If the slope of the TS curve is positive, it indicates that the PC1 mode and the TM010 mode in the cavity field are undercoupled. Reduce the length of the corresponding coupling rod to increase the coupling.

6. The online frequency compensation method for a drift tube linear accelerator according to claim 1, characterized in that, If the axial field flatness in step 6 is not within ±2% in step 7, then the eccentric angle of the cavity 3D model in step 6 will be adjusted, specifically as follows: Step 7.1: If the electric field of the unit is too high, rotate the corresponding eccentric piece toward the direction of the electric field to reduce the electric field. Step 7.2: If the electric field of the unit is too low, rotate the corresponding eccentric piece in the opposite direction of the electric field to increase the electric field. The feature is that, in step 7, if the axial field flatness in step 6 reaches within ±2%, a tuner is added based on the existing opening in the cavity, specifically: Step 7.3: Add a pickup tuner (11), a vacuum pump grid tuner (7 and 8), a fixed tuner (5), and a rod coupler (9) respectively. The pickup tuner is a new component. The vacuum pump grid tuner is an improvement on the existing vacuum pump grid. After the improvement, it becomes a vacuum pump grid tuner and is used as both a vacuum grid and a tuner. The fixed tuner is an improvement on the existing component. Specifically, the insertion depth of the fixed tuner is increased. The rod coupler is an improvement on the existing component. Specifically, the collar length of the rod coupler is increased. Step 7.4: Combine multiple schemes among the four tuners.

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