Beam phase distribution measuring and calculating method of isochronous cyclotron

Abstract

The invention discloses a method for measuring and calculating beam phase distribution of an isochronous cyclotron, which belongs to the technical field of production of cyclotrons and comprises the following steps of: moving a radial target to a central area of the cyclotron to enable the beam intensity to reach the maximum value; moving the radial target to a position corresponding to the beam extraction point in the radial direction, and adjusting the high frequency and the exciting current to enable the beam intensity to reach the maximum value; increasing the high frequency according to the step length until the beam current intensity is reduced to half; reducing the high frequency according to the step length until the beam current intensity is reduced to half; calculating beam phase slippage; enabling the radial target to be close to the central area of the accelerator according to the step length and repeating the process to obtain a plurality of beam phase slips; and making a beam phase distribution curve according to the distribution relation that the plurality of beam phase slippage changes along with the radius. The method has the characteristics of simplicity, convenience and easiness in operation, the workload is greatly reduced, the debugging process and the development period of the isochronous cyclotron are shortened, the efficiency is remarkably improved, and the cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of cyclotron production, and in particular relates to a method for measuring beam phase distribution of an isochronous cyclotron. Background Art

[0002] A cyclotron is a device that uses a magnetic field and an electric field to make charged particles perform a cyclotron motion and repeatedly accelerate them during the motion by a high-frequency electric field. It is often used to accelerate protons to bombard target materials at high speed to produce medical nuclides. Most of them are isochronous cyclotrons, and the magnet-based isochronous cyclotron is a commonly used isochronous cyclotron. Its core structure is as follows: Figure 1 As shown, it includes a magnetic yoke 4, a magnetic coil 6, a vacuum box 5, a magnetic pole 3, an ion source 2, a high-frequency resonance cavity 13 and a beam extraction pipeline 11. The magnetic pole 3, the ion source 2 and the high-frequency resonance cavity 13 are all located in the vacuum box 5. The ion source 2 is located in the accelerator central area 8 at the center of the accelerator. A high-frequency gap 14 for accelerating ions is formed between the adjacent high-frequency resonance cavity 13 and the magnetic pole 3. When used, the ion source 2 generates particles under the action of ionization. Under the action of the high-frequency resonance cavity 13 and the magnetic pole 3, a particle beam formed by multiple particles moves in a spiral shape at high speed in the cyclotron accelerator to form a beam running track 1. After the particle beam runs to the beam extraction point 9 near the outer peripheral edge of the accelerator, it quickly separates from the cyclotron accelerator from the beam extraction pipeline 11 and enters the target shooting system 12 to complete the function of producing nuclides by target shooting. An extraction beam track 10 is formed between the beam extraction point 9 and the beam extraction pipeline 11.

[0003] Theoretically, an isochronous cyclotron accelerator can overcome the phase slip problem between the cyclotron frequency and the high-frequency electric field caused by the relativistic effect of the accelerated particles. The key to overcoming this problem lies in the degree of realization of the ideal isochronous field. In an ideal isochronous field, particles can achieve true resonant acceleration without phase slip. However, in actual situations, due to the influence of various factors, the breaking of isochronism is unavoidable, which will cause the particles to lose synchronization with the resonant acceleration of the cyclotron period, resulting in a phase slip (abbreviated as phase slip); the phase slip of a particle is defined as the phenomenon that the cyclotron motion of the particle loses synchronization with the periodic change of the high-frequency electric field and causes the acceleration phase of the particle to slip in the direction of lag.

[0004] like Figure 2 As shown, taking the particle motion in two high-frequency cycles as an example, using T D represents the high-frequency phase cycle, and T represents the particle cyclotron motion cycle. Assuming that the particle enters the high-frequency gap from point A at a certain moment with the best phase (the acceleration voltage is the maximum value), since T<T DAfter one cycle, the particle enters the high-frequency phase at point B for another acceleration. At this time, the particle is still in the acceleration phase region (the acceleration voltage is positive), but has deviated from the optimal acceleration phase. Continuing to the second high-frequency cycle, the particle enters the high-frequency gap from point C. At this time, it is already in the deceleration phase region (the acceleration voltage is negative), and the particle begins to decelerate. Similarly, assuming that the particle still enters the high-frequency gap from point A with the optimal acceleration phase, when T>T D When the particle's acceleration phase slides to the right to points D and E, the particle is still in the acceleration phase, but it is foreseeable that continued phase sliding will cause the particle to enter the deceleration phase region. Therefore, when the particle's cyclotron frequency loses synchronization with the high-frequency frequency, the particle will experience phase slippage, and the accumulation of phase slippage will eventually lead to particle deceleration, which fundamentally limits the further increase of particle energy.

[0005] For isochronous cyclotron accelerators, phase slip mainly comes from magnets and high frequencies. In terms of magnets, during the manufacturing process of magnet components, due to factors such as inherent material inhomogeneity and precision limitations of processing equipment, the magnets deviate from the theoretical design model, resulting in the breakdown of the isochronous magnetic field, which in turn causes particles to slip during cyclotron motion; in the beam measurement stage, the stability of the excitation current will affect the isochronism of the magnetic field, causing particles to slip during cyclotron motion; the high vacuum required for particle cyclotron motion will cause structural deformation of the cover plate and other parts, causing the magnetic field in the beam measurement stage to deviate from the magnetic field in the magnetic measurement stage, causing particles to slip in the isochronous detuning area. In terms of high frequencies, due to the thermal effect of high-frequency feed power, uneven local heating of the cavity causes deformation of the high-frequency cavity, resulting in fluctuations in the acceleration voltage and resonant cavity frequency, or feed power fluctuations due to instability of the feed system, all of which will cause phase slips during particle cyclotron motion.

[0006] Therefore, measuring the phase distribution during the beam measurement stage of the isochronous cyclotron accelerator can effectively help engineering debugging personnel determine the basic condition of the magnet isochronism, determine the location and degree of beam loss, and provide a powerful basis for troubleshooting the cause of beam loss.

[0007] In the past, the phase distribution was measured mostly by capacitive induction phase probe method or RF-γ timing method. The capacitive induction phase probe method uses the central plane of the accelerator as the symmetry plane, installs multiple phase probes in the accelerator at equal distances along the radial direction, and uses the principle of double-balanced mixing to convert the induced signal on the phase probe into a DC voltage signal associated with the beam phase. Each phase probe contains an electrode made of copper plate to lead out the beam signal, and its induced signal voltage is proportional to the number of beam pulse charges. The electrode is designed with two layers of shielding around it, which protects the electrode from being bombarded by the beam and also serves as a shield to reduce high-frequency signal interference. The RF-γ timing method uses a radial target to intercept the beam to generate γ rays, and obtains the time information of the beam at the beam interception point by the time-of-flight method, and compares it with the high-frequency phase to obtain the phase information of the beam. This method can measure the offset between the actual phase of the beam and the high-frequency starting phase. For superconducting cyclotron accelerators, the method currently used is to obtain phase slip information only through beam intensity measurement. This method only requires measuring and comparing the changes in beam intensity and superconducting coil excitation current at different radii to calculate the actual degree of phase slip.

[0008] The above-mentioned traditional methods for measuring phase distribution have the following defects:

[0009] The use of capacitive induction phase probes to measure cyclotron beams requires the design of specialized detection circuits and complex dedicated signal processing circuits, and requires the complex signal processing of beam signals to reduce the interference of high-frequency signals on the measurement results.

[0010] The RF-γ timing method needs to detect the fast γ transition signal generated by the beam bombarding the target, which is easily interfered by the signals of other isotropic γ transitions. Therefore, it is not suitable for small-radius beam phase measurement and requires the use of additional γ detectors, making the signal processing system complex.

[0011] The phase sliding measurement method based on beam measurement is applicable to superconducting cyclotrons. Since superconducting cyclotrons mostly use tuning shimming coils to repair the magnetic field, the main magnetic field is contributed and integrated by the currents of each shimming coil respectively. The corresponding magnetic field variation satisfies a first-order approximation relationship with the shimming coil current near the shimming coil. For a cyclotron with a corresponding conventional magnet and no tuning coil, its magnetic field is mainly composed of the magnetizing magnetic field generated by the main coil current. The contribution of magnetization varies with the distance from the excitation main coil. Therefore, the local magnetic field variation does not satisfy a linear relationship with the change of the coil current. Therefore, this method is not applicable to isochronous cyclotrons using conventional magnets. Summary of the invention

[0012] The purpose of the present invention is to provide a simple and feasible method for calculating the beam phase distribution of an isochronous cyclotron accelerator in order to solve the above problems.

[0013] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0014] A method for measuring the beam phase distribution of an isochronous cyclotron accelerator, the isochronous cyclotron accelerator comprising a magnetic yoke, a magnetic coil, a vacuum box, a magnetic pole, an ion source, a high-frequency resonance cavity and a beam extraction pipeline, the magnetic pole, the ion source and the high-frequency resonance cavity are all located in the vacuum box, the ion source is located in the central area of ​​the accelerator at the central position of the accelerator, the beam extraction pipeline is close to the beam extraction point at the peripheral edge position of the accelerator, and the method for measuring the beam phase distribution of the isochronous cyclotron accelerator comprises the following steps:

[0015] Step 1, moving a radial target for measurement to the central area of ​​the accelerator, adjusting the parameters of the ion source, the excitation current of the magnetic coil and the high frequency of the high frequency resonant cavity, measuring the beam current intensity of the particle beam through the radial target, so that the beam current intensity of the particle beam in the central area of ​​the accelerator reaches a maximum value;

[0016] Step 2: Move the radial target radially to the radial position of the circle corresponding to the radius of the beam extraction point and set the position as R, adjust the high frequency and the excitation current, measure the beam intensity of the particle beam through the radial target, and make the beam intensity of the particle beam at the position R reach the maximum value I R , the corresponding high frequency is ω0;

[0017] Step 3: Assume that the step length of the frequency change is s, keep the excitation current constant, take ω0 as the initial frequency, and increase the high frequency by s. The beam intensity of the particle beam begins to decrease due to phase slip until the beam intensity measured by the radial target drops to I R / 2, the corresponding high frequency is ω + ;

[0018] Step 4: Keep the excitation current constant, take ω0 as the initial frequency, and reduce the high frequency frequency in steps of s. The beam intensity of the particle beam begins to decrease due to phase slip until the beam intensity measured by the radial target drops to I R / 2, the corresponding high frequency is ω - ;

[0019] Step 5: Calculate the beam phase slip φ at position R according to the following formula:

[0020]

[0021] Step 6: Assume that the radial movement step length of the radial target is t, move the radial target radially in a direction close to the central area of ​​the accelerator with a step length of t and assume that the corresponding position after the movement is R', adjust the high frequency and the excitation current, measure the beam intensity of the particle beam through the radial target, and make the beam intensity of the particle beam at the R' position reach the maximum value I R′ Repeat steps 3 to 5, always using ω0 as the initial frequency and using I R′ Replace I R , using ω +′ Replace ω + , using ω -′ Replace ω - , obtaining beam phase slips φ′ corresponding to a plurality of R′ positions until the radial target moves radially to the central region of the accelerator;

[0022] Step 7. Obtain the distribution relationship of beam phase slip with radius based on φ and multiple φ′, make a beam phase distribution curve, and complete the beam phase distribution calculation of the isochronous cyclotron accelerator, so as to provide a basis for later judging the isochronous condition of the isochronous cyclotron accelerator and repairing the magnetic field error introduced after the magnetic measurement stage due to vacuum effect, etc.

[0023] Preferably, in order to achieve more reliable measurement results, in step 3, s is 500 Hz; in step 6, t is 5 mm.

[0024] The beneficial effects of the present invention are:

[0025] The present invention realizes the measurement of beam phase slips at multiple radial positions of the accelerator by controlling the radial movement of the radial target, the high-frequency frequency change and the beam current intensity measurement, provides a basis for judging the isochronism of the isochronous cyclotron accelerator and repairing the magnetic field error introduced after the magnetic measurement stage due to the vacuum effect, etc., does not require a complex dedicated signal processing circuit, avoids the high workload caused by rework magnetic measurement and repeated disassembly of the high-frequency cavity, vacuum box and magnet assembly, has the characteristics of simplicity and easy operation, greatly reduces the workload, shortens the debugging process of the isochronous cyclotron accelerator, shortens the development cycle of the engineering project, significantly improves the efficiency and reduces the cost, and is particularly suitable for isochronous cyclotron accelerators based on conventional magnets. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the core structure of the isochronous cyclotron accelerator of the present invention;

[0027] Figure 2 It is a schematic diagram of the phase change relationship between the high frequency and the particle cyclotron frequency in the isochronous cyclotron accelerator of the present invention;

[0028] Figure 3It is a schematic diagram of a beam phase distribution curve obtained by using the beam phase distribution calculation method of the isochronous cyclotron accelerator of the present invention;

[0029] Figure 4 Schematic diagram of the relationship between the accelerator phase and the particle beam in the isochronous cyclotron accelerator of the present invention. In the figure, the beam line phase passes through the line of sinφ=±1, and the beam is lost, so it is represented by a dotted line;

[0030] Figure 5 It is a schematic diagram of the changing relationship between sinφ and Δω at different energy orbital radii in the isochronous cyclotron described in the present invention. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings:

[0032] like Figure 1 As shown, the core structure of the isochronous cyclotron accelerator of the present invention is the same as the structure described in the background technology content. The beam phase distribution measurement method of the isochronous cyclotron accelerator of the present invention includes the following steps:

[0033] Step 1, moving the radial target 7 for measurement to the central area 8 of the accelerator, adjusting the parameters of the ion source 2, the excitation current of the magnetic coil 6 and the high frequency of the high frequency resonant cavity 13, measuring the beam current intensity of the particle beam through the radial target 7, so that the beam current intensity of the particle beam in the central area 8 of the accelerator reaches a maximum value;

[0034] Step 2: radially move the radial target 7 to the radial position of the circle corresponding to the radius of the beam extraction point 9 and set the position as R, adjust the high frequency and the excitation current, measure the beam intensity of the particle beam through the radial target 7, and make the beam intensity of the particle beam at the position R reach the maximum value I R , the corresponding high frequency is ω0;

[0035] Step 3: Assume that the step length of the frequency change is s, preferably 500 Hz, keep the excitation current constant, take ω0 as the initial frequency, and increase the high frequency by s. The beam intensity of the particle beam begins to decrease due to phase slip until the beam intensity measured by the radial target 7 drops to I R / 2, the corresponding high frequency is ω + ;

[0036] Step 4: Keep the excitation current constant, take ω0 as the initial frequency, and reduce the high frequency frequency in steps of s. The beam intensity of the particle beam begins to decrease due to phase slip until the beam intensity measured by the radial target 7 drops to I R / 2, the corresponding high frequency is ω - ;

[0037] Step 5: Calculate the beam phase slip φ at position R according to the following formula:

[0038]

[0039] Step 6: Assume that the radial moving step length of the radial target 7 is t, and t is preferably 5 mm. The radial target 7 is radially moved in a direction close to the accelerator central area 8 with a step length of t, and the corresponding position after the movement is set as R'. The high-frequency frequency and the excitation current are adjusted, and the beam intensity of the particle beam is measured through the radial target 7, so that the beam intensity of the particle beam at the R' position reaches the maximum value I R′ Repeat steps 3 to 5, always using ω0 as the initial frequency and using I R′ Replace I R , using ω +′ Replace ω + , using ω -′ Replace ω - , a plurality of beam phase slips φ′ corresponding to the R′ positions are obtained until the radial target 7 moves radially to the accelerator central region 8;

[0040] Step 7: Based on φ and multiple φ′, the distribution relationship of beam phase slip with radius is obtained, and a beam phase distribution curve is prepared, such as Figure 3 As shown, according to the curve, the beam phase slip corresponding to different radial positions can be directly queried to complete the beam phase distribution calculation of the isochronous cyclotron accelerator, which provides a basis for later judging the isochronous condition of the isochronous cyclotron accelerator and repairing the magnetic field error introduced after the magnetic measurement stage due to vacuum effect.

[0041] In order to fully illustrate the feasibility of the beam phase distribution measurement method of the isochronous cyclotron accelerator described in the present invention, the derivation process of the calculation formula for calculating the beam phase slip φ at the R position is described in detail below.

[0042] When particles move in an ideal isochronous field, the cyclotron frequency of the particles should remain unchanged, and there should be no phase slip. In actual situations, after particles move out of the accelerator center area 8 and enter the main acceleration area, the distribution error of the average magnetic field and the error of the accelerating electric field frequency will cause the beam phase to move. Therefore, the phase slip of particles at radius r can be approximately expressed as:

[0043]

[0044] In the above formula, represents the phase change of each circle, and ω0 represents the beam intensity of the particle beam at position R reaching the maximum value I RThe high-frequency electric field frequency (i.e. angular frequency) at , ω represents the angular frequency of the particle cyclotron motion, B0 represents the ideal isochronous field, B(r) represents the actual magnetic field, and the energy obtained by the particle per circle is (here it is assumed that each circle is accelerated once):

[0045]

[0046] In the above formula, V0 is the high-frequency voltage peak, and φ is the phase of the cyclotron particle entering the high-frequency electric field; therefore,

[0047]

[0048] Among them, B0 is the mean field of the particle orbit at a certain radius in the ideal isochronous field, and B(r) is the mean field of the particle orbit at this radius in the actual magnetic field.

[0049] Integrating the above formula yields:

[0050]

[0051] Among them, φ i It represents the initial phase of the particle entering the acceleration region, assuming that the magnetic field changes very slowly and smoothly in the radial direction, in the non-relativistic case:

[0052]

[0053] According to

[0054]

[0055] have

[0056]

[0057] In an isochronous cyclotron accelerator, the angular frequency ω of the cyclotron motion is a constant, while B0 and ΔB(r) are functions of r, so the above formula can be simplified to:

[0058]

[0059] Among them, W max =eV0 is the maximum energy that the particle can obtain per turn. The distribution of the magnetic field has been determined, and the integral term on the right side of the above formula is a constant. At the same time, φ i ,E0,E max ,ω can be considered as constants during the measurement process. Therefore, sinφ∝Δω=ω0-ω, and it can be considered that:

[0060] sinφ=-kω0+b (1)

[0061] In fact, the beam is composed of a particle beam envelope distributed around a central beam with a phase of φ. The phase change of each beam trajectory in the envelope satisfies the above formula, such as Figure 4 As shown, 15 is the upper envelope of the beam, 16 is the center line of the beam, and 17 is the beam center line 16 passing through The phase is the tangent point with sinφ=-1, 18 is the phase of the beam envelope 15 is The intersection of 19 and sinφ=+1 is the phase of the beam center line 16. The intersection of 20 and sinφ=+1, the phase of 21 is The intersection of φ and sinφ=-1, 21 is the lower envelope of the beam, 22 is the current intensity I R / 2 corresponds to the dotted line.

[0062] refer to Figure 4 When measuring the beam, an imaginary central beam is used as a reference. When the phase of the central beam reaches When sinφ=±1, the imaginary central beam reaches the edge of the beam envelope, and the overall beam intensity should lose half. When the edge of the beam envelope reaches the intersections 20 and 18, the beam begins to lose. When the phase of the beam center line 16 reaches the position of sinφ=-1, that is, the intersection 17, the beam loss is I R / 2, and when the beam centerline 16 passes through sinφ=±1, i.e., intersection 19, the beam is completely lost. From this, we can get the equation group:

[0063] -1=-kω + +b (2)

[0064] 1=-kω - +b (3)

[0065] The combined equations (1) to (3) can be solved to obtain:

[0066]

[0067] This gives the final calculation formula:

[0068]

[0069] It should be noted that the main assumption of the above formula comes from the slow change of the average magnetic field to the radius, and the condition that the particle cyclotron orbit distance does not change. In fact, when the high frequency changes greatly, the particle motion distance may change greatly, and the orbital average field felt by the particle will mutate, which may cause sinφ and Δω to deviate from the linear region. For this reason, we use the LB-20 cyclotron physics model to verify this situation. The specific situation is shown in the attached figure. Figure 5 As shown, in the closed orbits of 10, 15, and 18 MeV, sinφ and Δω satisfy a linear relationship.

[0070] The above embodiments are only preferred embodiments of the present invention and are not limitations of the technical solutions of the present invention. Any technical solution that can be implemented on the basis of the above embodiments without creative work should be deemed to fall within the scope of protection of the patent of the present invention.

Claims

1. A method for measuring the beam phase distribution of an isochronous cyclotron accelerator, wherein the isochronous cyclotron accelerator comprises a magnetic yoke, a magnetic coil, a vacuum box, a magnetic pole, an ion source, a high-frequency resonant cavity and a beam extraction pipeline, wherein the magnetic pole, the ion source and the high-frequency resonant cavity are all located in the vacuum box, the ion source is located in the central area of ​​the accelerator at the central position of the accelerator, and the beam extraction pipeline is close to the beam extraction point at the peripheral edge of the accelerator, characterized in that: The beam phase distribution calculation method of the isochronous cyclotron accelerator comprises the following steps: Step 1, moving a radial target for measurement to the central area of ​​the accelerator, adjusting the parameters of the ion source, the excitation current of the magnetic coil and the high frequency of the high frequency resonant cavity, measuring the beam current intensity of the particle beam through the radial target, so that the beam current intensity of the particle beam in the central area of ​​the accelerator reaches a maximum value; Step 2: Move the radial target radially to the radial position of the circle corresponding to the radius of the beam extraction point and set the position as R, adjust the high frequency and the excitation current, measure the beam intensity of the particle beam through the radial target, and make the beam intensity of the particle beam at the position R reach the maximum value I R , the corresponding high frequency is ω0; Step 3: Assume that the step length of the frequency change is s, keep the excitation current constant, take ω0 as the initial frequency, and increase the high frequency by s. The beam intensity of the particle beam begins to decrease due to phase slip until the beam intensity measured by the radial target drops to I R / 2, the corresponding high frequency is ω + ; Step 4: Keep the excitation current constant, take ω0 as the initial frequency, and reduce the high frequency frequency in steps of s. The beam intensity of the particle beam begins to decrease due to phase slip until the beam intensity measured by the radial target drops to I R / 2, the corresponding high frequency is ω-; Step 5: Calculate the beam phase slip φ at position R according to the following formula: Step 6: Assume that the radial movement step length of the radial target is t, move the radial target radially in a direction close to the central area of ​​the accelerator with a step length of t and assume that the corresponding position after the movement is R', adjust the high frequency and the excitation current, measure the beam intensity of the particle beam through the radial target, and make the beam intensity of the particle beam at the R' position reach the maximum value I R′ Repeat steps 3 to 5, always using ω0 as the initial frequency and using I R′ Replace I R , using ω +′ Replace ω + , using ω _′ Replace ω _ , obtaining beam phase slips φ′ corresponding to a plurality of R′ positions until the radial target moves radially to the central region of the accelerator; Step 7: Obtain the distribution relationship of beam phase slip with radius according to φ and multiple φ′, make a beam phase distribution curve, and complete the beam phase distribution measurement of the isochronous cyclotron accelerator.

2. The method for calculating the beam phase distribution of an isochronous cyclotron accelerator according to claim 1, characterized in that: In step 3, s is 500 Hz; in step 6, t is 5 mm.

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