Superposition method and device of electron beam bunch string

Abstract

The invention discloses an electron beam bunch superposition method and device, and the beam bunch energy of the head part and the tail part of an electron beam bunch is different by adopting a beam energy absorption effect. And then a plurality of alpha magnets are utilized, so that the movement path of the high-energy beam bunches at the head of the beam bunch string is long, the movement path of the low-energy beam bunches at the tail of the beam bunch string is short, and longitudinal superposition and compression of the beam bunch string are achieved. And the alpha magnet can simultaneously realize an achromatic function, so that the transmission envelope after the beam bunch is compressed and superposed is not greatly increased. And the obtained shorter pulse beam can be used for scientific research application such as nuclear data measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of accelerators, in particular to a pulse superposition method and device. BACKGROUND

[0002] Electrons are accelerated to more than 20 MeV, and heavy metal targets are bombarded, which can produce neutrons and X-rays, so that radioactive isotope production and other scientific research such as nuclear data measurement can be carried out.

[0003] When nuclear data measurement is carried out, the neutron energy distribution produced by the bombardment of the electron beam on the heavy metal target is in the range of several eV to 20 MeV, so that the nuclear data of different neutron energies can be measured synchronously. In order to distinguish the neutron energy that produces nuclear reactions, the neutrons produced by the bombardment need to fly for a long enough distance, so that neutrons of different energies and speeds are separated in time through the flight distance, and then the neutrons react with the target material to be measured, so that the nuclear data of different neutron energies can be measured. In order to improve the resolution of the neutron energy, on the one hand, the flight distance of the neutrons needs to be increased, and on the other hand, the pulse length of the electron beam bombardment can be shortened. Increasing the flight distance of the neutrons will reduce the neutron flux, which is proportional to the square of the distance. In the case of constant pulse flow intensity, shortening the pulse time of the electron beam will reduce the total yield of neutrons, thereby reducing the neutron flux, which is proportional to the length of the pulse time. This will affect the efficiency of nuclear data measurement.

[0004] In order to improve the resolution of the neutron energy while minimizing the impact on the efficiency of nuclear data measurement, a micro-pulse beamlet superposition technology can be used. The electron beam output by the linear accelerator is composed of a plurality of micro-pulse beamlet strings. Because the linear accelerator uses microwave field acceleration, the electrons at a certain position on the axis of the acceleration tube are accelerated, and at the same time, the electrons at another position can be decelerated, so the electron beam can only be formed into individual micro-beamlets, so that all beamlets can be accelerated. The pulse length of each micro-beamlet is generally less than 1 ns. If ten to several dozen micro-beamlets are superimposed together, a high-flow micro-pulse with a pulse length of about 1 ns can be obtained. The pulse length required for high-precision nuclear data measurement is about 1 ns. SUMMARY

[0005] In view of the above-mentioned demand for micro-pulse electron beamlets for nuclear data measurement, the present patent proposes a method and device for superimposing electron beamlet strings.

[0006] A method for superimposing electron beamlet strings, comprising: The electron linear accelerator inputs microwave energy into the acceleration tube, thereby establishing an acceleration electric field; After the establishment of the acceleration electric field is completed and the energy gain of the acceleration tube is stable, the electron beamlet string is injected into the acceleration tube; The electron beam group absorbs microwave energy in the acceleration tube based on the beam current energy absorption effect until the energy gain of the acceleration tube is stabilized again, so that the electron beam groups at the head and tail have energy difference, the beam group energy at the head of the beam group string is high, and the beam group energy at the tail is low. By deflecting the electron beam group string with energy difference by the alpha magnet, the movement path of the beam group at the head in the alpha magnet is long, and the movement path of the beam group at the tail in the alpha magnet is short, so as to shorten the length of the electron beam group string and compress and superimpose the electron beam group, and obtain the compressed and superimposed electron beam group string.

[0007] Further, the pulse length of the injected electron beam group string is less than the injection equilibrium time. The injection equilibrium time is the time difference between the time when the electron beam group starts to be injected and the time when the energy gain of the acceleration tube is stabilized again.

[0008] Further, the pulse length of the compressed and superimposed electron beam group string is 1ns.

[0009] Further, the alpha magnet deflects the electron beam group with energy difference, specifically deflecting the electron beam group by 270 degrees.

[0010] An electron beam group string superimposition device for implementing the above-mentioned electron beam group string superimposition method, comprising: A plurality of alpha magnets arranged in an array in a planar space, so that the outgoing beam current of the electron beam group string is collinear with the incoming beam current.

[0011] Further, the superimposition device comprises: A plurality of alpha magnets arranged in an array in a three-dimensional space, so that the outgoing beam current of the electron beam group string is not collinear with the incoming beam current.

[0012] Further, the magnetic field gradient index of the alpha magnet is in the range of 0.5-1.2; when the magnetic field index is equal to 0.8, the deflection angle of the alpha magnet is 270 degrees, when the magnetic field gradient index is greater than 0.8, the beam current deflection angle is greater than 270 degrees, and when the magnetic field gradient index is less than 0.8, the beam current deflection angle is less than 270 degrees.

[0013] Further, the electron beam energy of the electron linear accelerator is greater than 20MeV, and the macro-pulse current is greater than 0.5A.

[0014] The beneficial effects of the present application are:

[0015] The present application first obtains beam groups with different energies by using the beam current energy absorption effect, the beam group energy at the head of the beam group string is high, and the beam group energy at the tail of the beam group string is low; then different energies of the beam group are passed through the alpha magnet, so that the paths of the beam groups with different energies are different, thereby realizing compression and superposition of the beam group string; and thirdly, a plurality of alpha magnets are arranged in an array in a three-dimensional space, so that the outgoing beam current of the electron beam group string is not collinear with the incoming beam current. α The magnetic field gradient index of the alpha magnet is in the range of 0.5-1.2; when the magnetic field index is equal to 0.8, the deflection angle of the alpha magnet is 270 degrees, when the magnetic field gradient index is greater than 0.8, the beam current deflection angle is greater than 270 degrees, and when the magnetic field gradient index is less than 0.8, the beam current deflection angle is less than 270 degrees. αMagnet, can make full use of site space and realize the superposition of the head and tail of the beam bunch string. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 For the beam energy absorption effect diagram; Figure 2 For the beam bunch trajectory diagram of different energy in the alpha magnet; Figure 3 For the alpha magnet structure and two-dimensional magnetic field distribution diagram; Figure 4 For the magnetic field distribution diagram on the symmetric plane of the alpha magnet; Figure 5 For the layout diagram of four 270-degree alpha magnets; Figure 6 For the layout diagram of multiple 270-degree alpha magnets; Figure 7 For the layout diagram of four alpha magnets with a deflection angle greater than 270 degrees. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0018] In view of the above-mentioned need for micro-pulse electron beam bunch in nuclear data measurement, the present application proposes a beam bunch string superposition method and device.

[0019] The method first utilizes the beam energy absorption effect of the electron linear accelerator. When the electron linear accelerator accelerates the electron beam, it first needs to input microwave energy into the acceleration tube to establish an acceleration electric field. The establishment of the acceleration electric field requires a certain time, which is called the electric field establishment time, generally not more than 1 microsecond.

[0020] The electric field establishment process is shown by the rising edge in Figure 1 , wherein the energy gain of the acceleration tube represents the energy that a single electron can obtain by passing through the acceleration tube at each time.

[0021] After the establishment of the accelerating electric field is completed, the energy gain curve of the accelerating tube becomes a straight line. If the electron beam group starts to inject at this time, the first injected electron beam group absorbs the microwave energy of the accelerating tube during the acceleration process, resulting in a small decrease in the accelerating electric field. The second injected electron beam group further reduces the microwave accelerating electric field, and the injection of each subsequent electron beam group will cause a small decrease in the microwave accelerating electric field, causing the energy gain curve of the accelerating tube to decrease until the input microwave energy is equal to the energy absorbed by the electron beam group from the accelerating electric field, i.e., the energy of the electron beam no longer decreases with the injection of subsequent beam groups, thereby reaching equilibrium, and the energy gain curve of the accelerating tube becomes a straight line again. The energy gain at this time is the steady-state energy gain. The time from the start of electron beam injection to the achievement of equilibrium is referred to as the injection equilibrium time. The electric field establishment time is generally equal to the injection equilibrium time.

[0022] In order to obtain a micro-pulse beam group of about 1 ns, the length of the electron beam group before superposition is not more than 100 ns, i.e., the time length of the electron beam group string needs to be less than the injection equilibrium time of the electron beam. Therefore, a higher electron beam micro-pulse flow intensity can be used, which is generally greater than 0.5 A, thereby further increasing the micro-pulse flow intensity after superposition, while ensuring that the decrease in the energy of the electron beam at the tail of the beam group string is sufficient.

[0023] If the microwave frequency of the electron linear accelerator is 2856 MHz, the single microwave period is 0.35 ns, and the length of a single micro-beam group is generally less than 0.1 ns, then a 10 ns electron beam group string contains 28 micro-beam groups. Superimposing the 10 ns electron beam group string with a single micro-beam group, the pulse flow intensity of the micro-beam group can be increased by 28 times.

[0024] The superposition of the electron beam group uses an alpha magnet. For an electron beam group string of less than 100 ns, due to the beam energy absorption effect, the energy of the head beam group of the beam group string is high, and the energy of the tail beam group of the beam group string is low. When the beam group string is deflected by about 270 degrees, the head beam group with high energy moves a long distance, and the tail beam group with low energy moves a short distance, as shown in Figure 2 , which makes the length of the electron beam group after deflection by about 270 degrees shorter. The beam deflected by the alpha magnet can also achieve achromatic dispersion, making the beam envelope not divergent.

[0025] Further, the structure and two-dimensional magnetic field distribution of the alpha magnet are shown in Figure 3 , and the one-dimensional magnetic field distribution of the alpha magnet along the X-axis on the symmetry plane is shown in Figure 4 , and the magnetic field gradient index takes a value in the range of 0.5-1.2. When the magnetic field gradient index is greater than 0.8, the beam deflection angle is greater than 270 degrees, and when the magnetic field gradient index is less than 0.8, the beam deflection angle is less than 270 degrees.

[0026] Further, in order to further compress the length of the electron bunch train, a series of four α magnets can be used, as shown in Figure 5 , which is the least number of α magnets to achieve the collinearity of the outgoing beam and the incoming beam. The α magnets can achieve the full use of the site space and the best effect of bunch compression, so that the superposition of the head and tail of the bunch train can be achieved.

[0027] Further, according to the needs, a series of multiple α magnets can be used to make the beam not only move in the same plane, but also move in three-dimensional space, so that more bunch compression effect can be obtained in a limited space, and the outgoing beam and the incoming beam can not be collinear.

[0028] The electron linear accelerator is three sections, and the steady-state energy gain of each section is about 12 MeV. The total output steady-state energy of the accelerator is about 35 MeV.

[0029] For the short electron beam, the energy gain of the electron beam is higher than the steady-state energy gain due to the unbalanced energy absorption effect of the beam. If the bunch time length of the electron beam is 20 ns, the path length is 6 meters, and there are 56 micro-bunches. The energy of the head bunch of the bunch train is about 50 MeV, and the energy of the tail bunch is about 44 MeV. In order to expand the energy difference between the head and tail of the bunch train, the macro-pulse current of the electron beam is greater than 0.5 A.

[0030] The trajectory of the α magnet used is shown in Figure 2 , wherein the movement path length of 50 MeV in the α magnet is about 1 meter, and the movement path length of 44 MeV in the α magnet is about 0.7 meter, that is, each α magnet can shorten the length of the bunch train by 0.3 meter. The bunch length of the order of ns is 0.3 meters, and the bunch train of 20 ns is compressed to about 1 ns, and the bunch length needs to be compressed by about 5.7 meters, so 19 α magnets are needed.

[0031] The one-dimensional distribution of the magnetic field of the α magnet on the symmetry plane is shown in Figure 4 , and the magnetic field gradient index is 0.8, which can achieve 270 degree deflection. The electron beam is injected into the α magnet from the (0, 0) coordinate position in Figure 3 , that is, the position shown by the circle in Figure 3 . The Z axis in Figure 3 is perpendicular to the paper and passes through the (0, 0) coordinate point, and the Z axis is positive outward from the paper. The incident direction of the beam is 45 degrees with the negative X axis and the positive Z axis in the XZ plane. The outgoing direction of the beam is 45 degrees with the negative X axis and the negative Z axis in the XZ plane.

[0032] The spatial arrangement of multiple α magnets is shown in Figure 5As shown in the figure, the layout of four α magnets is shown, wherein the incident beam direction is the positive direction of Z, the positive direction of Y is outwardly perpendicular to the paper, the X direction is perpendicular to the YZ plane, the first α magnet deflects the beam to the negative direction of X, the second α magnet deflects the beam to the positive direction of Z, the third α magnet deflects the beam to the positive direction of X, and the fourth α magnet deflects the beam to the positive direction of Z, so that the direction of the outgoing beam is collinear with the direction of the incident beam.

[0033] More α magnets can be inserted by using the method shown in Figure 6 , which is to insert in the same plane. In the method, the second α magnet deflects the beam to the negative direction of Z, the third α magnet deflects the beam to the negative direction of X, and the deflection directions of the subsequent α magnets are the same as described above. In addition, there are other similar methods for increasing the number of α magnets.

[0034] If the second α magnet deflects the beam to the positive direction of Y, the third α magnet deflects the beam to the positive direction of Z, the fourth α magnet deflects the beam to the negative direction of Y, the fifth α magnet deflects the beam to the positive direction of X, and the sixth α magnet deflects the beam to the positive direction of Z, the three-dimensional beam trajectory and the magnet arrangement can be realized, and the number of α magnets can be further increased by using this method.

[0035] If the deflection angle of the α magnet is not 270 degrees, in order to facilitate installation and debugging, the number of α magnets needs to be increased or decreased in pairs of two, such as Figure 7 , which is designed as four α magnets.

[0036] Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. It is intended that the specification and examples be considered as illustrative only of the preferred embodiments of the application and that other embodiments within the scope of the application should be apparent to those skilled in the art from consideration of the specification, examples, and practice of the application. Thus, it is intended that the application not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the application, but that the application will include all embodiments falling within the scope of the application. Accordingly, the application is not to be restricted except in the spirit of the claims that follow.

[0037] The above-described embodiments are merely illustrative of the present application, and the present application can be implemented in other specific ways or other specific forms without departing from the spirit or essential characteristics of the present application. Therefore, the described embodiments should be considered as illustrative in all aspects and not as restrictive. The scope of the present application should be explained by the appended claims, and any changes equivalent to the intent and scope of the claims should be included in the scope of the present application.

Claims

1. A method of electron beam bunch string superposition, characterized by, The method comprises the following steps: An electron linear accelerator inputs microwave energy into an acceleration tube, thereby establishing an acceleration electric field; After the establishment of the acceleration electric field and the stabilization of the energy gain of the acceleration tube, an electron bunch string is injected into the acceleration tube; The electron bunch absorbs the microwave energy in the acceleration tube based on the beam energy absorption effect until the energy gain of the acceleration tube is stabilized again, thereby making the head and tail electron bunches have an energy difference; The α magnet deflects the electron bunches with the energy difference, shortens the distance between the head and tail of the electron bunch string, and compresses and superimposes the electron bunches, thereby obtaining the compressed and superimposed electron bunch string.

2. The method of claim 1, wherein the pulse length of the injected electron bunch string is less than the injection equilibrium time. The injection equilibrium time is the time difference between the time when the injection of the electron bunch is started and the time when the energy gain of the acceleration tube is stabilized again.

3. The method of claim 1, wherein the pulse length of the compressed and superimposed electron bunch string is 1 ns.

4. The method of claim 1, wherein the α magnet can deflect the electron bunches with the energy difference, specifically by deflecting the electron bunches by 270 degrees.

5. An electron bunch string superimposition device for implementing the method of any one of claims 1-4, comprising: a plurality of α magnets arranged in an array in a planar space, so that the outgoing beam of the electron bunch string is collinear with the incoming beam. The method comprises the following steps: a plurality of α magnets arranged in an array in a three-dimensional space, so that the outgoing beam of the electron bunch string is not collinear with the incoming beam.

7. The device of claim 6, wherein the magnetic field gradient index of the α magnet ranges from 0.5 to 1.2; when the magnetic field index is 0.8, the deflection angle of the α magnet is 270 degrees; when the magnetic field gradient index is greater than 0.8, the beam deflection angle is greater than 270 degrees; and when the magnetic field gradient index is less than 0.8, the beam deflection angle is less than 270 degrees.

6. The method of claim 1-4, wherein, The electron beam energy of the electron linear accelerator is greater than 20 MeV, and the macro-pulse current is greater than 0.5 A. ​ ​ ​ 8. The electron-beam bunch string superposition device of claim 7, wherein ​

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