A method, apparatus and system for measuring an ultrashort electron bunch

Abstract

The application discloses a kind of measurement method, device and system of ultra-short electron bunch, the method uses terahertz simultaneously drive first resonator and second resonator, to generate first deflection electric field and second deflection electric field with equal angular frequency and mutually orthogonal direction respectively at its gap;With the advancing direction of electron bunch as z axis, it successively passes through the gap of the first resonator and second resonator, after first deflection electric field, second deflection electric field, reaches detector;According to the size of the projection beam spot, the length of the corresponding electron bunch is determined, and the spacing between the corresponding two electron bunches is determined according to the spacing of the two beam spot centers in the y axis direction. By adopting the resonant cavity with slit structure, the incident terahertz electric field is enhanced by resonance to provide a high gradient deflection electric field, which can generate a deflection field with a field strength of hundreds of megavolts per meter without the need for a very strong terahertz source, and can realize accurate measurement of ultra-short electron bunch while reducing cost.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of electron bunch measurement, and more particularly relates to a method, device and system for measuring ultra-short electron bunches. BACKGROUND

[0002] In recent years, with the development of accelerator science and technology, the application of ultra-short electron bunch trains with femtosecond length is increasingly widespread, including free electron lasers, tail field accelerators and high-power terahertz sources. In these applications, accurate understanding of the length of individual bunches and the spacing between bunches can further improve system performance. For example, in coherent Smith-Purcell radiation, the wavelength of electromagnetic radiation and the spacing between bunches are on the same order of magnitude, while the radiation power is negatively correlated with the length of the bunch. Therefore, the time structure information of the electron bunch train is crucial.

[0003] Currently, the traditional methods for diagnosing electron bunch trains mainly include autocorrelation method and zero-phase method. The autocorrelation method is an indirect measurement method and requires multiple measurements. Since the movement of the moving mirror of the Michelson interferometer in the measurement is mechanical motion, the response speed is slow, and the measurement resolution is about 100 femtoseconds. The zero-phase method can achieve a resolution of several femtoseconds, but requires the introduction of an additional radio frequency cavity and a deflection magnet. In practical applications, careful calibration of the initial bunch parameters is also required, which is complex to operate and costly. It is difficult to maintain high-resolution measurement for each bunch in the bunch train.

[0004] With the development of strong terahertz sources, terahertz waves have shown great potential in the diagnosis of ultra-short electron beams. The typical period of terahertz electromagnetic waves is on the order of picoseconds, and the linear region is on the order of 100 femtoseconds. The streak camera driven by terahertz can provide a high-gradient deflection field in the terahertz band to achieve sub-femtosecond resolution. However, due to the small spacing between bunches in the electron bunch train, the time information of the subsequent bunches will coincide on the detector. Therefore, the current single sub-wavelength resonant cavity driven by terahertz can only be used for the diagnosis of single bunch, and it is difficult to independently measure each bunch in the bunch train. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a method, device and system for measuring ultra-short electron bunches, which can accurately measure the length of each bunch in the ultra-short electron bunch train and the spacing between the bunches, solving the current diagnosis problem of ultra-short electron bunch trains.

[0006] To achieve the above-mentioned purpose, according to the first aspect of the present application, a method for measuring ultra-short electron bunches is provided, comprising:

[0007] S1, simultaneously driving the first resonator and the second resonator using terahertz to generate a first deflection electric field and a second deflection electric field with equal angular frequency and mutually orthogonal direction at the gap thereof, respectively;

[0008] S2, taking the advancing direction of the electron bunch as the z axis, making the electron bunch pass through the gap of the first resonator and the second resonator in sequence, and then reaching the detector after passing through the first deflection electric field and the second deflection electric field;

[0009] The first deflection electric field is used to apply a deflection force in the y direction to the electron bunch to make it deflect in the y axis direction, and the second deflection electric field is used to apply a deflection force in the transverse x axis direction to the electron bunch to make the projection beam spot on the detector separate along the x axis direction.

[0010] S3, determining the length ΔT of the corresponding electron bunch according to the size S of the projection beam spot and the formula Determining the interval Δt between the two electron bunches according to the interval Δy of the centers of the two beam spots in the y axis direction and the formula

[0011] Wherein, ω is the angular frequency of the deflection electric field, is the equivalent deflection electric field intensity, e is the elementary charge, P is the momentum of the electron bunch, D is the distance from the second resonator to the detection screen, T p is the time for the electron bunch to pass through the gap of the first resonator.

[0012] According to the second aspect of the present application, a measuring device for an ultra-short electron bunch string is provided, comprising: a processor, a first resonator, a second resonator and a detector placed in sequence along the advancing direction of the electron bunch;

[0013] The first resonator and the second resonator are used to generate a first deflection electric field and a second deflection electric field with equal angular frequency and mutually orthogonal direction at the gap thereof under the simultaneous driving of terahertz; wherein the first deflection electric field is used to apply a deflection force in the y direction to the electron bunch to make it deflect in the y axis direction; and the second deflection electric field is used to apply a deflection force in the transverse x axis direction to the electron bunch to make the projection beam spot on the detector separate along the x axis direction.

[0014] The detector is used to detect the information of the projection beam spot.

[0015] The processor is used to determine the length ΔT of the electron bunch according to the size S of the beam spot and the formula And determine the interval Δt between the two electron bunches according to the interval Δy of the centers of the two beam spots in the y axis direction and the formula

[0016] Wherein, ω is the angular frequency of the deflection electric field,​​ For equivalent deflection electric field intensity, e is elementary charge, P is momentum of electron bunch, D is distance from second resonator to detection screen, T is time of electron bunch passing through first resonator gap. p For equivalent deflection electric field intensity, e is elementary charge, P is momentum of electron bunch, D is distance from second resonator to detection screen, T is time of electron bunch passing through first resonator gap.

[0017] According to a third aspect of the present application, there is provided a measurement system of ultra-short electron bunches, comprising: a computer readable storage medium and a processor;

[0018] The computer readable storage medium is configured to store executable instructions.

[0019] The processor is configured to read the executable instructions stored in the computer readable storage medium, and execute the method according to the first aspect.

[0020] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0021] 1. The measurement method, device and system of ultra-short electron bunches provided by the present application provide only y-direction deflection force for the electron bunch by the upstream resonator, and only x-direction deflection force by the downstream resonator, so that the spot information finally presented on the detector is separated and does not interfere with each other, realizing the measurement of the time information of each electron bunch in the electron bunch string, simplifying the difficulty of beam dynamics analysis, and simultaneously acquiring the time structure information of each electron beam in the bunch string by using the orthogonal deflection fields provided by the upstream and downstream resonators and the field attenuation properties.

[0022] 2. The measurement method, device and system of ultra-short electron bunches provided by the present application use a resonant cavity with a gap structure to provide a high-gradient deflection electric field by resonant enhancement of the incident terahertz electric field, which can generate a deflection electric field with a field strength of hundreds of megavolts per meter without the need for a very strong terahertz source, thereby realizing accurate measurement of ultra-short electron bunches while reducing costs.

[0023] 3. The measurement method, device and system of ultra-short electron bunches provided by the present application use the same laser beam to drive the terahertz of the two resonators and the laser used to drive the electron source (such as a photocathode electron gun), thereby ensuring the synchronization of the electron bunch and the terahertz deflection field in the resonator, and the delay and phase of the two deflection electric fields can be accurately controlled through an optical system, thereby realizing accurate control of the bunch and the deflection field. This greatly improves the resolution of the bunch length and bunch spacing measurement, and realizes accurate measurement of ultra-short electron bunches. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The measurement method flowchart of ultra-short electron bunches provided by the embodiment of the present application;

[0025] Figure 2A schematic diagram of a measuring device of an ultra-short electron bunch provided by an embodiment of the present application;

[0026] Figure 3 A schematic diagram of an SRR provided by an embodiment of the present application;

[0027] Figure 4 A schematic diagram of a measuring process of an ultra-short electron bunch provided by an embodiment of the present application;

[0028] Figure 5 A schematic diagram of an ultra-short electron bunch sequentially passing through a first resonator and a second resonator and then reaching a detector provided by an embodiment of the present application;

[0029] Figure 6 A schematic diagram of a deflection electric field intensity distribution provided by an embodiment of the present application;

[0030] Figure 7 A schematic diagram of a deflection electric field intensity distribution provided by an embodiment of the present application;

[0031] Figure 8 A schematic diagram of an effect of a deflection electric field on an electron bunch provided by an embodiment of the present application;

[0032] Figure 9 A schematic diagram of a measuring device of an ultra-short electron bunch provided by an embodiment of the present application;

[0033] Figure 10 A schematic diagram of a resonator with a slit structure provided by an embodiment of the present application;

[0034] Figure 11 A schematic diagram of a measuring process of an ultra-short electron bunch provided by an embodiment of the present application;

[0035] Figure 12 A schematic diagram of a distribution of a bunch on a detector in a beam tracking result;

[0036] Figure 13 A schematic diagram of a relationship between a center offset of a bunch and a length of the bunch and a time delay of the bunch reaching the detector in a beam tracking result. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as there is no conflict.

[0038] An embodiment of the present application provides a measuring method of an ultra-short electron bunch, as shown inFigure 1 As shown, it includes:

[0039] S1, using terahertz to simultaneously drive the first resonator and the second resonator to generate a first deflection electric field and a second deflection electric field with equal angular frequencies and orthogonal directions at their gap, respectively.

[0040] Specifically, both the first and second resonators have gaps, i.e., they are resonators with slit structures. The gap lengths of the two resonators can be the same or different. The resonant cavity with the slit structure provides a high-gradient deflection electric field by resonating and enhancing the incident terahertz electric field. It can generate a deflection field with a field strength of hundreds of megavolts per meter without requiring a very strong terahertz source, thus reducing costs. This deflection electric field is concentrated at the slit and approximates a sine wave whose amplitude decays exponentially with time.

[0041] S2, with the direction of the electron beam's advance as the z-axis, allows it to pass through the first deflection electric field and the second deflection electric field in sequence before reaching the detector;

[0042] Wherein, the first deflection electric field is used to apply a deflection force in the y-direction to the electron beam cluster, causing it to deflect in the y-axis direction; the second deflection electric field is used to apply a deflection force in the transverse x-axis direction to the electron beam cluster, causing its projected beam spot on the detector to separate along the x-axis direction.

[0043] Specifically, the electron beam passes through the gap between the first and second resonators in sequence, is deflected in the y-axis direction under the action of the first deflection electric field, and is deflected in the x-axis direction under the action of the second deflection electric field before reaching the detector. The lens beam spot on the detector separates along the x-axis direction.

[0044] like Figures 2-5 As shown, where, Figure 2-4 The first and second resonators in the electron beam are split-ring resonators (SRR). Under the influence of terahertz waves, the first and second resonators generate mutually orthogonal THz deflection electric fields, which are orthogonal to the forward axis of the electron beam. The electron beam passes sequentially between the slits of the first and second resonators. The THz deflection electric field in the upstream resonator (i.e., the first deflection electric field) applies a vertical (y-direction) deflection force to the electron beam at each zero-phase point, converting its longitudinal distribution (z-direction) into a vertical lateral distribution. The attenuated THz deflection electric field in the downstream resonator horizontally deflects the electron beam, causing its projections on the downstream detector to separate along the x-direction. The resulting beam spot information on the detector is thus separated and does not interfere with each other, enabling the measurement of the electron beam's timing information. Figure 2-4 The first deflection electric field distribution generated by the first resonator in the middle is as follows: Figures 6-7 As shown, Figure 6The normalized distribution of the deflection electric field amplitude of the first deflection field along the z-axis, with the highest point corresponding to the peak of the deflection electric field. The shaded part is the gap; Figure 7 Shown is the electric field distribution on the XZ plane of y = 0.

[0045] S3, according to the projection beam spot size S (i.e. the root mean square length of the beam spot in the y-axis direction) and the formula determining the length of the electron bunch corresponding to the projection beam spot ΔT; wherein ω is the angular frequency of the deflection electric field (the angular frequencies of the first and second deflection electric fields are the same), is the equivalent deflection electric field strength, in order to simplify the calculation, the electric field strength E y (z) is equivalent to a uniform electric field strength in the gap range, i.e. e is the elementary charge, P is the momentum of the electron bunch, D is the distance from the outlet of the second resonator to the detection screen of the detector, T p is the transit time of the electron bunch passing through the gap of the first resonator;

[0046] According to the spacing Δy of any two adjacent projection beam spots in the y-axis direction and the formula determining the spacing Δt between two adjacent electron bunches corresponding to the any two adjacent projection beam spots.

[0047] Preferably, the electron bunch passes through the linear region of the first deflection electric field.

[0048] Preferably, the electron bunch passes through the zero-phase point of the first deflection electric field.

[0049] Preferably, the electron bunch passes through the extreme point of the second deflection electric field.

[0050] Specifically, in the two resonators, the upstream resonator only provides a deflection force in the vertical y direction. When the electron bunch passes through the THz deflection field of the resonator at different phases, the size of the deflection force it receives is different, thereby converting the longitudinal (i.e. z-axis) information of the electron bunch into transverse (i.e. xoy plane) distribution.

[0051] When the center of the electron beam just passes through the zero-crossing phase point (i.e. zero-phase point, at which the modulus of the electric field gradient is maximum) of the deflection electric field, the forces on the electrons in front of and behind the bunch (i.e. head and tail) are symmetrically opposite. Therefore, after the rotation of the electron beam, it is symmetrically distributed on the detector.

[0052] If the center of the electron beam on the detector is offset, it is caused by the difference between the center of the electron beam and the zero-crossing phase, i.e. the arrival time information of the electron beam. Thus, the spacing of the bunch string can be calculated.

[0053] The downstream resonator provides deflection force only in the x-direction, causing successive electron beam clusters to deviate from the axis in the horizontal x-direction. Each beam cluster leaving the upstream resonator passes sequentially through the extreme point of the deflection field of the downstream resonator, generating momentum in the x-axis direction. The deflection field decays over time, and the degree of deviation of the measured beam cluster from the axis changes proportionally. Ultimately, the beam spots presented on the detector separate from each other, and the beam spot information presented on the detector is thus separated and does not interfere with each other, realizing the measurement of the time information of the electron beam.

[0054] The resonator's geometric parameters are adjusted to control its resonant frequency based on the spacing and number of bundles in the bundle string, so that the frequency of the deflection field meets the measurement requirements.

[0055] like Figure 8 As shown ( Figure 8 The deflection field in the image is the first deflection field. If the electron beam passes through the deflection field with zero phase, the electrons at the center of the cluster will not experience a net lateral deflection, and their lateral coordinates will be zero upon reaching the detector. The head and tail of the electron cluster experience lateral forces in opposite directions. If the attenuation of the deflection field is ignored, the electrons will be symmetrically distributed about the y-axis upon reaching the detector after passing through the second resonator. If the electron beam does not pass through the deflection field with zero phase, the electrons at the center of the cluster will experience a net lateral deflection, and their electrons will be asymmetrically distributed about the y-axis upon reaching the detector after passing through the second resonator. Regardless of whether the electrons are symmetrically distributed about the y-axis on the detector, the distance between clusters can be deduced from the distance between the cluster centers in the y-direction on the detector.

[0056] Regardless of whether the electron beam passes through the zero-phase region, it is essential to ensure that the electron beam passes through the linear region of the deflection electric field (i.e., the region near the zero-phase point where the field strength changes approximately linearly, and the rate of change of the field strength in this region remains essentially constant, close to a constant). Figure 2 The gray line segment in the electric field waveform diagram of the first resonator is shown. This region can be selected by those skilled in the art based on the actual measurement situation, for example, the region where the field strength change rate is k±0.1, so that the electron beam information can be accurately deduced from the beam spot of the electron beam on the detector.

[0057] Preferably, the length of the gap of the second resonator in the y-direction is greater than the length of the gap of the first resonator in the y-direction.

[0058] It is understandable that after the electron beam passes through the first deflection electric field, it is deflected in the y direction due to the deflection force in the y direction. Therefore, in order to ensure that the head and tail of the electron beam after the first deflection are both affected by the second deflection electric field, the length of the gap of the second resonator in the y direction should be greater than the length of the gap of the first resonator in the y direction.

[0059] The embodiment of the present application provides a kind of ultra-short electron bunch string measurement device, comprising: processor, first resonator, second resonator, detector are placed in turn along the direction of electron bunch advancement.

[0060] Specifically, as shown in Figure 9 First resonator, second resonator are arranged along the direction of electron advancement axis.

[0061] The first resonator, second resonator are used to generate first, second deflection electric field with equal angular frequency and mutually orthogonal direction at its gap under the driving of terahertz at the same time;Wherein, the first deflection electric field is used to exert y direction deflection force on the electron bunch, so that it deflects in y axis direction;Second deflection electric field is used to exert transverse x axis direction deflection force on the electron bunch, so that the projection beam spot on the detector separates along x axis direction.

[0062] It can be understood that the distance between the first resonator and the second resonator should ensure that the deflection electric field of the two resonators does not affect each other.

[0063] The detector is used to detect the information of projection beam spot;

[0064] The processor is used to determine the length ΔT of the electron bunch according to beam spot size S and formula And determine the interval Δt between the electron bunches according to the interval Δy of the center of two beam spots in y axis direction and formula

[0065] Wherein, ω is the angular frequency of deflection electric field, Equivalent deflection electric field intensity, e is elementary charge, P is the momentum of electron bunch, D is the distance from the second resonator to the detection screen, T p The transit time of electron bunch through the gap of first resonator.

[0066] Preferably, the device further comprises: laser source, electron source, first beam splitter, second beam splitter, first terahertz generator, second terahertz generator, frequency adjustment device, first delay device and second delay device;

[0067] The laser emitted by the laser source is divided into two beams by the first beam splitter, wherein the first beam is adjusted in frequency by the frequency adjustment device, and then drives the electron source to generate electron bunch;Second beam is divided into two beams again by second beam splitter, and is used to drive the first terahertz generator and the second terahertz generator to generate terahertz respectively.

[0068] The first delay device is used to regulate and control the second beam, and the second delay device is used to regulate and control the beam driving the second terahertz generator. ​

[0069] Specifically, the terahertz for driving the two resonators and the laser for driving the electron source (such as a photocathode electron gun) come from the same laser beam, to ensure the synchronization of the electron beam and the terahertz deflection field in the resonator, the time delay and phase of the two deflection fields can be accurately controlled through the optical path system, to realize accurate control of the beam and the deflection field, thereby improving the resolution of the measurement.

[0070] As shown in Figure 9 , the laser generated by the laser is partially used to excite the electron beam after being tripled, and the rest is divided into two parts and converted into terahertz to excite two resonators after passing through the terahertz generator, to generate a deflection field in the resonator. The phase between the electron beam and the two deflection fields can be accurately controlled through the time delay device.

[0071] Preferably, the first resonator and the second resonator are both split ring resonators, and the split ring resonant cavity thereof is a metal subwavelength structure. The first resonator and the second resonator can also be plate-shaped with a slit structure as shown in Figures 10-11 . It can be understood that the present application does not specifically limit the structure of the first resonator and the second resonator, as long as the resonator has a slit structure and can generate a terahertz electric field at the slit under the action of terahertz.

[0072] Preferably, Figure 10 The values of the parameters in the table 1 are shown in the table 1

[0073] Table 1

[0074]

[0075] Specifically, the split ring resonator (SRR) amplifies the incident THz wave in the split ring resonant cavity, providing a high gradient deflection THz field. The enhancement coefficient of the SRR resonant cavity is determined by its geometric parameters and materials.

[0076] The method provided by the embodiment of the present application is further described below in combination with a simulation example.

[0077] Taking the split ring resonator as shown in Figure 3 as an example, the split ring resonator driven by terahertz is simulated and analyzed, and the three-dimensional electromagnetic field distribution in the resonator can be obtained, and thus the beam behavior is analyzed in combination with the beam tracking software.

[0078] A schematic diagram of the two split ring resonant cavities is shown in Figure 3 , and the parameters are shown in Table 2. The thickness of the gap in the y-axis direction of the downstream SRR is increased to accommodate all the electrons, so that each electron is subjected to uniform deflection. The materials of the two resonant cavities are copper, the resonant frequencies are both designed to be 0.3 terahertz, and the distance along the axis is 1 centimeter.

[0079] Table 2

[0080]

[0081]

[0082] Two THz drive pulses from the same laser excite two SRRs respectively, and get enhanced THz resonance fields; the field enhancement factors of the two SRRs are about 22 and 15 respectively. The design targets of the two resonance fields are 100 MV / m and 200 MV / m. The required THz drive pulse field strengths are 4.55 MV / m and 13.33 MV / m respectively.

[0083] The six bunch trains pass through the gaps of the two SRRs in turn, and the phases of the two orthogonal deflection fields are controlled so that the bunch trains pass through the zero phase point of the first deflection field and the peak point of the second deflection field. The attenuated deflection fields make the projections of the bunch trains on the downstream detector separate from each other, and the distribution of the beam spots on the detector is as shown in Figure 12 .

[0084] After being subjected to the deflection force in the vertical y direction provided by the upstream SRR, the longitudinal information of the electron beam is converted into the transverse distribution. The relationship between the bunch train length and the transverse size of the beam spot measured by the detector is:

[0085]

[0086] As shown in Figure 13 , the center offset information of the electron beam on the detector represents the arrival time information of the electron beam, i.e. the interval of the bunch trains. Moreover, since the bunch trains pass through the nearly linear region near the zero phase point, the change of the arrival time of the electron beam within a certain range will not affect the bunch train length information.

[0087] In the case where the SRR geometric parameters and materials are determined, the energy distribution of the THz drive pulse determines the field strengths of the two deflection fields. The first deflection field determines the resolution of the measurement of the bunch train length and the interval of the bunch trains. As can be seen from Figure 12 , the attenuation of the field strength in the vertical y direction makes the beam spot size of the bunch trains decrease in turn. In this example, the time resolution of the measurement of the bunch train is determined by the resolution of the measurement of the sixth bunch train, the resolution of the measurement of the bunch length is 2.5 femtoseconds, and the resolution of the measurement of the interval is 1 femtosecond. The second deflection field determines the number of the bunch trains in the bunch train that can be measured. As can be seen from Figure 12 , the attenuation of the field strength in the horizontal x direction makes the beam spots separate from each other, and the distance between the beam spots is constantly reduced. When the distance between the beam spots is reduced to the extent that the information of two bunch trains cannot be distinguished, the measurement cannot be completed. Therefore, according to the resolution of the measurement and the measurement requirement of the number of the bunch trains, the energy size of the THz drive pulse should be reasonably distributed so as to maximize the efficiency.

[0088] An embodiment of the present application provides a kind of measurement system of ultra-short electron bunch, comprising: computer readable storage medium and processor;

[0089] The computer readable storage medium is used to store executable instructions;

[0090] The processor is used to read the executable instructions stored in the computer readable storage medium, executes the method as described in any of the above embodiments.

[0091] Those skilled in the art readily understand that the above only describes the preferred embodiments of the present application and is not used to limit the present application, any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method of measuring an ultrashort electron bunch, characterized by, Comprising: S1, using terahertz to simultaneously drive the first resonator and the second resonator to generate a first deflection electric field and a second deflection electric field with equal angular frequency and mutually orthogonal direction at the gap thereof, respectively; S2, taking the forward direction of the electron bunch as the z-axis, making it pass through the gap of the first resonator and the second resonator in turn, and after the first deflection electric field and the second deflection electric field, reaching the detector; Wherein, the first deflection electric field is used to apply a deflection force in the y direction to the electron bunch, so that it deflects in the y-axis direction; the second deflection electric field is used to apply a deflection force in the transverse x-axis direction to the electron bunch, so that the projected beam spot on the detector separates along the x-axis direction; S3, the size S of the projection beam spot and the formula determining the length ΔT of the corresponding electron bunch; according to the spacing Δy of the two beam spot centers in the y-axis direction and the formula determining the spacing Δt between the corresponding two electron bunches; where ω is the angular frequency of the deflection electric field, is the equivalent deflection electric field strength, e is the elementary charge, P is the momentum of the electron bunch, D is the distance of the second resonator to the detection screen, T p is the time of the electron bunch passing through the gap of the first resonator.

2. The method of claim 1, wherein, The electron bunch passes through the linear region of the first deflection electric field.

3. The method of claim 1 or 2, wherein, The electron bunch passes through the zero phase point of the first deflection electric field.

4. The method of claim 3, wherein, The electron bunch passes through the extreme point of the second deflection electric field.

5. The method of claim 1 or 4, wherein, The length of the gap of the second resonator in the y direction is greater than the length of the gap of the first resonator in the y direction.

6. A device for measuring a train of ultrashort electron bunches, characterized in that Comprising: A processor, a first resonator, a second resonator and a detector placed in turn along the forward direction of the electron bunch; The first resonator and the second resonator are used to generate a first deflection electric field and a second deflection electric field with equal angular frequency and mutually orthogonal direction at the gap thereof under the simultaneous driving of terahertz; wherein, the first deflection electric field is used to apply a deflection force in the y direction to the electron bunch, so that it deflects in the y-axis direction; the second deflection electric field is used to apply a deflection force in the transverse x-axis direction to the electron bunch, so that the projected beam spot on the detector separates along the x-axis direction; The detector is used to detect the information of the projected beam spot; The processor is configured to determine a length ΔT of the electron bunches according to a beam spot size S and a formula and determine a spacing Δt between the electron bunches according to a spacing Δy of the two beam spot centers in a y-axis direction and a formula ​ where ω is the angular frequency of the deflection electric field, is the equivalent deflection electric field strength, e is the elementary charge, P is the momentum of the electron bunch, D is the distance of the second resonator to the detection screen, T p is the time of the electron bunch passing through the gap of the first resonator.

7. The apparatus of claim 6, wherein, Further comprising: A laser source, an electron source, a first beam splitter, a second beam splitter, a first terahertz generator, a second terahertz generator, a frequency adjustment device, a first delay device and a second delay device; The laser emitted by the laser source is divided into two beams by the first beam splitter, wherein the first beam is used to drive the electron source to generate an electron bunch after adjusting the frequency by the frequency adjustment device; the second beam is divided into two beams again by the second beam splitter, which are used to drive the first terahertz generator and the second terahertz generator to generate terahertz, respectively; The first delay device is used to regulate and control the second beam, and the second delay device is used to regulate and control the beam driving the second terahertz generator.

8. The apparatus of claim 6 or 7, wherein, The first resonator and the second resonator are both split ring resonators.

9. A system for measuring ultra-short electron bunches, characterized in that Comprising: A computer readable storage medium and a processor; The computer readable storage medium is used to store executable instructions; The processor is used to read the executable instructions stored in the computer readable storage medium, and execute the method as claimed in any one of claims 1-5.

Images