Time transverse coupling modulation attosecond mode-locked X-ray pulse generation method and device

Abstract

The invention relates to the technical field of data processing, and discloses a time transverse coupling modulated attosecond mode-locked X-ray pulse generation method and device, and the method comprises the steps: obtaining a first electron beam through a linear accelerator, carrying out the acceleration processing, obtaining a second electron beam, and transmitting the second electron beam to a first dispersion section; the first dispersion section processes the second electron beam to obtain a third electron beam and sends the third electron beam to the laser modulation section; the laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam, and sends the fourth electron beam to the second dispersion section; the second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam, and sends the fifth electron beam to a mode-locked amplifier; and the mode-locked amplifier processes the fifth electron beam and outputs an attosecond mode-locked X-ray pulse sequence. By performing energy modulation and spatial distribution modulation on the electron beam, mutual interference between pulse signals can be effectively avoided, and the signal-to-noise ratio of a pulse string is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of data processing, and particularly to a method and device for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation. Background Art

[0002] In existing free electron laser technologies, generally, an external laser is used to modulate the energy distribution of an electron beam to form a periodic energy chirp for synthesizing a pulse train. However, high-energy and low-energy electrons that are not effectively separated still radiate synchronously in the undulator, resulting in significant background noise; in addition, methods such as nonlinear compression, plasma, or metal sheets are usually used to apply energy modulation to the electron bunch, but this limits the pulse duration.

[0003] Therefore, there is still room for improvement and development in the prior art. Summary of the Invention

[0004] The main objective of the present invention is to provide a method and device for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation, aiming to solve the problems in the prior art that the pulse signal generated by the free electron laser technology has significant background noise and the pulse duration is limited.

[0005] To achieve the above objective, the present invention provides a method for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation. The method for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation is applied to a device for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation. The device for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation includes a linear accelerator, a first dispersion section, a laser modulation section, a second dispersion section, and a mode-locking amplifier. The method for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation includes the following steps: The linear accelerator acquires a first electron beam, accelerates the first electron beam to obtain a second electron beam, and sends the second electron beam to the first dispersion section; The first dispersion section processes the second electron beam to obtain a third electron beam, and sends the third electron beam to the laser modulation section; The laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam, and sends the fourth electron beam to the second dispersion section; The second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam, and sends the fifth electron beam to the mode-locking amplifier; The mode-locking amplifier processes the fifth electron beam and outputs an attosecond mode-locked X-ray pulse train.

[0006] Optionally, in the method for generating attosecond mode-locked X-ray pulses with temporal transverse coupling modulation, the phase space expression of the second electron beam is: ; where is the phase space expression of the second electron beam, is the horizontal coordinate, is the horizontal divergence, is the vertical coordinate, is the vertical divergence, is the longitudinal coordinate, is the relative energy deviation.

[0007] Optionally, in the method for generating attosecond mode-locked X-ray pulses with temporal transverse coupling modulation, the first dispersion section processes the second electron beam to obtain a third electron beam and sends the third electron beam to the laser modulation section, which specifically includes: The first dispersion section receives the second electron beam sent by the linear accelerator and obtains a first transfer matrix, where the expression of the first transfer matrix is: ; where is the first transfer matrix, is the length of the first dispersion section, is the dispersion generated in the first dispersion section, is the momentum compression generated in the first dispersion section; The first dispersion section processes the second electron beam through the first transfer matrix to obtain a third electron beam, where the phase space expression of the third electron beam is: ; The first dispersion section sends the third electron beam to the laser modulation section.

[0008] Optionally, in the method for generating attosecond mode-locked X-ray pulses with temporal transverse coupling modulation, the laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam and sends the fourth electron beam to the second dispersion section, which specifically includes: The laser modulation section receives the seed laser sent by the seed laser generator and receives the third electron beam sent by the first dispersion section; The laser modulation section calculates the undulator parameters of the laser modulation section according to the average Lorentz factor of the third electron beam, the laser wavelength of the seed laser, and the undulator period of the modulation section, and calculates the energy change of the third electron beam according to the undulator parameters and the laser power; The laser modulation section obtains a second transfer matrix, and processes the second transfer matrix and the third electron beam according to the energy change to obtain a fourth electron beam; The laser modulation section sends the fourth electron beam to the second dispersion section.

[0009] Optionally, in the attosecond mode-locked X-ray pulse generation method with time transverse coupling modulation, the expression of the resonance relationship between the average Lorentz factor of the third electron beam, the laser wavelength of the seed laser, the modulation section undulator period of the laser modulation section, and the undulator parameters of the laser modulation section is: ; Wherein, is the laser wavelength of the seed laser, is the modulation section undulator period, is the undulator parameter, is the average Lorentz factor; The expression of the energy change of the third electron beam is: ; Wherein, is the energy change, is the maximum value of the Lorentz factor change.

[0010] Optionally, in the attosecond mode-locked X-ray pulse generation method with time transverse coupling modulation, the second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam, and sends the fifth electron beam to the mode-locking amplifier, specifically including: The second dispersion section receives the fourth electron beam sent by the laser modulation section and obtains a third transfer matrix; The second dispersion section processes the fourth electron beam according to the third transfer matrix to obtain an initial transfer matrix; The second dispersion section simplifies the initial transfer matrix to obtain a target transfer matrix; The second dispersion section outputs the fifth electron beam according to the target transfer matrix and sends the fifth electron beam to the mode-locking amplifier.

[0011] Optionally, in the attosecond mode-locked X-ray pulse generation method with time transverse coupling modulation, the expression of the target transfer matrix is: ; Wherein, is the target transfer matrix, is the sum of the lengths of the first dispersion section, the laser modulation section, and the second dispersion section, is the energy chirp, is the length of the dispersion section, is the bending angle value.

[0012] In addition, to achieve the above object, the present invention also provides an attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation, wherein the attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation includes: A linear accelerator for obtaining a first electron beam, accelerating the first electron beam to obtain a second electron beam, and sending the second electron beam to a first dispersion section; A first dispersion section for processing the second electron beam to obtain a third electron beam and sending the third electron beam to a laser modulation section; A laser modulation section for performing energy modulation processing on the third electron beam to obtain a fourth electron beam and sending the fourth electron beam to a second dispersion section; A second dispersion section for performing spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam and sending the fifth electron beam to a mode-locked amplifier; A mode-locked amplifier for processing the fifth electron beam and outputting an attosecond mode-locked X-ray pulse train.

[0013] Optionally, in the attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation, the attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation further includes: An electron source for generating a first electron beam and sending the first electron beam to the linear accelerator.

[0014] Optionally, in the attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation, the attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation further includes: A seed laser generator for generating a seed laser and sending the seed laser to the laser modulation section.

[0015] In the present invention, the linear accelerator acquires a first electron beam, accelerates the first electron beam to obtain a second electron beam, and sends the second electron beam to the first dispersion section; the first dispersion section processes the second electron beam to obtain a third electron beam, and sends the third electron beam to the laser modulation section; the laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam, and sends the fourth electron beam to the second dispersion section; the second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam, and sends the fifth electron beam to the mode-locked amplifier; the mode-locked amplifier processes the fifth electron beam and outputs an attosecond mode-locked X-ray pulse train. By performing energy modulation and spatial distribution modulation on the electron beam, the present invention can effectively avoid the mutual interference between pulses, significantly improve the signal-to-noise ratio of the pulse train, and finally obtain an attosecond mode-locked X-ray pulse train with high purity and short pulses. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a flowchart of a preferred embodiment of the method for generating attosecond mode-locked X-ray pulses with time transverse coupling modulation according to the present invention; Figure 2 is a schematic diagram of the overall structure of a preferred embodiment of the method for generating attosecond mode-locked X-ray pulses with time transverse coupling modulation according to the present invention; Figure 3 is a schematic diagram of the principle of a preferred embodiment of the method for generating attosecond mode-locked X-ray pulses with time transverse coupling modulation according to the present invention; Figure 4 is a schematic diagram of the energy distribution, transverse distribution, and current intensity distribution of the electron bunches in a preferred embodiment of the method for generating attosecond mode-locked X-ray pulses with time transverse coupling modulation according to the present invention; Figure 5 is a schematic diagram of the FEL simulation results of a preferred embodiment of the device for generating attosecond mode-locked X-ray pulses with time transverse coupling modulation according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] Two key technical problems faced by existing Free Electron Lasers (FEL) technology in the field of attosecond pulse generation: 1. Insufficient suppression of background noise in mode-locked FEL pulse trains: Traditional mode-locked FEL schemes modulate the energy distribution of electron beams through external lasers to form periodic energy chirps for synthesizing pulse trains. However, high-energy and low-energy electrons that are not effectively separated still emit synchrotron radiation in the undulator, resulting in significant background noise. In traditional schemes, the power of noise in the time domain usually reaches more than 10% of the power of the FEL pulse train, and the time-domain noise also causes spectral broadening and random noise in the frequency domain. 2. Pulse duration limitation of the fresh bunch technology: Existing fresh slicing schemes usually use methods such as nonlinear compression, plasma, and metal sheets to apply energy modulation to electron bunches. The modulation range is comparable to the electron bunch length, which leads to the accuracy of subsequent time-transverse modulation being near the electron bunch length range, restricting the shortest pulse duration obtained to above 1 fs.

[0018] To solve the above problems, the present invention discloses a method for generating attosecond mode-locked X-ray pulses with time-transverse coupling modulation. Through the collaborative design of electron beam energy modulation and orbit control technology, it specifically solves the technical problems of insufficient noise suppression in mode-locked FEL pulse trains and too long pulse duration of fresh bunches. The specific implementation method is as follows: 1. Introduce a wiggler for laser modulation in the magnetic bend system to convert more refined energy modulation into time-transverse modulation (including generating energy modulation in the laser modulation section and converting energy modulation into spatial distribution modulation in the second dispersion section): When the bunch advances in the undulator, the effective interaction length on the axis is shortened to 150 nm within a single laser wavelength. By independently controlling the orbits of high-energy and low-energy electrons, the interference between multi-color pulses in traditional fresh slicing is avoided, further improving the purity of monochromatic pulses. Further, the high-energy and low-energy electrons are laterally separated by 240 μm through the magnetic bend system, and only electrons with specific energies are allowed to participate in the laser process through orbit switching in the undulator section, suppressing the FEL gain of off-axis electrons to below 0.01 GW, optimizing the spectral bandwidth to 2.1%, and precisely controlling the mode interval to 1.55 eV, significantly improving the signal-to-noise ratio of the pulse train.

[0019] 2. To address the problem of the excessively long duration of the fresh bunch pulse, the present invention realizes the generation of ultrashort pulses through the optimization of the momentum compaction factor and the design of the mode-locking amplifier: A second dispersion section is set downstream of the modulator (for example, the momentum compaction factor ξ is set to 90 μm), converting the electron beam energy modulation into a finer time-lateral modulation, and shortening the effective interaction length on the axis to 150 nm. Combining with the design of the slip amount of the mode-locking amplifier (for example, the slip amount in the first stage is set to 40 nm), the interaction period between the electron beam and the radiation field is compressed to the sub-femtosecond level. The simulation results show that the duration of a single pulse can be reduced to 250 attoseconds, the maximum power can reach 2 GW, and the average power can reach 0.5 GW.

[0020] Through precise electron beam manipulation and optimization of undulator parameters, the present invention breaks through the performance bottleneck of existing attosecond FEL technology, providing a high-purity, ultrashort-pulse two-color attosecond mode-locking FEL light source for ultrafast scientific research, which is particularly suitable for precision detection in fields such as electron dynamics and quantum control.

[0021] The attosecond mode-locking X-ray pulse generation method with time-lateral coupling modulation according to a preferred embodiment of the present invention, as Figure 1 shown, the attosecond mode-locking X-ray pulse generation method with time-lateral coupling modulation includes the following steps: Step S10: The linear accelerator acquires a first electron beam, accelerates the first electron beam to obtain a second electron beam, and sends the second electron beam to the first dispersion section.

[0022] As Figure 2As shown in the figure, the present invention relates to an attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation, which includes an electron linear accelerator, a first dispersion section, a first modulation section, a second dispersion section and a mode-locking amplifier. Among them, the specific generation process of the attosecond mode-locked X-ray pulse sequence is as follows: 1. The first electron beam generated by the electron source is accelerated by the electron linear accelerator to form a second electron beam; 2. The second electron beam forms a third electron beam after passing through the first dispersion section; 3. The seed laser generator is used to generate seed laser, which and the third electron beam are both injected into the first modulation section to make the third electron beam generate periodic energy modulation to obtain a fourth electron beam; 4. After the fourth electron beam passes through the second dispersion section, its periodic energy modulation is converted into periodic time-transverse modulation to obtain a fifth electron beam; 5. The fifth electron beam enters the mode-locking amplifier to generate an attosecond free electron laser pulse sequence. Among them, the mode-locking amplifier is composed of multiple units, and each unit is composed of a wiggler and a delay structure. With such a structure, an attosecond pulse sequence with adjustable soft X-ray wavelength in the range of 1 nm to 10 nm can be generated. Among them, the shortest pulse width of each attosecond pulse is between 250 attoseconds and several femtoseconds. The generated attosecond pulse sequence can provide a new tool for ultrafast scientific research, especially suitable for two-color synchronous pump-probe experiments, and has important application potential in the fields of electron dynamics, quantum control, etc.

[0023] The particle accelerator in the present invention is used to generate a relativistic electron beam, and can realize continuous adjustment of the bunch length (i.e., the beam length of the electron beam) between 200 femtoseconds and 3 picoseconds. The particle accelerator can be either an accelerator with a beam energy of 50-150 MeV or an accelerator with a beam energy above 1 GeV. The particle accelerator is a photocathode electron gun, which includes a photocathode, a band accelerating tube, a band accelerating tube and a magnetic compression section. The laser hits the photocathode for photoemission, and the band accelerating tube and the band accelerating tube accelerate the electron beam. The band accelerating tube and the magnetic compression section realize the control of the beam length of the electron beam. Specifically, the beam length is added with an energy chirp in the longitudinal direction of the electron beam through the phase of the band accelerating tube. After the electron beam with the energy chirp passes through the magnetic compression section, the electron beam will be compressed. The bunch length of the compressed electron beam is related to the energy chirp of the electron beam and the length of the magnetic compression section. If it is necessary to adjust the beam length of the electron beam, it can be realized by adjusting the phase of the band accelerating tube to change the energy chirp of the electron beam.

[0024] Specifically, the phase space expression of the second electron beam is: ; Among them, is the phase space expression of the second electron beam, is the horizontal coordinate, is the horizontal divergence, is the vertical coordinate, is the vertical divergence, is the longitudinal coordinate, is the relative energy deviation.

[0025] For the linear optical analysis of the solution proposed by the present invention, the present invention uses a beam transport matrix to process the six-dimensional phase space of an electron bunch. The phase space of the electron bunch is defined by a six-dimensional vector (the dimension of the transport matrix is 6×6, and the number of rows in the phase space of the electron bunch is the number of electrons), where , and are the horizontal, vertical and longitudinal coordinates respectively, and are the horizontal and vertical divergences respectively, is the relative energy deviation with respect to the reference particle. The transverse structure generated by laser modulation can be carried out in the horizontal or vertical direction. The main difference is that the magnetic field in the direction in the planar undulator decays much faster than the magnetic field in the direction. For the operation in the direction, the centers of the wiggler and the undulator are on the same horizontal plane. Therefore, this adjustment can be easily applied to the existing seeded free electron laser. For simplicity, the change in the vertical direction is omitted here, that is, the following uses to represent the phase space of the electron bunch, and it is assumed that the , , of the first electron bunch are Gaussian distributions.

[0026] Step S20, the first dispersion section processes the second electron beam to obtain a third electron beam and sends the third electron beam to the laser modulation section.

[0027] As shown in Figure 3 ( Figure 3 where L in is the length of the dispersion section,

[0028] Specifically, the first dispersion section receives the second electron beam sent by the linear accelerator and obtains a first transfer matrix, where the expression of the first transfer matrix is: ; where is the first transfer matrix, is the length of the first dispersion section, is the dispersion generated in the first dispersion section, is the momentum compaction generated in the first dispersion section.

[0029] The first dispersion section processes the second electron beam through the first transfer matrix to obtain a third electron beam, where the phase space expression of the third electron beam is: ; the first dispersion section sends the third electron beam to the laser modulation section.

[0030] After obtaining the second electron beam, the present invention first passes it through a dispersion section composed of two bending magnets (i.e., the first dispersion section in the present invention), and the transfer matrix of the first dispersion section is , where is the length of the first dispersion section, is the dispersion generated in the first dispersion section, is the momentum compaction generated in the first dispersion section (where the first dispersion section and the second dispersion section have opposite polarities and are both composed of 2 dipole magnets. Let the length of the dispersion section be L , the angle between the center connection line and the advancing direction of the electron beam is , then the dispersion and the momentum compaction factor can be expressed as and ). Second electron beam phase space After passing through the transfer matrix , a third electron beam is obtained, and the phase space of the third electron beam is .

[0031] Step S30: The laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam and sends the fourth electron beam to the second dispersion section.

[0032] After the third electron beam undergoes energy modulation, a fourth electron beam is obtained, and the phase space of the fourth electron beam is , where is the transfer matrix corresponding to the laser modulation part.

[0033] Specifically, the laser modulation section receives the seed laser sent by the seed laser generator and receives the third electron beam sent by the first dispersion section; the laser modulation section calculates the undulator parameters of the laser modulation section according to the average Lorentz factor of the third electron beam, the laser wavelength of the seed laser, and the undulator period of the modulation section of the laser modulation section, and calculates the energy change of the third electron beam according to the undulator parameters and the laser power.

[0034] Among them, the expression of the resonance relationship between the average Lorentz factor of the third electron beam, the laser wavelength of the seed laser, the undulator period of the modulation section of the laser modulation section, and the undulator parameters of the laser modulation section is: ; Among them, is the laser wavelength of the seed laser, is the undulator period of the modulation section, is the undulator parameter, is the average Lorentz factor.

[0035] Among them, an example of generating the seed laser is as follows: a traveling-wave femtosecond optical parametric amplifier, which is pumped by a titanium-sapphire laser amplifier with a repetition rate of 1 kHz near 800 nm, uses a monoclinic BiB3O6 crystal, and is realized through a two-stage continuum seed injection scheme, achieving a total energy output exceeding 1 mJ, and the intrinsic conversion efficiency corresponding to the second stage is about 32%. The tuning range extends from 1.1 μm to 2.9 μm. The high parametric gain and wide amplification bandwidth of this crystal enable the pump pulse width to be maintained. Even at such a high output level, the pulse widths of the signal light and the idler light pulses are still less than 140 femtoseconds.

[0036] For the interaction process between the seed laser and the electron bunch in the laser modulation section: in the laser modulation section, the seed laser and the electron bunch swing forward in the same direction. In a specific embodiment, the first modulation section can be an undulator (an undulator refers to an array of magnets that generate a periodic magnetic field). The third electron beam satisfies the following resonance relationship in the first modulation section: ; Among them, is the laser wavelength of the seed laser, is the undulator period of the modulation section, is the undulator parameter. In the present invention, the preferred modulation section undulator is a planar undulator with a period equal to 15 cm and a total length of 0.9 m. The length of the modulation section undulator is usually in the range of 0.3 m to 2 m.

[0037] The laser modulation section calculates the energy change of the third electron beam according to the average Lorentz factor.

[0038] Among them, the expression for the energy change of the third electron beam is: ; Among them, is the energy change, is the maximum value of the change in the Lorentz factor.

[0039] The laser modulation section obtains a second transfer matrix, and processes the second transfer matrix and the third electron beam according to the energy change to obtain a fourth electron beam; the laser modulation section sends the fourth electron beam to the second dispersion section.

[0040] When the average Lorentz factor of the electron bunch is , and the maximum value of the change in the Lorentz factor is , the energy of the electron bunch changes from to .

[0041] The third electron beam interacts with the seed laser with a wavelength of in the modulator and obtains an energy change . For electrons within a seed wavelength range, only electrons near the maximum energy modulation position will generate laser in the undulator section. Comparing these electrons with the reference electrons that are not energy-modulated, the present invention assumes that the energy modulation is still linear, then the energy chirp is: ; Therefore, the energy change of these electrons can be written as: , among which, , the transfer matrix corresponding to the electron beam in the laser modulation part can be described as: ; Among them, is the length of the modulator, is the momentum compression generated in the undulator.

[0042] Step S40: The second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam, and sends the fifth electron beam to the mode-locking amplifier.

[0043] After the fourth electron beam passes through the second dispersion section, a fifth electron beam is obtained, and the phase space of the fifth electron beam is , among which, is the transfer matrix of the second dispersion section.

[0044] Specifically, the second dispersion section receives the fourth electron beam sent by the laser modulation section and obtains a third transfer matrix; the second dispersion section processes the fourth electron beam according to the third transfer matrix to obtain an initial transfer matrix; the second dispersion section performs a simplification process on the initial transfer matrix to obtain a target transfer matrix.

[0045] Among them, the expression of the target transfer matrix is: ; Among them, is the target transfer matrix, is the sum of the lengths of the first dispersion section, the laser modulation section, and the second dispersion section, is the energy chirp, is the length of the dispersion section, is the bending angle value of the dispersion section.

[0046] The second dispersion section outputs the fifth electron beam according to the target transfer matrix and sends the fifth electron beam to the mode-locking amplifier.

[0047] Since the momentum compression of the modulator is usually much smaller than that of the subsequent dispersion section (i.e., the second dispersion section), is ignored in the following calculations (i.e., the momentum compression generated in the undulator), The expression of ; Among them, L 2 is the length of the downstream dispersion section, and are the diffusion degree and the compression generated by the momentum at the downstream dispersion section respectively, then the transfer matrix of the entire beam line before the inserter is: ; Among them, , since the first dispersion section and the second dispersion section can be regarded as jointly constituting a bending magnet, and the polarities of the first dispersion section and the second dispersion section are opposite, so and . Considering the structure of the actual device, and The relationship between them can be expressed as and , among them, , the simplified entire transfer matrix can be expressed as: .

[0048] Subsequently, the transverse position of the electron after the downstream bend can be expressed as: ; In the equation of , this term indicates that the transverse modulation is closely related to the longitudinal energy modulation. The dispersion section - laser modulation section - dispersion section structure proposed by the present invention obtains time - transverse coupled modulation on the laser wavelength scale through energy modulation on the laser wavelength scale. After such a bunch is combined with the subsequent mode - locking amplifier, an attosecond mode - locked free - electron laser can be generated.

[0049] Step S50: The mode - locking amplifier processes the fifth electron beam and outputs an attosecond mode - locked X - ray pulse sequence.

[0050] In the mode - locking amplifier, assuming there are undulators and the subsequent delay structure, the finally obtained spectral shape is approximately: ; where is the normalized frequency, is the initial noise, is the slip length between the FEL and the bunch in one undulator, is the total slip length in one undulator and one delay structure.

[0051] Example demonstration: The electron beam extracted from the linear accelerator has the following parameters: energy of 2.53 GeV, average current of 3 kA, root - mean - square normalized emittance of 0.4 mm - mrad, and an uncorrelated energy spread of 180 keV. An external laser with a wavelength of = 800 nm, peak power of 50 GW, beam waist size of 0.5 mm, and root - mean - square pulse duration of 100 fs is used. The electron beam interacts with the external laser in the undulator and obtains an energy modulation amplitude of 10 MeV. The total length of the wiggler is 0.9 m and contains 6 periods. The emission wavelength of the mode - locking amplifier is selected to be 2 nm. The undulator period is selected to be 3 cm. When the modulation amplitude is 10 MeV, does not need to be very large. Here, take = 0.03 m, and the separation between the high - energy electrons and the reference particles can be calculated as ≈ 120 μm, indicating that the separation between the high - energy electrons and the low - energy electrons is about 240 μm.

[0052] The initial beam waist radius and the divergence angle are 60 μm and 1.3 μrad respectively. The length The length is 5 m, the deflection angle of each dipole magnet is 6 mrad, and the ELEGANT code is used to track the transport of the electron beam in the modulator and the dispersion section. The simulation takes into account the second-order transport effects. The FEL laser process is simulated using the time-dependent mode of GENESIS, and these simulations consider the actual transfer matrix of each component. Various nonlinear effects, such as coherent synchrotron radiation, incoherent synchrotron radiation, and longitudinal space charge effects, are also included in these simulations. After setting the structure of the dispersion section - laser modulation section - dispersion section, the energy distribution (i.e., (a) in Figure 4 ), the transverse distribution of the electron bunch (i.e., (b) in Figure 4 ), and the current intensity distribution (i.e., (c) in Figure 4 ) are as shown in Figure 4 .

[0053] As shown in (a) of Figure 4 , the sinusoidal energy modulation is overcompressed, and the highest density is located in the regions with the highest and lowest beam energies. In (b) of Figure 4 , after passing through the downstream bend, the centers of the highest-energy electrons and the lowest-energy electrons are 200 μm apart, which is sufficient to adopt a two-stage fresh-slice laser scheme. The length of the on-axis region within a single period in the undulator section is approximately 150 nm. Figure 4 In (c) of

[0054] It shows that the peak current is enhanced to 5 kA, and the peak current of the on-axis beam is approximately 4.2 kA. Taking the peak current as 4200 A, Planck's constant as H, and the speed of light as c, the following can be estimated under the above parameters: In the obtained spectrum, the energy difference between adjacent modes of the spectral comb is: ; The Pierce parameter of the FEL is estimated to be m, where is the undulator period of the mode-locked undulator, and the coherence time is calculated as as, where is the central wavelength of the X-ray free electron laser obtained in the mode-locked undulator (which can also be called the amplifier).

[0055] The simulation results of the FEL are as shown in Figure 5 . In the mode-locked amplifier, the performance of the FEL at the end of 13 undulators is given in (a) of Figure 5 and (b) of Figure 5 . Figure 5 In (a) of The pulse interval of a train of pulses is Ts = 1.33 fs, the total pulse energy is 4.23 μJ, the average peak power is 0.517 GW, and the average pulse duration is = 248 attoseconds. Among them, the slippage caused by the delay magnet results in a slightly larger total number of pulses than the number of electron periods, which is also the reason why the peak power of the pulses near the beam tail is lower than the average value.

[0056] In addition, the peak power of the free electron laser from the off-axis beam is suppressed below 0.01 GW. In Figure 5 (b), the spectral width (full width at half maximum) is 2.10%, consisting of about 40 modes, the mode interval is 0.25% (1.55 eV), and the average bandwidth of each individual mode is 0.41 eV. Advantages of the present invention: 1. High-purity and low-noise output: Horizontally separate high-energy and low-energy electrons through the magnetic bend system (this process is jointly completed by the first dispersion section - modulation section - second dispersion section, and the relationship between the horizontal separation and the corresponding parameters can be deduced through the transfer matrix. Under the given parameters in the example, the calculated horizontal separation reaches 240 μm) by 240 μm, and only allow electrons with specific energies to participate in the laser process in the undulator section, effectively suppressing the spontaneous emission (SASE) noise of off-axis electrons. Experiments show that the FEL peak power of off-axis electrons is suppressed below 0.01 GW, the spectral full width at half maximum is optimized to 2.1%, the mode interval precisely matches the external laser wavelength (1.55 eV), significantly improving the coherence and signal-to-noise ratio of the pulse train. 2. Breakthrough in ultra-short pulse duration: Through the optimization of the momentum compression factor ( = 90 μm) and the design of the mode-locked amplifier, shorten the effective interaction length between the electron beam and the radiation field to 150 nm, compress the single-pulse duration to 250 attoseconds, which is nearly one order of magnitude better than the traditional fresh slice scheme (about 2 femtoseconds), providing a finer time resolution for detecting ultrafast processes such as electron dynamics. 3. High compatibility and scalability: The horizontal modulation selects the horizontal direction, facilitating direct integration into existing seeded FEL devices; if vertical modulation is adopted, the external laser power requirement can be further reduced (for example, sufficient horizontal separation can be achieved through a smaller energy modulation amplitude). In addition, the present invention can also be extended to multi-color pulse generation or isolated attosecond pulse output, providing a technical basis for the development of new light sources.

[0057] Furthermore, as Figure 2 shown, based on the above attosecond mode-locked X-ray pulse generation method of time transverse coupling modulation, the present invention also correspondingly provides a device for generating attosecond mode-locked X-ray pulses by time transverse coupling modulation, wherein the device for generating attosecond mode-locked X-ray pulses by time transverse coupling modulation includes: A linear accelerator for obtaining a first electron beam, accelerating the first electron beam to obtain a second electron beam, and sending the second electron beam to the first dispersion section; The first dispersion section is configured to process the second electron beam to obtain a third electron beam and send the third electron beam to the laser modulation section; The laser modulation section is configured to perform energy modulation processing on the third electron beam to obtain a fourth electron beam and send the fourth electron beam to the second dispersion section; The second dispersion section is configured to perform spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam and send the fifth electron beam to the mode-locked amplifier; The mode-locked amplifier is configured to process the fifth electron beam and output an attosecond mode-locked X-ray pulse train.

[0058] Wherein, the attosecond mode-locked X-ray pulse generating device with time transverse coupling modulation further includes: An electron source is configured to generate a first electron beam and send the first electron beam to the linear accelerator.

[0059] Wherein, the attosecond mode-locked X-ray pulse generating device with time transverse coupling modulation further includes: A seed laser generator is configured to generate a seed laser and send the seed laser to the laser modulation section.

[0060] In summary, the present invention provides a method and device for generating attosecond mode-locked X-ray pulses with time transverse coupling modulation. The method includes: the linear accelerator acquires a first electron beam, performs acceleration processing on the first electron beam to obtain a second electron beam, and sends the second electron beam to the first dispersion section; the first dispersion section processes the second electron beam to obtain a third electron beam and sends the third electron beam to the laser modulation section; the laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam and sends the fourth electron beam to the second dispersion section; the second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam and sends the fifth electron beam to the mode-locked amplifier; the mode-locked amplifier processes the fifth electron beam and outputs an attosecond mode-locked X-ray pulse train. By performing energy modulation and spatial distribution modulation on the electron beam, the present invention can effectively avoid mutual interference between pulses, significantly improve the signal-to-noise ratio of the pulse train, and finally obtain an attosecond mode-locked X-ray pulse train with high purity and short pulses.

[0061] It should be noted that, in this article, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or terminal including a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such a process, method, article or terminal. Without more limitations, an element defined by the statement "including one..." does not exclude the existence of additional identical elements in the process, method, article or terminal including that element.

[0062] Of course, those of ordinary skill in the art can understand that all or part of the processes of implementing the above-described embodiments of the method can be completed by instructing relevant hardware (such as a processor, a controller, etc.) through a computer program. The program can be stored in a computer-readable storage medium readable by a computer. When the program is executed, it can include the processes of the above-described method embodiments. The computer-readable storage medium can be a memory, a magnetic disk, an optical disk, etc.

[0063] It should be understood that the application of the present invention is not limited to the above examples. For those of ordinary skill in the art, improvements or transformations can be made according to the above description. All such improvements and transformations should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for generating attosecond mode-locked X-ray pulses with temporal transverse coupling modulation, characterized in that, The method for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation is applied to an apparatus for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation. The apparatus for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation includes a linear accelerator, a first dispersion section, a laser modulation section, a second dispersion section, and a mode-locking amplifier; The method for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation includes: The linear accelerator obtains a first electron beam, accelerates the first electron beam to obtain a second electron beam, and sends the second electron beam to the first dispersion section; The first dispersion section processes the second electron beam to obtain a third electron beam, and sends the third electron beam to the laser modulation section; The laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam, and sends the fourth electron beam to the second dispersion section; The second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam, and sends the fifth electron beam to the mode-locking amplifier; The mode-locking amplifier processes the fifth electron beam and outputs an attosecond mode-locked X-ray pulse train.

2. The method for generating an attosecond mode-locked X-ray pulse by time transverse coupling modulation according to claim 1, wherein The phase space expression of the second electron beam is: ； Among them, is the phase space expression of the second electron beam, is the horizontal coordinate, is the horizontal divergence, is the vertical coordinate, is the vertical divergence, is the longitudinal coordinate, is the relative energy deviation.

3. The method for generating attosecond mode-locked X-ray pulses by temporal transverse coupling modulation according to claim 2, characterized in that, The first dispersion section processes the second electron beam to obtain a third electron beam, and sends the third electron beam to the laser modulation section, specifically including: The first dispersion section receives the second electron beam sent by the linear accelerator and obtains a first transfer matrix, where the expression of the first transfer matrix is: ； Among them, is the first transmission matrix, is the length of the first dispersion section, is the dispersion generated in the first dispersion section, is the momentum compression generated in the first dispersion section; The first dispersion section processes the second electron beam through the first transfer matrix to obtain a third electron beam, where the phase space expression of the third electron beam is: ; The first dispersion section sends the third electron beam to the laser modulation section.

4. The method for generating an attosecond mode-locked X-ray pulse by time transverse coupling modulation according to claim 2, characterized in that, The laser modulation section performs energy modulation processing on the third electron beam to obtain a fourth electron beam, and sends the fourth electron beam to the second dispersion section, specifically including: The laser modulation section receives the seed laser sent by the seed laser generator and receives the third electron beam sent by the first dispersion section; The laser modulation section calculates the undulator parameters of the laser modulation section according to the average Lorentz factor of the third electron beam, the laser wavelength of the seed laser, and the undulator period of the modulation section of the laser modulation section, and calculates the energy change of the third electron beam according to the undulator parameters and the laser power; The laser modulation section obtains a second transfer matrix and processes the second transfer matrix and the third electron beam according to the energy change to obtain a fourth electron beam; The laser modulation section sends the fourth electron beam to the second dispersion section.

5. The method for generating an attosecond mode-locked X-ray pulse by temporal transverse coupling modulation according to claim 4, wherein The expression of the resonance relationship among the average Lorentz factor of the third electron beam, the laser wavelength of the seed laser, the undulator period of the modulation section of the laser modulation section, and the undulator parameters of the laser modulation section is: ； Among them, is the laser wavelength of the seed laser, is the period of the undulator in the modulation section, is the undulator parameter, is the average Lorentz factor; The expression of the energy change of the third electron beam is: ； wherein, is the energy change, is the maximum value of the change in the Lorentz factor.

6. The method for generating an attosecond mode-locked X-ray pulse by time transverse coupling modulation according to claim 1, wherein The second dispersion section performs spatial distribution modulation processing on the fourth electron beam to obtain a fifth electron beam, and sends the fifth electron beam to the mode-locking amplifier, specifically including: The second dispersion section receives the fourth electron beam sent by the laser modulation section and obtains a third transfer matrix; The second dispersion section processes the fourth electron beam according to the third transfer matrix to obtain an initial transfer matrix; The second dispersion section performs a simplification process on the initial transfer matrix to obtain a target transfer matrix; The second dispersion section outputs the fifth electron beam according to the target transfer matrix and sends the fifth electron beam to the mode-locking amplifier.

7. The method for generating an attosecond mode-locked X-ray pulse by temporal transverse coupling modulation according to claim 6, characterized in that, The expression of the target transfer matrix is: ； Among them, is the target transmission matrix, is the sum of the lengths of the first dispersion section, the laser modulation section, and the second dispersion section, is the energy chirp, is the length of the dispersion section, is the bending angle value.

8. An attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation, characterized in that, The time transverse coupling modulation attosecond mode-locked X-ray pulse generation device includes: A linear accelerator for obtaining a first electron beam, accelerating the first electron beam to obtain a second electron beam, and sending the second electron beam to a first dispersion section; The first dispersion section for processing the second electron beam to obtain a third electron beam and sending the third electron beam to a laser modulation section; The laser modulation section for performing an energy modulation process on the third electron beam to obtain a fourth electron beam and sending the fourth electron beam to a second dispersion section; The second dispersion section for performing a spatial distribution modulation process on the fourth electron beam to obtain a fifth electron beam and sending the fifth electron beam to a mode-locking amplifier; The mode-locking amplifier for processing the fifth electron beam to output an attosecond mode-locked X-ray pulse train.

9. The attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation according to claim 8, characterized in that, The time transverse coupling modulation attosecond mode-locked X-ray pulse generation device further includes: An electron source for generating a first electron beam and sending the first electron beam to the linear accelerator.

10. The attosecond mode-locked X-ray pulse generation device with time transverse coupling modulation according to claim 8, characterized in that, The time transverse coupling modulation attosecond mode-locked X-ray pulse generation device further includes: A seed laser generator for generating a seed laser and sending the seed laser to the laser modulation section.

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