A Terahertz X-ray Generator Based on Free Electron Laser

Abstract

This invention relates to a terahertz-X-ray generation device based on a free-electron laser, comprising an ultrafast laser system and, in sequence, an injector, a laser electron beam modulation system, a linear acceleration section, an X-ray radiation section, a terahertz radiation section, and an X-ray delay system. The ultrafast laser system includes an ultrafast laser, a pulse broadening and beam splitting system, a first beam splitter, and a first mirror. The ultrafast laser is configured to generate ultrafast laser pulses. The first beam splitter divides the ultrafast laser pulses into a first ultrafast laser path and a second ultrafast laser path. The first ultrafast laser path is transmitted to the injector, and the second ultrafast laser path is transmitted to the pulse broadening and beam splitting system. Upon receiving the first ultrafast laser path, the injector generates an electron beam. The pulse broadening and beam splitting system generates a double-pulse laser based on the second ultrafast laser path and transmits the double-pulse laser to the laser electron beam modulation system. The terahertz-X-rays generated by this invention exhibit high pulse energy, narrow spectral bandwidth, and good tunability.

Description

Technical Field

[0001] This invention relates to the field of terahertz optical instrument technology, and more specifically to a terahertz-X-ray generating device based on a free-electron laser. Background Technology

[0002] Terahertz waves are electromagnetic waves with frequencies higher than microwaves but lower than visible light. They possess superior characteristics such as broadband speed, high resolution, low energy, high penetration, and transient response, making them valuable for applications in cutting-edge scientific research and industrial technology. In recent years, terahertz-X-ray pump-detection technology has played an increasingly important role in cutting-edge scientific research, serving as a key technology for studying nanomaterials, biomolecular structure dynamics, and strongly interacting quantum systems. However, this technology also places extremely high demands on the radiation characteristics of terahertz and X-rays.

[0003] In recent years, terahertz generation technology based on linear accelerators has developed rapidly. A scheme based on free-electron laser bundle compression can simultaneously generate terahertz radiation and X-rays, making terahertz-X-ray pump detection possible. Specifically, this scheme accelerates and compresses a high-energy electron beam generated by an injector through a linear segment, compressing the electron beam to within a radiation wavelength, and then generating X-rays and terahertz pulses through superradiation in both the X-ray and terahertz radiation segments. However, this scheme has the following drawbacks: 1) It lacks a high-gain free-electron laser amplification process, limiting the peak radiation power and resulting in relatively low pulse energy; 2) Due to the great difficulty in compressing the bundle to below 100 femtoseconds, generating terahertz radiation above 10 THz is also very difficult, thus resulting in poor tunability of the radiation spectrum; 3) The radiation spectrum is relatively broad. Summary of the Invention

[0004] To address the problems in the prior art, the present invention provides a terahertz-X-ray generating device based on a free-electron laser, which can generate X-rays while producing terahertz radiation with high pulse energy and narrow spectral bandwidth, and has good tunability.

[0005] This invention provides a terahertz-X-ray generation device based on a free-electron laser, comprising an ultrafast laser system and, in sequence, an injector, a laser electron beam modulation system, a linear acceleration section, an X-ray radiation section, a terahertz radiation section, and an X-ray delay system. The ultrafast laser system includes an ultrafast laser, a pulse broadening and beam splitting system, a first beam splitter, and a first reflector. The ultrafast laser is configured to generate ultrafast laser pulses. The first beam splitter divides the ultrafast laser pulses into a first ultrafast laser path and a second ultrafast laser path. The first ultrafast laser path is transmitted to the injector, and the second ultrafast laser path is transmitted to the pulse broadening and beam splitting system. Upon receiving the first ultrafast laser path, the injector generates an electron beam. The pulse broadening and beam splitting system generates a double-pulse laser based on the second ultrafast laser path and transmits the double-pulse laser to the laser electron beam modulation system.

[0006] Furthermore, the injector includes a photocathode electron gun and an S-band or X-band acceleration structure located downstream of the photocathode electron gun. The photocathode electron gun is configured to receive the first ultrafast laser to generate an electron beam, and the S-band or X-band acceleration structure is configured to accelerate the electron beam.

[0007] Furthermore, the pulse broadening and beam splitting system includes a second beam splitter, which splits the second ultrafast laser into a first sub-laser and a second sub-laser. An adjustable delay unit, a second reflector, and a first beam combiner are arranged sequentially along the optical path of the first sub-laser. A third reflector, a first beam combiner, and a pulse broadener are arranged sequentially along the optical path of the second laser.

[0008] Furthermore, the laser electron beam modulation system includes a modulation segment undulator.

[0009] Furthermore, the modulation segment undulator is an undulator with a period of 50 mm and a magnetic gap that is continuously adjustable from 24 mm to 80 mm.

[0010] Furthermore, the linear acceleration section includes a first acceleration unit, a magnetic compressor, and a second acceleration unit connected in sequence. The first acceleration unit is configured to accelerate the electron beam from the laser electron beam modulation system. The accelerated electron beam is converted from energy modulation to density modulation by the magnetic compressor. The density-modulated electron beam is then used by the second acceleration unit to generate an electron beam capable of producing X-rays.

[0011] Furthermore, the X-ray radiation section is an oscillator with a period of 16 cm and a magnetic gap that is continuously adjustable from 4 mm to 80 mm.

[0012] Furthermore, the terahertz radiation section is an oscillator with a switchable period of 40 cm and 80 cm and a continuously adjustable magnetic gap of 9 mm to 80 mm.

[0013] Furthermore, the X-ray delay system includes a third beam splitter for separating X-rays and terahertz radiation, and a first multilayer film mirror, a second multilayer film mirror, a third multilayer film mirror, a fourth multilayer film mirror, and a second beam combiner arranged sequentially along the optical path of the X-rays, and a fourth mirror, a fifth mirror, and the second beam combiner arranged sequentially along the optical path of the terahertz radiation.

[0014] Furthermore, both the third beam splitter and the second beam combiner are plane mirrors with a central aperture size of 1 mm.

[0015] The pulse broadening and beam splitting system of this invention can generate a continuously tunable terahertz signal from 0.1 terahertz to 30 terahertz using chirped laser beat frequency technology. The frequency of the terahertz signal can be adjusted by regulating the pulse spacing and energy chirp of the dual-pulse laser, exhibiting strong tunability. The terahertz signal interacts with the electron beam in the laser electron beam modulation system, thereby forming an energy modulation in the longitudinal phase space of the electron beam with the same frequency characteristics as the terahertz signal. This energy modulation is converted into density modulation in the terahertz band in the magnetic compressor of the linear acceleration section. In this invention, the density-modulated electron beam is accelerated to 1.3 GV to 1.5 GV in the linear acceleration section. This electron beam can generate X-ray free electron laser in the X-ray radiation section. Since the wavelengths of terahertz and X-rays differ by more than three orders of magnitude, the electron beam that has been irradiated with X-rays can continue to generate terahertz radiation. This electron beam can generate 0.1 to 30 terahertz radiation through fundamental wave radiation and 0.2 to 60 terahertz radiation through second harmonic radiation. The terahertz radiation of this invention is generated by a high-gain free-electron laser, achieving millijoule-level terahertz pulse output with a narrow spectral bandwidth and good coherence. Therefore, this invention's terahertz-X-ray generation device based on a free-electron laser can simultaneously generate X-ray free-electron lasers while producing continuously adjustable, millijoule-level coherent terahertz radiation from 0.1 to 60 terahertz. By adjusting the delay between X-rays and terahertz radiation using an X-ray delay system, the requirements of X-ray-pumped detection experiments can be met. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the terahertz-X-ray generating device based on a free electron laser according to the present invention.

[0017] Figure 2 yes Figure 1 A schematic diagram of the pulse broadening and beam splitting system in the image.

[0018] Figure 3 yes Figure 1 A schematic diagram of the laser electron beam modulation system.

[0019] Figure 4 yes Figure 1 A schematic diagram of the linear acceleration segment in the diagram.

[0020] Figure 5 yes Figure 1 A schematic diagram of the structure of the X-ray delay system. Detailed Implementation

[0021] The preferred embodiments of the present invention are given below with reference to the accompanying drawings and described in detail.

[0022] This invention provides a terahertz-X-ray generation device based on a free-electron laser, used for terahertz-X-ray pump detection experiments, such as... Figure 1 As shown, the system includes an injector 1, a laser electron beam modulation system 2, a linear acceleration section 3, an X-ray radiation section 4, a terahertz radiation section 5, and an X-ray delay system 6, connected in sequence. The laser pulses required by the injector 1 and the laser electron beam modulation system 2 are provided by an ultrafast laser system, which includes an ultrafast laser 7, a pulse broadening and beam splitting system 8, a first beam splitter 9, and a first reflector 10. Specifically, the ultrafast laser 7, the first beam splitter 9, and the first reflector 10 are arranged sequentially along the optical path and parallel to the transmission axis of the electron beam generated by the injector 1. Simultaneously, the first reflector 10, the pulse broadening and beam splitting system 8, and the laser electron beam modulation system 2 are also arranged sequentially along the optical path.

[0023] The ultrafast laser 7 is configured to generate ultrafast laser pulses adjustable from 30 femtoseconds to 200 femtoseconds. A first beam splitter 9 divides the ultrafast laser pulses into a first ultrafast laser 71 and a second ultrafast laser 72. The first ultrafast laser 71 is transmitted to the injector 1, and the second ultrafast laser 72 is transmitted to the pulse broadening and beam splitting system 8. Upon receiving the first ultrafast laser 71, the injector 1 generates a high-energy electron beam. The pulse broadening and beam splitting system 8 generates a modulated terahertz-band dual-pulse laser based on the second ultrafast laser 72.

[0024] Injector 1 is a photocathode injector, comprising a photocathode electron gun and an S-band or X-band acceleration structure located downstream of the photocathode electron gun. The photocathode of the photocathode electron gun receives the first ultrafast laser 71 to generate a high-quality electron beam, and the S-band or X-band acceleration structure accelerates the electron beam, making the electron beam energy at the injector 1 outlet adjustable between 50 MeV and 150 MeV.

[0025] like Figure 2As shown, the pulse broadening and beam splitting system 8 includes a second beam splitter 81, which splits the second ultrafast laser 72 into a first sub-laser 721 and a second sub-laser 722. The system also includes an adjustable delay unit 82, a second reflector 83, and a first beam combiner 85 arranged sequentially along the optical path of the first sub-laser 721, and a third reflector 84, the first beam combiner 85, and a pulse broadener 86 arranged sequentially along the optical path of the second sub-laser 722. The first sub-laser 721, after passing through the adjustable delay unit 82, forms an adjustable delay with the second sub-laser 722, then passes through the second reflector 83 to the first beam combiner 85 where it merges with the second sub-laser 722 to obtain a dual-pulse laser 73. The merged dual-pulse laser 73 then undergoes pulse broadening by the pulse broadener 86, and the length of the broadened pulse is adjustable.

[0026] The broadened double-pulse laser beat frequency can be used to obtain a terahertz signal, the frequency of which is:

[0027]

[0028] Where μ is the energy chirp coefficient of the high-energy electron beam, and τ is the optical pulse delay. Therefore, the terahertz signal frequency can be adjusted by changing the optical pulse delay and energy chirp of the dual-pulse laser 73.

[0029] like Figure 3 As shown, the laser electron beam modulation system 2 is configured to generate an energy modulation signal consistent with the frequency characteristics of the dual-pulse laser 73 in the longitudinal distribution of the electron beam generated by the injector 1, thereby obtaining an electron beam with terahertz energy modulation. The laser electron beam modulation system 2 includes a modulation segment undulator 21. In this embodiment, the modulation segment undulator 21 is a undulator with a period of 50 mm and a continuously adjustable magnetic gap from 24 mm to 80 mm (an undulator is a magnet structure whose magnetic poles change periodically along the longitudinal direction). In this invention, the electron beam and the aforementioned dual-pulse laser 73 are transmitted to the vacuum tube at the magnetic center of the undulator in a sinusoidal periodic motion. The lateral and longitudinal (time) positions of the dual-pulse laser 73 and the electron beam in the undulator should coincide as much as possible, so that the energy space of the electron beam is modulated.

[0030] This invention utilizes a pulse broadening and beam splitting system 8 to adjust the pulse spacing and energy chirp of a dual-pulse laser 73, thereby regulating the frequency of energy modulation in the electron beam and thus adjusting the terahertz signal frequency. Specifically, the energy chirp of the dual-pulse laser 73 is changed by adjusting the dispersion parameters of the pulse broadener 86, and the pulse spacing of the dual-pulse laser 73 is changed by adjusting the adjustable delay unit 82, thus achieving adjustment of the energy modulation frequency in the electron beam. Because a relativistic electron beam is used in the modulation process, the space charge effect has a relatively small impact on the terahertz structure, ensuring the feasibility of generating and maintaining terahertz energy modulation.

[0031] like Figure 4 As shown, the linear acceleration section 3 includes a first acceleration unit 31, a magnetic compressor 32, and a second acceleration unit 33. The first acceleration unit 31 is configured to receive an electron beam from the laser electron beam modulation system 2 and accelerate the electron beam to 200 MeV to 300 MeV. The accelerated electron beam is converted from energy modulation to density modulation by the magnetic compressor 32, forming micro-clusters in the terahertz band. The density-modulated electron beam is accelerated to 1.3 GV to 1.5 GV by the second acceleration unit 33. This energy electron beam can generate X-rays.

[0032] Refer again Figure 1 X-ray radiation section 4 is configured to generate an X-ray free-electron laser based on the electron beam generated by linear acceleration section 3, and terahertz radiation section 5 is configured to generate a terahertz free-electron laser based on the electron beam generated after the X-ray free-electron laser. X-ray radiation section 4 is an undulator with a period of 16 cm and a continuously adjustable magnetic gap from 4 mm to 80 mm, while terahertz radiation section 5 is an undulator with a switchable period of 40 cm and 80 cm and a continuously adjustable magnetic gap from 9 mm to 80 mm.

[0033] The principle of generating X-ray-terahertz free-electron lasers is as follows: When the electron beam moves periodically within a undulator, if the radiating section of the undulator satisfies the resonance relationship, the light generated in the previous cycle of the electron beam is coherently enhanced with the light generated in the next cycle. Simultaneously, this effect reacts to the electron beam, causing it to form micro-clusters with a period consistent with the radiation wavelength. Specifically, the magnetic gap of the radiating section undulator is related to the magnetic field strength of the undulator and can be used to adjust the resonant wavelength. The resonant wavelength of the radiating section undulator is:

[0034]

[0035] Where, λ n λ is the resonant wavelength of the undulator, γ is the relativistic factor, n is the harmonic number, and λ is the resonant wavelength of the undulator. u is the period of the oscillator, and K is the dimensionless parameter of the oscillator.

[0036] When the electron beam itself exhibits micro-clusters, it rapidly generates strong radiation within the undulator, which can be further amplified in subsequent undulators. Since the X-ray and terahertz radiation bands differ by more than three orders of magnitude, the energy dispersion required for terahertz radiation is far greater than that for X-rays. Therefore, the high-energy electron beam radiating X-rays can continue to generate terahertz radiation. Thus, through X-ray radiation segment 4 and terahertz radiation segment 5, continuously tunable, high-pulse-energy coherent terahertz radiation and X-ray free-electron lasers can be generated simultaneously. At this point, the X-rays and terahertz radiation are on the same optical path, with the X-rays positioned in front.

[0037] like Figure 5 As shown, the X-ray delay system 6 is configured to receive the X-ray-terahertz free-electron laser generated by the terahertz radiation section 5 and delay the X-rays so that the terahertz radiation is positioned in front of the X-rays, satisfying the requirements of terahertz-X-ray pump detection. Specifically, the X-ray delay system 6 includes a third beam splitter 61, which separates the X-rays and terahertz radiation. The X-ray delay system 6 also includes a first multilayer mirror 62, a second multilayer mirror 63, a third multilayer mirror 64, a fourth multilayer mirror 65, and a second beam combiner 66 arranged sequentially along the optical path of the X-rays, and a fourth mirror 67, a fifth mirror 68, and a second beam combiner 66 arranged sequentially along the optical path of the terahertz radiation. Specifically, X-rays pass through the first multilayer mirror 62, the second multilayer mirror 63, the third multilayer mirror 64, and the fourth multilayer mirror 65, and then converge with terahertz radiation through the second combiner 66. The terahertz radiation, after passing through the fourth mirror 67 and the fifth mirror 68, also converges with the X-rays through the second combiner 66. Compared to terahertz radiation, X-rays are delayed due to the longer optical path. Thus, the X-rays and terahertz radiation can form an adjustable delay between 100 picoseconds and 30 nanoseconds, which can be adjusted according to the requirements of terahertz-X-ray pump detection.

[0038] In this embodiment, both the third beam splitter 61 and the second beam combiner 66 are plane mirrors with a central aperture of 1 mm. The divergence angle of the X-rays generated by the X-ray radiation section 4 is approximately 10 μr to 50 μr, and the divergence angle of the terahertz radiation generated by the terahertz radiation section 5 is approximately 1 mR to 10 mR. When both X-rays and terahertz radiation pass through the third beam splitter 61 and the second beam combiner 66 simultaneously, the X-rays can pass through completely, while the terahertz radiation is reflected. Furthermore, the multilayer reflector reduces the intensity of the X-rays and limits their wavelength range.

[0039] Therefore, this invention simultaneously generates X-rays and terahertz radiation on a single undulator line and can effectively control the time delay between X-rays and terahertz radiation, meeting the requirements of terahertz-X-ray pump detection experiments. Compared with existing technologies, the device of this invention has advantages such as high electron beam utilization, high radiance, a wide terahertz radiation frequency adjustment range, simple adjustment, and narrow spectral bandwidth.

[0040] Therefore, the method for generating terahertz X-rays based on free-electron lasers includes the following steps:

[0041] Step S1: Provide the above-mentioned terahertz-X-ray generating device based on free electron laser, and use the ultrafast laser 7 to generate ultrafast laser pulses of 30 femtoseconds to 200 femtoseconds. The first beam splitter 9 divides the ultrafast laser pulses into a first ultrafast laser 71 and a second ultrafast laser 72.

[0042] Step S1 further includes: the injector 1 uses the first ultrafast laser 71 to generate an electron beam in the photocathode electron gun through the photoelectric effect, and then the electron beam is immediately accelerated to relativistic energy by the accelerating structure. By changing the accelerating field voltage and phase, the energy of the electron beam can be adjusted between 50 megaelectron volts and 150 megaelectron volts.

[0043] Step S1 further includes: the second ultrafast laser 72 generates a dual-pulse laser with a terahertz signal through a pulse broadening and beam splitting system 8. The pulse spacing and pulse length (i.e., energy chirp) of the dual-pulse laser can be continuously adjusted. As mentioned above, the frequency of the terahertz signal can be adjusted by adjusting the pulse spacing and energy chirp of the dual-pulse laser.

[0044] Step S2: The dual-pulse laser is transmitted to the laser electron beam modulation system 2. The electron beam interacts with the dual-pulse laser to generate energy modulation in the longitudinal distribution of the electron beam that is consistent with the frequency characteristics of the dual-pulse laser.

[0045] In step S3, the electron beam is transmitted to the first acceleration unit 31 in the linear acceleration section 3 to accelerate the electron beam to 200 megaelectron volts to 300 megaelectron volts. The energy modulation of the electron beam is converted into density modulation by the magnetic compressor 32 to form micro-clusters in the terahertz band. The electron beam is then accelerated to 1.3 to 1.5 gigaelectron volts by the second acceleration unit 33.

[0046] In step S4, the electron beam with terahertz density modulation generates X-rays and terahertz radiation through X-ray radiation section 4 and terahertz radiation section 5. At this time, the X-rays are on the same optical path in the terahertz spectrum and are located in front.

[0047] Step S5: X-rays and terahertz radiation are transmitted to the X-ray delay system 6, so that the X-rays are introduced with a time delay, thereby placing the terahertz radiation in front of the X-rays. The X-rays and terahertz radiation form an adjustable delay between 100 picoseconds and 30 nanoseconds, which meets the requirements of terahertz-X-ray pump detection.

[0048] The beneficial effects of this invention are:

[0049] 1) The device provided by the present invention can adjust the frequency of the terahertz signal by adjusting the pulse spacing and energy chirp of the dual-pulse laser, and generate terahertz radiation of 0.1 terahertz to 60 terahertz. It has strong tunability, narrow spectral bandwidth and good coherence.

[0050] 2) The device provided by the present invention can generate coherent terahertz radiation at the millijoule level and X-ray free electron laser at the same time through the process of high-gain free electron laser, thus meeting the requirements of terahertz-X-ray pump detection.

[0051] 3) The device provided by the present invention can achieve precise terahertz-X-ray pump detection by adjusting the delay of X-rays and terahertz radiation through an X-ray delay system.

[0052] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims of this patent. All aspects not described in detail in this invention are conventional technical content.

Claims

1. A terahertz-X-ray generating device based on a free-electron laser, characterized in that, The system includes an ultrafast laser system and, in sequence, an injector, a laser electron beam modulation system, a linear acceleration section, an X-ray radiation section, a terahertz radiation section, and an X-ray delay system. The ultrafast laser system comprises an ultrafast laser, a pulse broadening and beam splitting system, a first beam splitter, and a first reflector. The ultrafast laser is configured to generate ultrafast laser pulses. The first beam splitter splits the ultrafast laser pulses into a first ultrafast laser path and a second ultrafast laser path. The first ultrafast laser path is transmitted to the injector, and the second ultrafast laser path is transmitted to the pulse broadening and beam splitting system. Upon receiving the first ultrafast laser path, the injector generates an electron beam. The pulse broadening and beam splitting system generates a double-pulse laser based on the second ultrafast laser path and transmits the double-pulse laser to the laser electron beam modulation system.

2. The terahertz-X-ray generating device based on a free-electron laser according to claim 1, characterized in that, The injector includes a photocathode electron gun and an S-band or X-band acceleration structure located downstream of the photocathode electron gun. The photocathode electron gun is configured to receive the first ultrafast laser to generate an electron beam, and the S-band or X-band acceleration structure is configured to accelerate the electron beam.

3. The terahertz-X-ray generating device based on a free-electron laser according to claim 1, characterized in that, The pulse broadening and beam splitting system includes a second beam splitter, which splits the second ultrafast laser into a first sub-laser and a second sub-laser. An adjustable delay unit, a second reflector, and a first beam combiner are arranged sequentially along the optical path of the first sub-laser. A third reflector, a first beam combiner, and a pulse broadener are arranged sequentially along the optical path of the second sub-laser.

4. The terahertz-X-ray generating device based on a free-electron laser according to claim 1, characterized in that, The laser electron beam modulation system includes a modulation segment oscillator.

5. The terahertz-X-ray generating device based on a free-electron laser according to claim 4, characterized in that, The modulation segment undulator is an undulator with a period of 50 mm and a magnetic gap that is continuously adjustable from 24 mm to 80 mm.

6. The terahertz-X-ray generating device based on a free-electron laser according to claim 1, characterized in that, The linear acceleration section includes a first acceleration unit, a magnetic compressor, and a second acceleration unit connected in sequence. The first acceleration unit is configured to accelerate the electron beam from the laser electron beam modulation system. The accelerated electron beam is converted from energy modulation to density modulation by the magnetic compressor. The density-modulated electron beam is then used by the second acceleration unit to generate an electron beam capable of producing X-rays.

7. The terahertz-X-ray generating device based on a free-electron laser according to claim 1, characterized in that, The X-ray radiation section is an oscillator with a period of 16 cm and a continuously adjustable magnetic gap from 4 mm to 80 mm.

8. The terahertz-X-ray generating device based on a free-electron laser according to claim 1, characterized in that, The terahertz radiation section is an oscillator with a switchable period of 40 cm and 80 cm and a continuously adjustable magnetic gap of 9 mm to 80 mm.

9. The terahertz-X-ray generating device based on a free-electron laser according to claim 1, characterized in that, The X-ray delay system includes a third beam splitter for separating X-rays and terahertz radiation, and a first multilayer reflector, a second multilayer reflector, a third multilayer reflector, a fourth multilayer reflector, and a second beam combiner arranged sequentially along the optical path of the X-rays. A fourth reflector, a fifth reflector, and the second beam combiner are arranged sequentially along the optical path of the terahertz radiation.

10. The terahertz-X-ray generating device based on a free-electron laser according to claim 9, characterized in that, Both the third beam splitter and the second beam combiner are plane mirrors with a central aperture size of 1 mm.

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