Terahertz radiation generation system and method

Abstract

The application relates to a terahertz radiation generation system and method. The system comprises a photocathode electron gun for receiving a laser pulse train and generating an electron beam; an acceleration tube for accelerating the electron beam to a relativistic energy; a first focusing solenoid located at an outlet of the photocathode electron gun for increasing a charge density of the electron beam; a second focusing solenoid surrounding the acceleration tube for adjusting the charge density of the electron beam to a target charge density when the electron beam is accelerated in the acceleration tube; a beamline tube for transmitting the energy-modulated electron beam; a dispersion section comprising an upper dispersion section and a lower dispersion section, the beamline tube passing between the upper dispersion section and the lower dispersion section for converting the energy-modulated electron beam into a density-modulated electron beam when the energy-modulated electron beam is transmitted in the beamline tube; and a radiation device for receiving the density-modulated electron beam and generating terahertz radiation through the density-modulated electron beam. The system can work on a general linear accelerator, thereby reducing the demand for the accelerator.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of terahertz radiation sources, and in particular to a terahertz radiation generation system and method. BACKGROUND

[0002] High-brightness, narrow-band tunable terahertz light sources have a strong demand and wide application prospects in scientific research and industrial fields, such as resonance excitation of molecular rotation and lattice vibration, ultrafast spin control, and terahertz imaging, etc.

[0003] In the terahertz light source technology developed in recent years, accelerator-based terahertz light sources have the advantages of high peak (high average) power and wide frequency tunability. These accelerator light sources use undulator super radiation to generate terahertz narrow-band tunable terahertz radiation, but since the current accelerator light sources usually need to be modified to the beam line when in use, they are not suitable for ordinary electron linear accelerators, resulting in low versatility. SUMMARY

[0004] Therefore, it is necessary to provide a terahertz radiation generation system and method aiming at the above technical problems.

[0005] In a first aspect, the present application provides a terahertz radiation generation system. The system comprises a photocathode electron gun, an acceleration tube, a first focusing solenoid, a second focusing solenoid, a beam line tube, a dispersion section, and a radiation device, wherein:

[0006] The photocathode electron gun is configured to receive a laser pulse train and generate an electron beam through the laser pulse train;

[0007] The acceleration tube is configured to accelerate the electron beam to a relativistic energy;

[0008] The first focusing solenoid is located at the outlet of the photocathode electron gun and is configured to increase the charge density of the electron beam so that the electron beam oscillates for half a plasma period before the outlet of the acceleration tube;

[0009] The second focusing solenoid surrounds the acceleration tube and is configured to adjust the charge density of the electron beam to a target charge density when the electron beam is accelerated in the acceleration tube, so that the electron beam is subjected to its own space charge force at the target charge density to obtain an energy-modulated electron beam;

[0010] The beam line tube is configured to transmit the energy-modulated electron beam;

[0011] The dispersion section includes an upper half dispersion section and a lower half dispersion section, and the beamline tube passes through between the upper half dispersion section and the lower half dispersion section, for converting the energy-modulated electron beam into the density-modulated electron beam when the energy-modulated electron beam is transmitted in the beamline tube.

[0012] The radiation device is configured to receive the density-modulated electron beam and generate terahertz radiation by the density-modulated electron beam.

[0013] In one of the embodiments, the terahertz radiation has a target bandwidth and a target tuning range, and the system further includes a laser, a frequency doubling module, and a stacking module, wherein:

[0014] The laser is configured to generate infrared laser pulses.

[0015] The frequency doubling module is configured to convert the infrared laser pulses into ultraviolet laser pulses.

[0016] The stacking module is configured to stack the ultraviolet laser pulses to obtain the laser pulse train, the laser pulse train having a target number of sub-pulses, and a target accumulation frequency, wherein the terahertz radiation generated by the electron beam obtained by the laser pulse train having the target number of sub-pulses has the target bandwidth, and the target accumulation frequency is the center of the target tuning range.

[0017] In one of the embodiments, the frequency doubling module includes a first barium borate (BBO) crystal and a second BBO crystal, and the stacking module includes at least one α-BBO crystal, wherein:

[0018] The first BBO crystal is configured to double the frequency of the infrared laser pulses to obtain intermediate laser pulses formed by a first part of the infrared laser pulses and a second part of the infrared laser pulses, the second part being the infrared laser pulses other than the first part, and the intermediate laser pulses having a frequency twice that of the infrared laser pulses.

[0019] The second BBO crystal is configured to sum the second part of the infrared laser pulses and the intermediate laser pulses to obtain ultraviolet laser pulses, the ultraviolet laser pulses having a frequency three times that of the infrared laser pulses.

[0020] The α-BBO crystal is configured to stack the ultraviolet laser pulses to obtain the laser pulse train.

[0021] In one of the embodiments, the energy of the terahertz radiation is a target energy, the frequency doubling module further comprises a grating pair, and the stacking module further comprises an aperture,

[0022] The grating pair is configured to adjust a pulse width of the infrared laser pulse to a target pulse width before the infrared laser pulse enters the first BBO crystal, so that the pulse width of the laser pulse train is also the target pulse width.

[0023] The aperture is configured to adjust a spot size of the laser pulse train to a target spot size.

[0024] The energy of the terahertz radiation generated by the electron beam with the laser pulse train having the target pulse width and the target spot size is the target energy.

[0025] In one of the embodiments, the frequency of the terahertz radiation is a target frequency, and the system further comprises a phase shifter,

[0026] The phase shifter is configured to adjust an acceleration phase of the electron beam in the acceleration tube to a target acceleration phase, so that the frequency of the terahertz radiation generated by the electron beam having the target acceleration phase reaches the target frequency.

[0027] In one of the embodiments, the system further comprises a beam diagnosis module, and the beam diagnosis module comprises an S-band deflection cavity, an analyzing magnet, and two beam profile viewers,

[0028] The S-band deflection cavity is configured to associate a longitudinal distribution of the electron beam with a transverse distribution of the electron beam.

[0029] The analyzing magnet is configured to associate an energy distribution of the electron beam with the transverse distribution of the electron beam.

[0030] The beam profile viewers are configured to observe the transverse distribution of the electron beam, and obtain the longitudinal distribution of the electron beam and the energy distribution of the electron beam according to the transverse distribution of the electron beam.

[0031] In one of the embodiments, the system further comprises a terahertz detector, and the terahertz detector comprises a Michelson interferometer, a first Golay detector, and a second Golay detector,

[0032] The Michelson interferometer is configured to split the terahertz radiation.

[0033] The first Golay detector is located at an outlet of the Michelson interferometer, and is configured to measure an autocorrelation spectrum of the terahertz radiation according to the split terahertz radiation.

[0034] The second Golay detector is configured to measure the energy of the terahertz radiation.

[0035] In a second aspect, the present application also provides a method for generating terahertz radiation. The method comprises:

[0036] receiving a laser pulse train by a photocathode electron gun, and generating an electron beam by electrons generated by the laser pulse train bombarding a photocathode of the photocathode electron gun;

[0037] increasing a charge density of the electron beam by a first focusing solenoid so that the electron beam oscillates for half a plasma period before an exit of an acceleration tube;

[0038] accelerating the electron beam to a relativistic energy by the acceleration tube, while adjusting the charge density of the electron beam by a second focusing solenoid to obtain the electron beam with a target charge density, so that the electron beam with the target charge density is affected by its own space charge force to obtain an energy modulated electron beam;

[0039] transmitting the energy modulated electron beam by a beamline tube, while converting the energy modulated electron beam into a density modulated electron beam by the dispersive section;

[0040] receiving the density modulated electron beam by a radiation device, and generating terahertz radiation by coherent radiation generated by the density modulated electron beam when passing through the radiation device;

[0041] The system is the terahertz radiation generation system of any one of claims 1 to 7.

[0042] In one of the embodiments, the method further comprises:

[0043] generating infrared laser pulses by a laser;

[0044] converting the infrared laser pulses into ultraviolet laser pulses by a frequency doubling module;

[0045] stacking the ultraviolet laser pulses into the laser pulse train with a target number of sub-pulses and a target stacking frequency, wherein the terahertz radiation generated by the electron beam obtained by the laser pulse train with the target number of sub-pulses has a target bandwidth, and the target stacking frequency is a center of the target tuning range.

[0046] In one of the embodiments, the terahertz radiation has a target frequency, and the method further comprises:

[0047] The accelerating phase of the electron beam in the accelerating tube is adjusted to a target accelerating phase by a phase shifter, so that the frequency of terahertz radiation generated by the electron beam with the target accelerating phase reaches the target frequency.

[0048] The above-mentioned terahertz radiation generation system and method increase the charge density of the electron beam to a target charge density through the first focusing solenoid and the second focusing solenoid, so that the electron beam oscillates at half of the plasma period at the outlet of the accelerating tube to form a current spike, and the energy of the electron beam is modulated by the space charge force generated by the current spike to form an energy-modulated electron beam. The energy-modulated electron beam is converted into a density-modulated electron beam by the dispersive section, and the density-modulated electron beam generates terahertz radiation through the radiation device. Since the energy of the electron beam is modulated by the space charge force generated by the electron beam itself, there is no need for complex beamline design, and there is no requirement for the repetition frequency of the beamline. It can be applied to general linear accelerators and can be extended to high-repetition-frequency beamlines, thus reducing the demand for accelerators. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 A schematic diagram of a terahertz radiation generation system in an embodiment;

[0050] Figure 2 A schematic diagram of a terahertz radiation generation system in an embodiment;

[0051] Figure 3 A flowchart of a terahertz radiation generation method in an embodiment;

[0052] Figure 4 A flowchart of a terahertz radiation generation method in an embodiment. DETAILED DESCRIPTION

[0053] To make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below. In the following description, many specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present application, so the present application is not limited to the specific implementation disclosed below.

[0054] The serial numbers of components in this document, such as "first", "second", etc., are only used to distinguish the described objects, and do not have any sequence or technical meaning. Unless otherwise specified, "connection" and "coupling" in this application include direct and indirect connections (couplings). In the description of this application, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing this application and simplifying the description, and therefore cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting this application.

[0055] In this application, unless otherwise explicitly specified and limited, the first feature is "on" or "under" the second feature, which can be direct contact between the first and second features, or indirect contact between the first and second features through an intermediate medium. Moreover, the first feature "above", "above" and "above" the second feature can be directly above or obliquely above the first feature, or only indicate that the horizontal height of the first feature is higher than that of the second feature. The first feature "below", "below" and "below" the second feature can be directly below or obliquely below the first feature, or only indicate that the horizontal height of the first feature is less than that of the second feature.

[0056] As shown in Figure 1 The embodiment of the present application provides a terahertz radiation generation system 10. The terahertz radiation generation system 10 includes a photocathode electron gun 110, an acceleration tube 120, a first focusing solenoid 130, a second focusing solenoid 140, a beam line tube 150, a dispersion section 160 and a radiation device 170. The photocathode electron gun 110 is used to receive a laser pulse train and generate an electron beam through the laser pulse train. The acceleration tube 120 is used to accelerate the electron beam to a relativistic energy. The first focusing solenoid 130 is located at the outlet of the photocathode electron gun and is used to increase the charge density of the electron beam so that the electron beam oscillates for half a plasma period before the outlet of the acceleration tube. The second focusing solenoid 140 surrounds the acceleration tube 110 and is used to adjust the charge density of the electron beam to a target charge density when the electron beam is accelerated in the acceleration tube 110, so that the electron beam is subjected to its own space charge force at the target charge density to obtain an energy-modulated electron beam. The beam line tube 150 is used to transmit the energy-modulated electron beam. The dispersion section 160 includes an upper half dispersion section and a lower half dispersion section, and the beam line tube 150 passes between the upper half dispersion section and the lower half dispersion section, and is used to convert the energy-modulated electron beam into a density-modulated electron beam when the energy-modulated electron beam is transmitted in the beam line tube 150. The radiation device 170 is used to receive the density-modulated electron beam and generate terahertz radiation through the density-modulated electron beam.

[0057] In the light cathode electron gun 110, the laser pulse train is directly transmitted to the light cathode, and the laser pulse train generates electrons by bombarding the light cathode. The electrons form an initial electron beam with a certain density modulation under the action of a microwave field in the light cathode electron gun 110. After the electron beam leaves the light cathode electron gun 110, it enters the first focusing solenoid 130, which focuses the electron beam to increase the charge density of the electron beam, so that the electron beam can oscillate for half a plasma period before the exit of the accelerating tube 120, and the electron beam oscillating for half a plasma period can generate a periodic current spike in the subsequent transmission process. At the same time, the first focusing solenoid 130 can also maintain the emittance of the electron beam by laterally focusing the electron beam. Then the electron beam enters the accelerating tube 120, and the nonlinear space charge oscillation is carried out during the transmission process. The accelerating tube 120 increases the energy of the electron beam to a relativistic energy. At the same time, the second focusing solenoid 140 focuses the electron beam to increase the charge density of the electron beam to a target charge density, so that the electron beam can be affected by the space charge force under the target charge density, and the electron beam with the best energy modulation is obtained. The value of the target charge density can be determined by a person skilled in the art according to actual needs. After the electron beam leaves the accelerating tube 120, it enters the beamline tube 150. When the electron beam is transmitted in the beamline tube 150, the electron beam oscillating for half a plasma period can generate a current spike, and the space charge force generated by the current spike can cause the energy distribution of the electron beam to periodically change in the longitudinal direction, that is, an energy-modulated electron beam is formed. The energy-modulated electron beam passes through the dispersion section 160, and due to the influence of the dispersion section 160, the energy-modulated electron beam appears spatially concentrated, and forms a density spike in the longitudinal distribution, that is, a density-modulated electron beam is formed. The density-modulated electron beam passes through the radiation device 170 to generate coherent transition radiation (that is, terahertz radiation) with a wave band in the terahertz band. Subsequently, the terahertz radiation can be collected by a terahertz radiation collection device.

[0058] For example, the light cathode electron gun 110 can be a light cathode microwave electron gun operating in the S-band microwave, which can specifically include a copper cathode (that is, a light cathode), a microwave cavity, an electron gun solenoid, a microwave feed device, and a sampling coupling device. The accelerating tube 120 can be an S-band 40-unit normal-temperature traveling wave accelerating tube, wherein the phase shift of each unit is 2π / 3, and the energy of the electron beam at the exit of the accelerating tube 120 can reach about 35 MeV. The dispersion section 160 can be a chicane structure composed of four dipole magnets and three drift sections, wherein the power supply of the dipole magnet is provided by a direct-current stabilized power supply. The radiation device 170 can be an aluminum film assembled on a pulling device, or a waveguide. For example, the waveguide can have a length of 1 m and include 8 periods, wherein the length of a single period is 10 cm.

[0059] The terahertz radiation generation system provided in this application embodiment increases the charge density of the electron beam to a target charge density through a first focusing solenoid and a second focusing solenoid. This causes the electron beam to oscillate for half a plasma cycle at the exit of the accelerating tube, forming a current spike. The space charge force generated by the current spike modulates the energy of the electron beam, forming an energy-modulated electron beam. The energy-modulated electron beam is then converted into a density-modulated electron beam through a dispersion section, and the density-modulated electron beam generates terahertz radiation through a radiation device. Because this application embodiment modulates the energy of the electron beam through the space charge force generated by the electron beam itself, it eliminates the need for complex beamline design and avoids requirements on the beamline's repetition rate. It is applicable to general linear accelerators and can operate under conditions of high repetition rate and large charge quantity, exhibiting excellent scalability. The generated density beam is very high and not limited by the space charge force, and the process of generating the density beam does not damage the electron beam quality, thus reducing the requirements for the accelerator.

[0060] In one embodiment, such as Figure 2 As shown, the terahertz radiation has a target bandwidth and a target tuning range. The system also includes a laser 180, a frequency doubling module, and a stacking module. The laser 180 is used to generate infrared laser pulses, the frequency doubling module is used to convert the infrared laser pulses into ultraviolet laser pulses, and the stacking module is used to stack the ultraviolet laser pulses to obtain a laser pulse train. The laser pulse train has a target number of sub-pulses, and the stacking frequency of the laser pulse train is the target stacking frequency. The bandwidth of the terahertz radiation generated by the electron beam obtained through the laser pulse train with the target number of sub-pulses is the target bandwidth, and the target stacking frequency is the center of the target tuning range.

[0061] In this embodiment, the target bandwidth and target tuning range can be set by those skilled in the art according to actual needs. Since the number of sub-pulses in the initial incident laser pulse train of the photocathode electron gun is related to the bandwidth of the terahertz radiation generated by the electron beam based on the laser pulse train, the target bandwidth can be adjusted by changing the number of sub-pulses in the laser pulse train. The stacking frequency of the initial incident laser pulse train of the photocathode electron gun (i.e., the repetition frequency of the sub-pulses in the laser pulse train) is equal to the center of the tuning range of the terahertz radiation generated by the electron beam based on the laser pulse train; therefore, the target tuning range can be adjusted by changing the stacking frequency of the laser pulse train.

[0062] For example, the infrared laser pulses can be generated by the laser 180, the infrared laser pulses can be incident on the frequency doubling module, the infrared laser pulses can be converted into ultraviolet laser pulses by frequency doubling, the ultraviolet laser pulses can be incident on the stacking module, and the ultraviolet laser pulses can be stacked into a laser pulse train by stacking. The laser pulse train includes a plurality of sub-pulses, and each of the sub-pulses is an ultraviolet laser pulse. Since the number of sub-pulses is related to the bandwidth of the terahertz radiation, the more the number of sub-pulses, the narrower the bandwidth of the terahertz radiation, and therefore, the number of sub-pulses can be changed by changing the structure of the stacking module to control the bandwidth of the terahertz radiation. For example, when the ultraviolet laser pulses are stacked by the α-BBO crystal, the number of sub-pulses can be changed by changing the number of α-BBO crystals, and therefore, the bandwidth of the terahertz radiation can be changed. Since the stacking frequency of the laser pulse train is the initial frequency of the density modulation of the electron beam, and the initial frequency of the density modulation of the electron beam is the center of the tuning range of the terahertz radiation, the repetition frequency of the sub-pulses in the laser pulse train can be adjusted by changing the stacking scheme, and therefore, the stacking frequency of the laser pulse train and the center of the tuning range of the terahertz radiation can be changed. For example, when the ultraviolet laser pulses are stacked by the α-BBO crystal, the repetition frequency of the sub-pulses can be changed by changing the thickness of the α-BBO crystal, and the repetition frequency of the sub-pulses can also be changed by pulse splitting stacking, chirped pulse beating, and the like.

[0063] After the laser pulse train is obtained, the laser pulse train can be transmitted to the photocathode of the photocathode electron gun without diffraction by the laser transmission system 190. The laser transmission system can be composed of a pair of image transfer lenses, and the laser pulse train can be reflected to the photocathode of the photocathode electron gun through the electromotive mirror frame of the laser incidence cavity on the beam pipe. The feeding microwave in the photocathode electron gun 110 and the klystron in the accelerating tube 120 can be controlled to be locked with the laser 180 and to ensure the time synchronization of the laser and the electron beam.

[0064] For example, the laser 180 can be an 800 nm Ti:Sapphire femtosecond mode-locked laser, the laser includes an oscillator and a regenerative amplifier, the repetition frequency of the laser pulses is 10 Hz, the bandwidth of the laser is 35 nm, and the highest energy of the generated infrared single pulse is 20 mJ. The laser 180 can generate infrared laser pulses with a wavelength of 800 nm, and the infrared laser pulses can be converted into ultraviolet laser pulses with a wavelength of 266 nm by tripling after the infrared laser pulses are incident on the stacking module. The ultraviolet laser pulses can be stacked to form a laser pulse train including 16 sub-pulses, and the interval between each of the sub-pulses is 0.5 ps, that is, the stacking frequency of the laser pulse train is 2 THz.

[0065] The system for generating terahertz radiation provided in the embodiments of the present application can obtain terahertz radiation with a target bandwidth and a target tuning range by adjusting the number of sub-pulses in the laser pulse train and the repetition frequency of the sub-pulses. Therefore, the system can generate terahertz radiation with a narrow band and tunable in the range of 1-10 THz, and improve the quality of the terahertz radiation.

[0066] In one embodiment, the frequency doubling module includes a first BBO crystal and a second BBO crystal, and the stacking module includes at least one α-BBO crystal. The first BBO crystal is used to double the frequency of the infrared laser pulse to obtain intermediate laser pulses formed by a first part of the infrared laser pulse and a second part of the infrared laser pulse, and the frequency of the intermediate laser pulses is twice the frequency of the infrared laser pulse. The second BBO crystal is used to sum the infrared laser pulse and the intermediate laser pulse to obtain ultraviolet laser pulses, and the frequency of the ultraviolet laser pulses is three times the frequency of the infrared laser pulse. The α-BBO crystal is used to stack the ultraviolet laser pulses to obtain the laser pulse train.

[0067] In the embodiments of the present application, the BBO crystal refers to a barium borate crystal, and the α-BBO crystal refers to a crystal form of the barium borate crystal. The infrared laser pulse first enters the first BBO crystal, and the frequency doubling is performed in the first BBO crystal to generate intermediate laser pulses with a frequency twice that of the infrared laser pulse. Due to the limitation of the frequency doubling efficiency of the BBO crystal, only a part of the infrared laser pulse (the first part of the infrared laser pulse) is doubled to the intermediate laser pulse after passing through the first BBO crystal. The other part of the infrared laser pulse (the second part of the infrared laser pulse) enters the second BBO crystal together with the intermediate laser pulse, and the sum frequency is generated in the second BBO crystal to generate ultraviolet laser pulses with a frequency three times that of the infrared laser pulse. The ultraviolet laser pulses further enter the α-BBO crystal, and the stacking is performed in the α-BBO crystal to form the laser pulse train.

[0068] For example, four α-BBO crystals can be provided, and the thickness of each crystal is halved in turn, that is, the ultraviolet laser pulses enter the thickest first α-BBO crystal, the second α-BBO crystal with a thickness half of the first α-BBO crystal, the third α-BBO crystal with a thickness half of the second α-BBO crystal, and the fourth α-BBO crystal with a thickness half of the third α-BBO crystal in turn to form a laser pulse train with 16 sub-pulses. For example, the thickness of each α-BBO crystal can be 4.72 mm, 2.36 mm, 1.18 mm, and 0.59 mm, and the stacking frequency of the generated laser pulse train is 2 THz. Different numbers of α-BBO crystals can be provided to change the number of sub-pulses, and different thicknesses of α-BBO crystals can be provided to change the repetition frequency of the sub-pulses.

[0069] The system for generating terahertz radiation provided by the embodiments of the present application adjusts the number of sub-pulses and the repetition frequency of the sub-pulses in the laser pulse train through the BBO crystal and the α-BBO crystal, and obtains terahertz radiation with a target bandwidth and a target tuning range. Therefore, terahertz radiation with a narrow band and tunable in the range of 1-10 THz can be generated, and the quality of the terahertz radiation is improved.

[0070] In one embodiment, the energy of the terahertz radiation is a target energy, the frequency doubling module further includes a grating pair, and the stacking module further includes an aperture, the grating pair is used to adjust the pulse width of the infrared laser pulse to a target pulse width before the infrared laser pulse enters the first BBO crystal, so that the pulse width of the laser pulse train is also the target pulse width. The aperture is used to adjust the spot size of the laser pulse train to a target spot size. The energy of the terahertz radiation generated by the electron beam obtained through the laser pulse train with the target pulse width and the target spot size is the target energy.

[0071] In the embodiments of the present application, the target energy can be set by a person skilled in the art according to the requirements. Generally, the higher the target energy is, the better the properties of the terahertz radiation are. Before the infrared laser pulse enters the first BBO crystal, the infrared laser pulse can be compressed or widened through the dispersion effect of the grating pair, so that the pulse width of the infrared laser pulse is adjusted to the target pulse width, and the pulse width of the subsequent ultraviolet laser pulse converted from the infrared laser pulse and the sub-pulse of the laser pulse train stacked from the ultraviolet laser pulse are also the target pulse width.

[0072] When the ultraviolet laser pulses are stacked, an aperture can be arranged at the object point of the optical path to adjust the spot size of the laser pulse train in the transverse direction to a target spot size.

[0073] The pulse width of the sub-pulse and the spot size of the laser pulse train will affect the nonlinear space charge oscillation process of the electron beam and the amplitude of the space charge oscillation, and further affect the density bunching factor of the electron beam at the radiation device 170. The density bunching factor is related to the coherence and energy of the terahertz radiation, and the terahertz radiation with a specific coherence and a target energy can be obtained by adjusting the pulse width of the sub-pulse to the target pulse width and adjusting the spot size of the laser pulse train to the target spot size.

[0074] Further, the field strength of the solenoid in the photocathode electron gun 110 and the field strength of the first focusing solenoid 130 and the second focusing solenoid 140 can also be set to a target field strength. The field strength of the solenoid will also affect the density bunching factor of the electron beam, and the terahertz radiation with better coherence and higher energy can be obtained by setting the field strength of the solenoid to the target field strength.

[0075] The system for generating terahertz radiation provided in the embodiments of the present application can obtain terahertz radiation with a target energy by adjusting the pulse width of the sub-pulse in the laser pulse train and the spot size of the laser pulse train. Therefore, terahertz radiation with high brightness and good coherence can be generated, and the quality of the terahertz radiation is improved.

[0076] In one embodiment, the frequency of the terahertz radiation is a target frequency, and the system further includes a phase shifter configured to adjust an acceleration phase of the electron beam in the acceleration tube 120 to a target acceleration phase, so that the frequency of the terahertz radiation generated by the electron beam with the target acceleration phase reaches the target frequency.

[0077] In the embodiments of the present application, the target frequency can be set by a person skilled in the art according to actual needs, and the acceleration phase of the electron beam can be changed by the phase shifter to adjust the frequency of the terahertz radiation generated finally, so that the terahertz wave is continuously tuned.

[0078] The acceleration phase of the electron beam in the acceleration tube 120 can be adjusted by the phase shifter. The energy chirp and the bunch length of the electron beam with different acceleration phases at the outlet of the acceleration tube 120 are also different. The electron beam with positive energy chirp is compressed in the longitudinal direction after passing through the dispersion section 160, and the electron beam with negative energy chirp is widened in the longitudinal direction after passing through the dispersion section 160, further changing the bunch length of the electron beam. The bunch length directly affects the density modulation period of the electron beam, and the density modulation period is equivalent to the frequency of the terahertz radiation generated by the electron beam, so the bunch length of the electron beam can be changed by changing the acceleration phase, and then the frequency of the terahertz radiation generated by the electron beam is changed.

[0079] If the radiation device 170 is a device related to the resonance condition and the energy of the electron beam (for example, an undulator), the energy of the acceleration tube 120 and the acceleration phase of the electron beam in the acceleration tube 120 can also be adjusted by the attenuator and the phase shifter to change the energy of the electron beam. When passing through the radiation device related to the resonance condition and the energy of the electron beam, the energy of the electron beam will affect the resonance wavelength of the terahertz radiation, so the frequency of the terahertz radiation generated by the electron beam can also be changed, so that the frequency of the terahertz radiation reaches the target frequency.

[0080] The system for generating terahertz radiation provided in the embodiments of the present application can change the acceleration phase of the electron beam by the phase shifter, and obtain terahertz radiation with a target frequency, so that the terahertz radiation can be continuously tuned in a tuning range.

[0081] In one embodiment, as Figure 2As shown, the terahertz radiation generation system 10 further comprises a beam diagnosis module 210, which comprises an S-band deflection cavity, an analyzing magnet and two beam profile viewers. The S-band deflection cavity is used to associate the longitudinal profile of the electron beam with the transverse profile of the electron beam. The analyzing magnet is used to associate the energy profile of the electron beam with the transverse profile of the electron beam. The beam profile viewers are used to observe the transverse profile of the electron beam, and obtain the longitudinal profile of the electron beam and the energy profile of the electron beam according to the transverse profile of the electron beam.

[0082] In the embodiments of the present application, the beam diagnosis module 210 can be arranged after the radiation device 170, and is used to detect the quality of the electron beam. The S-band deflection cavity is a microwave deflection cavity operating in the S-band, and can separate the electrons at different positions in the longitudinal direction (the longitudinal direction is the propagation direction of the electron beam) of the electron beam bunch, and form the distribution of the electrons in the transverse direction (the transverse direction is the direction perpendicular to the propagation direction of the electron beam), so that the longitudinal profile of the electron beam can be determined by observing the transverse profile of the electron beam. The analyzing magnet can separate the electrons with different energies, so that the electrons with different energies form the distribution in the transverse direction, so that the energy profile of the electron beam can be determined by observing the transverse profile of the electron beam. The beam profile viewers can comprise pulling devices, YAG (yttrium aluminum garnet) screens and cameras, and the two beam profile viewers are respectively used to observe the electron beam processed by the S-band deflection cavity and the electron beam processed by the analyzing magnet, so as to obtain the longitudinal profile of the electron beam and the energy profile of the electron beam according to the observed transverse profile of the electron beam.

[0083] The terahertz radiation generation system provided by the embodiments of the present application associates the longitudinal profile of the electron beam with the transverse profile of the electron beam by the S-band deflection cavity, associates the energy profile of the electron beam with the transverse profile of the electron beam by the analyzing magnet, and observes the longitudinal profile of the electron beam and the energy profile of the electron beam by the beam diagnosis device, so as to determine the properties of the electron beam, and more accurately adjust the parameters in the system according to the properties of the electron beam.

[0084] In one embodiment, the terahertz radiation generation system 10 further comprises a terahertz detector 220, which comprises a Michelson interferometer, a first Golay detector and a second Golay detector. The Michelson interferometer is used to split the terahertz radiation. The first Golay detector is located at the outlet of the Michelson interferometer, and is used to measure the autocorrelation spectrum of the terahertz radiation according to the split terahertz radiation. The second Golay detector is used to measure the energy of the terahertz radiation.

[0085] In the embodiments of the present application, the terahertz detector 220 can be arranged after the radiation device 170, and is used to detect the quality of the terahertz radiation. The Michelson interferometer is used to divide the terahertz radiation into two beams. The first Golay detector is arranged at the outlet of the Michelson interferometer, and can measure the autocorrelation of the terahertz radiation according to the terahertz radiation divided into two beams. The second Golay detector can be arranged at the outlet of the radiation device 170, and is used to detect the energy of the terahertz radiation.

[0086] The terahertz radiation generation system provided by the embodiments of the present application can detect the properties of the terahertz radiation by the terahertz detector, and can more accurately adjust the parameters in the system according to the properties of the terahertz radiation.

[0087] In one embodiment, as shown in Figure 3 A terahertz radiation generation method is provided, and the method is applied to a terahertz radiation generation system. The terahertz radiation generation system is the terahertz radiation generation system in any of the preceding embodiments. The method includes the following steps:

[0088] In step 302, the laser pulse train is received by the photocathode electron gun, and the electrons generated by the laser pulse train bombarding the photocathode of the photocathode electron gun are obtained to obtain an electron beam.

[0089] In step 304, the charge density of the electron beam is increased by the first focusing solenoid, so that the electron beam oscillates for half a plasma period before the outlet of the acceleration tube.

[0090] In step 306, the electron beam is accelerated to a relativistic energy by the acceleration tube, and the charge density of the electron beam is adjusted by the second focusing solenoid to obtain an electron beam with a target charge density, so that the electron beam with the target charge density is affected by its own space charge force to obtain an energy-modulated electron beam.

[0091] In step 308, the energy-modulated electron beam is transmitted by the beamline tube, and the energy-modulated electron beam is converted into a density-modulated electron beam by the dispersive section.

[0092] In step 310, the density-modulated electron beam is received by the radiation device, and the terahertz radiation is obtained by the coherent radiation generated by the density-modulated electron beam when passing through the radiation device.

[0093] In the embodiments of the present application, the laser pulse train incident on the system can be selected by those skilled in the art according to actual needs. The properties of the laser pulse train will affect the properties of the generated electron beam, and further affect the properties of the terahertz radiation generated by the electron beam. The laser pulse train incident can be selected according to the energy, frequency, bandwidth, etc. required by the terahertz radiation.

[0094] The laser pulse train is directly transmitted to the photocathode of the photocathode electron gun, and the laser pulse train generates electrons by bombarding the photocathode, and the electrons form an electron beam under the action of a microwave field in the photocathode electron gun. After the electron beam leaves the photocathode electron gun, it enters a first focusing solenoid, which focuses the electron beam to increase the charge density of the electron beam so that the electron beam can oscillate for half a plasma period before exiting the accelerating tube. Meanwhile, the first focusing solenoid can also maintain the emittance of the electron beam by laterally focusing the electron beam. Subsequently, the electron beam enters the accelerating tube, which increases the energy of the electron beam to relativistic energy, and the second focusing solenoid increases the charge density of the electron beam to the target charge density by focusing the electron beam, so that the electron beam can be affected by the space charge force under the target charge density, and the electron beam with the best energy modulation is obtained. The value of the target charge density can be determined by a person skilled in the art according to actual needs. After the electron beam exits the accelerating tube, it enters the beamline tube. When the electron beam is transmitted in the beamline tube, the electron beam oscillating for half a plasma period can generate a current spike, and the space charge force generated by the current spike can cause the energy distribution of the electron beam to periodically change in the longitudinal direction, that is, an energy-modulated electron beam is formed. The energy-modulated electron beam passes through the dispersion section, and due to the influence of the dispersion section, the energy-modulated electron beam appears spatially concentrated, and forms a density spike in the longitudinal distribution, that is, a density-modulated electron beam is formed. The density-modulated electron beam passes through the radiation device to generate coherent transition radiation (i.e., terahertz radiation) with a wavelength in the terahertz band, and the terahertz radiation can be subsequently collected by a terahertz radiation collection device.

[0095] The method for generating terahertz radiation provided by the embodiments of the present application increases the charge density of the electron beam to the target charge density through the first focusing solenoid and the second focusing solenoid, so that the electron beam oscillates for half a plasma period at the exit of the accelerating tube to form a current spike, and the space charge force generated by the current spike modulates the energy of the electron beam to form an energy-modulated electron beam. Then, the energy-modulated electron beam is converted into a density-modulated electron beam by the dispersion section, and the density-modulated electron beam generates terahertz radiation by the radiation device. Since the energy of the electron beam is modulated by the space charge force generated by the electron beam itself, there is no need for complex beamline design, and there is no requirement for the repetition frequency of the beamline, which can be applied to general linear accelerators and can be extended to high-repetition-frequency beamlines, thereby reducing the demand for accelerators.

[0096] In one embodiment, as shown in Figure 4 the terahertz radiation has a target bandwidth and a target tuning range, and the method further includes:

[0097] Step 402: generating an infrared laser pulse by a laser;

[0098] Step 404, converting the infrared laser pulse into an ultraviolet laser pulse through a frequency doubling module;

[0099] Step 406, stacking the ultraviolet laser pulse into a laser pulse train with a target number of sub-pulses and a target accumulation frequency through a stacking module, wherein the target accumulation frequency is the center of a target tuning range, and the target bandwidth of the terahertz radiation generated by the electron beam obtained through the laser pulse train with the target number of sub-pulses is the target bandwidth.

[0100] In the embodiments of the present application, the infrared laser pulse can be generated by a laser, and the infrared laser pulse can be converted into an ultraviolet laser pulse through a stacking module, and then the ultraviolet laser pulse can be stacked into a laser pulse train with a target number of sub-pulses and a target accumulation frequency to obtain terahertz radiation with a target bandwidth and a target tuning range. The specific conversion of the infrared laser pulse into the ultraviolet laser pulse and the stacking of the ultraviolet laser pulse can be referred to the related description of the foregoing embodiments, which will not be described herein again.

[0101] The method for generating terahertz radiation provided in the embodiments of the present application can obtain terahertz radiation with a target bandwidth and a target tuning range by adjusting the number of sub-pulses in the laser pulse train and the repetition frequency of the sub-pulses. Therefore, the terahertz radiation with a narrow band and a tunable range of 1-10 THz can be generated, and the quality of the terahertz radiation is improved.

[0102] In one embodiment, the frequency of the terahertz radiation is a target frequency, and the above method further comprises:

[0103] The acceleration phase of the electron beam in the acceleration tube is adjusted to a target acceleration phase through a phase shifter, so that the terahertz radiation generated by the electron beam with the target acceleration phase has a target frequency.

[0104] In the embodiments of the present application, the acceleration phase of the electron beam in the acceleration tube can be adjusted through a phase shifter, so that the electron beam with the target acceleration phase can have a specific energy chirp and form an electron beam bunch with a specific length after passing through the dispersion section. The length of the electron beam bunch directly affects the density modulation frequency of the electron beam, so that the terahertz radiation with the target frequency can be obtained by adjusting the acceleration phase of the electron beam in the acceleration tube. The specific adjustment of the acceleration phase can be referred to the related description of the foregoing embodiments, which will not be described herein again.

[0105] The method for generating terahertz radiation provided in the embodiments of the present application can obtain terahertz radiation with a target frequency by changing the acceleration phase of the electron beam through a phase shifter, and the terahertz radiation can be tuned in a tuning range.

[0106] It should be understood that, although the steps in the flowcharts involved in the embodiments described above are shown in sequence according to the arrows, the steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified herein, the execution of the steps is not strictly limited in sequence, and the steps can be executed in other sequences. Moreover, at least some of the steps in the flowcharts involved in the embodiments described above can include multiple steps or multiple stages, which are not necessarily executed at the same time, but can be executed at different times, and the execution sequence of the steps or stages is not necessarily sequential, but can be executed in rotation or alternation with at least some of the other steps or the steps or stages in the other steps.

[0107] The technical features of the above embodiments can be combined in any manner. For brevity, not all possible combinations of the technical features in the above embodiments are described, but as long as the combinations of the technical features do not contradict each other, they should be considered within the scope of the present disclosure.

[0108] The above embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the scope of the patent of the present application. It should be noted that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A system for generating terahertz radiation, characterized by, The system comprises a photocathode electron gun, an acceleration tube, a first focusing solenoid, a second focusing solenoid, a beam line tube, a dispersion section and a radiation device, wherein: The photocathode electron gun is configured to receive a laser pulse train and generate an electron beam through the laser pulse train; The acceleration tube is configured to accelerate the electron beam to a relativistic energy; The first focusing solenoid is located at an outlet of the photocathode electron gun and is configured to increase a charge density of the electron beam so that the electron beam oscillates for half a plasma period before an outlet of the acceleration tube; The second focusing solenoid surrounds the acceleration tube and is configured to adjust the charge density of the electron beam to a target charge density when the electron beam is accelerated in the acceleration tube, so that the electron beam is affected by its own space charge force at the target charge density to obtain an energy-modulated electron beam; The beam line tube is configured to transmit the energy-modulated electron beam; The dispersion section comprises an upper half dispersion section and a lower half dispersion section, and the beam line tube passes through between the upper half dispersion section and the lower half dispersion section, and is configured to convert the energy-modulated electron beam into a density-modulated electron beam when the energy-modulated electron beam is transmitted in the beam line tube; The radiation device is configured to receive the density-modulated electron beam and generate terahertz radiation through the density-modulated electron beam; and the radiation device is an aluminum film or an undulator assembled on a pulling device.

2. The system of claim 1, wherein, The terahertz radiation has a target bandwidth and a target tuning range, and the system further comprises a laser, a frequency doubling module and a stacking module, wherein: The laser is configured to generate an infrared laser pulse; The frequency doubling module is configured to convert the infrared laser pulse into an ultraviolet laser pulse; The stacking module is configured to stack the ultraviolet laser pulse to obtain the laser pulse train, the laser pulse train has a target number of sub-pulses, and the accumulation frequency of the laser pulse train is a target accumulation frequency, wherein the terahertz radiation generated by the electron beam obtained through the laser pulse train with the target number of sub-pulses has the target bandwidth, and the target accumulation frequency is the center of the target tuning range.

3. The system of claim 2, wherein, The frequency doubling module comprises a first barium borate (BBO) crystal and a second BBO crystal, and the stacking module comprises at least one α-BBO crystal, wherein: The first BBO crystal is configured to double the frequency of the infrared laser pulse to obtain an intermediate laser pulse formed by a first part of the infrared laser pulse and a second part of the infrared laser pulse, the second part being the laser pulse other than the first part, and the frequency of the intermediate laser pulse being twice the frequency of the infrared laser pulse; The second BBO crystal is configured to generate an ultraviolet laser pulse by frequency mixing the second part of the infrared laser pulse and the intermediate laser pulse, the frequency of the ultraviolet laser pulse being three times the frequency of the infrared laser pulse; The α-BBO crystal is configured to stack the ultraviolet laser pulse to obtain the laser pulse train.

4. The system of claim 3, wherein, The terahertz radiation has a target energy, the frequency doubling module further comprises a grating pair, and the stacking module further comprises an aperture, The grating pair is configured to adjust a pulse width of the infrared laser pulse to a target pulse width before the infrared laser pulse enters the first BBO crystal, so that the pulse width of the laser pulse train is also the target pulse width. The aperture is configured to adjust a spot size of the laser pulse train to a target spot size. The terahertz radiation has a target energy, the frequency doubling module further comprises a grating pair, and the stacking module further comprises an aperture, 5. The system of claim 1, wherein, The system further comprises a phase shifter, The phase shifter is configured to adjust an acceleration phase of the electron beam in the acceleration tube to a target acceleration phase, so that the terahertz radiation generated by the electron beam with the target acceleration phase has a target frequency.

6. The system of claim 1, wherein, The system further comprises a beam diagnosis module, the beam diagnosis module comprises an S-band deflection cavity, an analyzing magnet and two beam distribution observers, The S-band deflection cavity is configured to associate a longitudinal distribution of the electron beam with a transverse distribution of the electron beam. The analyzing magnet is configured to associate an energy distribution of the electron beam with the transverse distribution of the electron beam. The beam distribution observer is configured to observe the transverse distribution of the electron beam, and obtain the longitudinal distribution of the electron beam and the energy distribution of the electron beam according to the transverse distribution of the electron beam.

7. The system of claim 1, wherein, The system further comprises a terahertz detector, the terahertz detector comprises a Michelson interferometer, a first Golay detector and a second Golay detector, The Michelson interferometer is configured to split the terahertz radiation; The first Golay detector is located at an outlet of the Michelson interferometer, and is configured to measure an autocorrelation spectrum of the terahertz radiation according to the split terahertz radiation; The second Golay detector is configured to measure an energy of the terahertz radiation.

8. A method of generating terahertz radiation, characterized by, The method is applied to a terahertz radiation generation system, and the method comprises: An electron beam is obtained by receiving a laser pulse train by a photocathode electron gun and bombarding electrons generated at a photocathode of the photocathode electron gun by the laser pulse train; A first focusing solenoid is used to increase a charge density of the electron beam, so that the electron beam oscillates for half a plasma period before an outlet of an acceleration tube; The electron beam is accelerated to a relativistic energy by the acceleration tube, and a second focusing solenoid is used to adjust the charge density of the electron beam, so that the electron beam with a target charge density is obtained, so that the electron beam with the target charge density is affected by its own space charge force, and the energy-modulated electron beam is obtained; The energy-modulated electron beam is transmitted by a beamline tube, and the energy-modulated electron beam is converted into a density-modulated electron beam by the dispersive section. The density-modulated electron beam is received by a radiation device, and terahertz radiation is obtained by coherent radiation generated by the density-modulated electron beam when passing through the radiation device; the radiation device is an aluminum film or a waveguide installed on a pulling device. The system is the terahertz radiation generation system of any one of claims 1 to 7.

9. The method of claim 8, wherein, The terahertz radiation has a target bandwidth and a target tuning range, and the method further comprises: generating an infrared laser pulse by a laser; converting the infrared laser pulse into an ultraviolet laser pulse by a frequency doubling module; stacking the ultraviolet laser pulse into a laser pulse train with a target number of sub-pulses and a target accumulation frequency by a stacking module, wherein the terahertz radiation generated by the electron beam with the laser pulse train having the target number of sub-pulses has the target bandwidth, and the target accumulation frequency is the center of the target tuning range.

10. The method of claim 8, wherein, The terahertz radiation has a target frequency, and the method further comprises: adjusting the acceleration phase of the electron beam in the acceleration tube to a target acceleration phase by a phase shifter, so that the terahertz radiation generated by the electron beam with the target acceleration phase has the target frequency.

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