CN117954943A - Efficient Bessel terahertz radiation generating device and generating method - Google Patents
Efficient Bessel terahertz radiation generating device and generating method Download PDFInfo
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Abstract
The invention provides a high-efficiency Bessel terahertz radiation generating device, which comprises a linear accelerator, a laser electron beam modulation system and a terahertz radiation section which are sequentially arranged on a first axis, and a pulse widening and beam splitting system and an axicon which are positioned at the upstream of the laser electron beam modulation system and sequentially arranged on a second axis along a transmission path of femtosecond laser; the pulse widening and beam splitting system and the axicon form Bessel terahertz laser; the laser electron beam modulation system utilizes Bessel terahertz laser to carry out energy modulation on an electron beam provided by a linear accelerator to generate a longitudinal terahertz density modulated electron beam with transverse Bessel modulation; the magnetic field strength of the terahertz radiation section is adjusted so that the electron beam generates bessel terahertz radiation. The invention also provides a corresponding method. The radiation generating device and the radiation generating method not only improve the radiation efficiency of terahertz radiation generated based on free electron laser, but also improve the transmission distance of terahertz waves in free space.
Description
Technical Field
The invention belongs to the field of terahertz optical instruments, and particularly relates to a high-efficiency Bessel terahertz radiation generating device and a generating method.
Background
Terahertz waves refer to a segment of electromagnetic waves having a frequency between millimeter waves and infrared light. Because of the characteristics of wide band, low energy, high penetrability, transient state, high characteristic resolution and the like, the terahertz wave has important application value in the industrial fields of pharmacy, communication, astronomy, security check, medical imaging, quality detection and the like. In addition, the photon energy of the terahertz wave is matched with the fine energy structure of many molecular systems for exciting resonance, so the terahertz wave plays an important role in researches in many leading-edge scientific fields.
In recent years, terahertz generation technology based on linear accelerators has been rapidly developed. Terahertz generation methods based on advanced linac-free electron lasers have gradually become the dominant approach to generate strong-field terahertz radiation, which can generate terahertz radiation with continuously adjustable wavelength. However, the terahertz wave has long wavelength, the terahertz radiation diffraction phenomenon is obvious and the transverse diffusion is serious in the free space transmission process, which brings a plurality of difficulties to the scheme of generating the terahertz radiation based on the free electron laser: firstly, terahertz is limited by diffraction and diffusion effects in the amplifying process, and only a small part of terahertz light energy in a central area continuously interacts with an electron beam, so that the terahertz radiation amplifying efficiency is very limited and cannot be fully amplified; secondly, after the terahertz radiation is saturated and output, the terahertz radiation can be rapidly diffused, and in an optical system with a limited aperture, the terahertz radiation energy loss on the terahertz transmission line is huge. These difficulties present challenges to the leading edge scientific application of terahertz waves.
In order to solve the problems of low generation efficiency and large transmission loss of terahertz free electron laser, the problems of obvious terahertz radiation diffraction phenomenon and serious transverse diffusion are required to be solved, and the terahertz radiation can be continuously adjustable, so that no mature technology exists at present. The present invention therefore proposes to solve this problem by modulating the electron beam with an undiffracted beam-bessel beam.
Disclosure of Invention
The invention aims to provide a high-efficiency Bessel terahertz radiation generating device and a high-efficiency Bessel terahertz radiation generating method, which are used for generating coherent Bessel terahertz radiation with continuously adjustable wavelength and high pulse energy, so that the radiation efficiency of generating terahertz radiation based on free electron laser can be improved, and the transmission distance of terahertz waves in free space can be improved.
In order to achieve the above object, the present invention provides a high-efficiency bessel terahertz radiation generating apparatus, including a linear accelerator, a laser electron beam modulation system and a terahertz radiation section, which are sequentially arranged on a first axis, and a pulse widening and beam splitting system and axicon, which are positioned upstream of the laser electron beam modulation system and sequentially arranged on a second axis along a transmission path of a femtosecond laser; the pulse widening and beam splitting system and the axicon are used for forming Bessel terahertz laser; the laser electron beam modulation system is configured to perform energy modulation on an electron beam provided by the linear accelerator by using Bessel terahertz laser, and then generate a longitudinal terahertz density modulated electron beam with transverse Bessel modulation; the magnetic field strength of the terahertz radiation section is adjusted so that the electron beam generates bessel terahertz radiation.
The pulse stretching and beam splitting system comprises a pulse stretcher, a beam splitting sheet, an adjustable delayer, a first reflecting mirror, a second reflecting mirror and a beam combining sheet, wherein the pulse stretcher, the beam splitting sheet, the adjustable delayer and the first reflecting mirror are sequentially arranged on a third axis; the pulse stretching and beam splitting system stretches the femtosecond laser by using a pulse stretcher, and divides the femtosecond laser into a first laser beam and a second laser beam by using a beam splitting piece; the first laser beam passes through the adjustable delay device and the first reflecting mirror and then reaches the beam combining piece, and the second laser beam passes through the second reflecting mirror and then reaches the beam combining piece; the beam combining sheet combines the first laser beam and the second laser beam into beat frequency laser beams; the axicon is configured to receive the beat frequency laser beam such that wave vectors of the beat frequency laser beam are rotationally symmetrically distributed on a conical surface of a cone and have the same included angle as an initial propagation direction of the femtosecond laser to form the Bessel terahertz laser.
The bottom surface radius of the axicon is set to 3-30 mm, the base angle is set to 0.5-5 degrees, and the thickness is set to 3-10 mm.
The laser electron beam modulation system comprises a modulation undulator and a dispersion section, wherein the modulation undulator is used for modulating the transverse and longitudinal distribution of the electron beam in the modulation undulator by utilizing Bessel terahertz laser so as to form transverse Bessel modulation and longitudinal terahertz energy modulation in the electron beam, and the dispersion section is used for converting the longitudinal energy modulation of the electron beam into the longitudinal terahertz density modulation electron beam with the transverse Bessel modulation.
The laser electron beam modulation system further comprises a laser injection system, and the laser electron beam modulation system comprises two first deflection magnets on a first optical axis, a laser reflector between the two first deflection magnets and two second deflection magnets deviating from the first optical axis, wherein the laser reflector is used for reflecting Bessel terahertz laser light onto the first optical axis.
The linear accelerator adopts a photocathode electron gun and comprises an S wave band, an X wave band and a C wave band; the linear accelerator is a low-energy accelerator with energy of 50 to 150 megaelectron volts or a high-energy accelerator with energy of 0.5GeV to 1.5GeV which is continuously adjustable; the cluster length of the linac can be adjusted by adjusting the phase of the X-band.
The period of the modulating undulator is 3 cm when the linac is a low energy accelerator and 8 cm when the linac is a high energy accelerator.
The terahertz radiation section is provided with 2 radiation undulators with the length of 5 meters; the undulator period was set to 6 cm when the linac was a low energy accelerator and to 20 cm when the linac was a high energy accelerator.
The femtosecond laser is an ultrafast laser pulse with the pulse length of 30 to 200 femtoseconds, the pulse energy of 50 to 1 millijoule and the wavelength of 800 nanometers.
In another aspect, the present invention provides a method for generating a high-efficiency bessel terahertz radiation generating apparatus, including:
S1: providing a high efficiency bessel terahertz radiation generating apparatus as described above;
S2: the pulse widening and beam splitting system and the axicon form Bezier terahertz laser by using femtosecond laser, the Bezier terahertz laser and an electron beam provided by a linear accelerator are injected into a laser electron beam modulation system together, so that the laser electron beam modulation system utilizes the Bezier terahertz laser to carry out energy modulation on the transverse and longitudinal distribution of the electron beam, and then a longitudinal terahertz density modulated electron beam with transverse Bezier modulation is generated;
s3: passing the electron beam with transverse Bessel modulation and longitudinal terahertz density modulation through a terahertz radiation section, and adjusting the magnetic field intensity of the terahertz radiation section to enable the electron beam to generate Bessel terahertz radiation, wherein the Bessel terahertz radiation has a diffraction-free characteristic.
Therefore, the high-efficiency Bessel terahertz radiation generating device provided by the invention not only improves the radiation efficiency in the terahertz free electron laser amplifying process, but also increases the transmission distance of the generated terahertz radiation, and solves the key problem in the terahertz free electron laser. Compared with other schemes, the device provided by the invention has the advantages of high electron beam utilization rate, high radiation brightness, large terahertz radiation frequency adjusting range, simplicity in adjustment, narrow spectral bandwidth, long transmission distance and the like.
Drawings
Fig. 1 is a schematic structural view of a high-efficiency bessel terahertz radiation generating apparatus of the present invention.
Fig. 2 is a schematic structural diagram of a pulse stretching and beam splitting system of the high-efficiency bessel terahertz radiation generating apparatus of the present invention.
Fig. 3 is a schematic structural view of an axicon of the high-efficiency bessel terahertz radiation generating apparatus of the present invention.
Fig. 4 is a schematic structural diagram of a laser electron beam modulation system of the high-efficiency bessel terahertz radiation generating apparatus of the present invention.
Detailed Description
The invention is further illustrated in the following examples and figures, which should not be taken to limit the scope of the invention.
Fig. 1 is a diagram showing a connection relationship of a high-efficiency bessel terahertz radiation generating apparatus according to one embodiment of the invention. Fig. 2 is a schematic structural diagram of a pulse stretching and beam splitting system of the high-efficiency bessel terahertz radiation generating apparatus shown in fig. 1, fig. 3 is a schematic structural diagram of an axicon of the pulse stretching and beam splitting system of the high-efficiency bessel terahertz radiation generating apparatus shown in fig. 1, and fig. 4 is a schematic structural diagram of a laser electron beam modulating system of the high-efficiency bessel terahertz radiation generating apparatus shown in fig. 1.
As shown in fig. 1, the high-efficiency bessel terahertz radiation generating apparatus of the present invention includes a linear accelerator 1, a laser electron beam modulation system 2, and a terahertz radiation section 3, which are sequentially arranged on a first axis (i.e., along an electron beam transmission axis), and a pulse widening and splitting system 5 and axicon 6, which are located upstream of the laser electron beam modulation system 2 and sequentially arranged on a second axis along a transmission path of a femtosecond laser 4. The second axis is perpendicular to the first axis and is positioned on the same horizontal plane.
Specifically, the femtosecond laser 4 is an ultrafast laser pulse with a pulse length of 30 to 200 femtoseconds, a pulse energy of 50 to 1 millijoule and a wavelength of 800 nanometers. In the present embodiment, the pulse length of the femtosecond laser 4 is set to 30 femtoseconds, and the pulse energy is set to 100 microjoules.
The linac 1 is arranged to provide a relativistic electron beam. As shown in fig. 1, the linac 1 employs a photocathode electron gun and includes accelerator segments of different acceleration frequency bands, i.e., S-band, X-band, and C-band. In the present embodiment, the linac 1 employs a high-energy accelerator in which the energy of the electron beam is continuously adjustable from 0.5GeV to 1.5GeV, and the beam cluster length of the electron beam provided by the linac 1 can be continuously adjustable from 200 femtoseconds to 2 picoseconds by adjusting the output phase of the accelerator section of the X-band.
In other embodiments, the linac 1 may be either a low energy accelerator with energy in the range of 50 to 150 mev, or a continuously variable high energy accelerator with energy in the range of 0.5GeV to 1.5 GeV.
When the output phase of the accelerator section of the X wave band is regulated, the acceleration field of the electron beam is different when passing through the acceleration structure of the X wave band, so that different energy chirps are introduced into the energy space of the electron beam, and when the electron beam with different energy chirps passes through the same magnetic compression section, the electron beam group is compressed to different beam group lengths. The energy chirp of the electron beam is achieved by adjusting the output phase of the accelerator section of the X-band of the linac 1.
As shown in fig. 2, the pulse stretching and splitting system 5 includes a pulse stretcher 51, a beam splitter 52, an adjustable retarder 54, and a first mirror 55 sequentially arranged on a third axis, and a second mirror 53 and a beam combiner 56 downstream of the beam splitter 52 and arranged on a fourth axis.
Thus, the pulse stretching and splitting system 5 is arranged to receive the femtosecond laser 4 and to generate the beat laser beam 44. Specifically, the pulse stretching and beam splitting system 5 stretches the femtosecond laser 4 by using a pulse stretcher 51, and splits the stretched femtosecond laser 41 into a first laser beam 42 and a second laser beam 43 by using a beam splitting piece 52; the first laser beam 42 passes through the adjustable delay 54 and the first reflecting mirror 55 to reach the beam combining sheet 56, and the second laser beam 43 passes through the second reflecting mirror 53 to reach the beam combining sheet 56; the beam combining sheet 56 combines the first laser beam 42 and the second laser beam 43 into a beat laser beam 44.
The beat frequency laser beam 44 includes a signal in a terahertz wave band, and the beat frequency of the signal in the terahertz wave band is:
Where μ is the laser chirp coefficient and τ is the optical pulse delay.
Accordingly, the beat frequency of the signal of the terahertz wave band can be adjusted by changing the pulse interval and the energy chirp of the first laser beam 42 and the second laser beam 43; the energy chirp of the first laser beam 42 and the second laser beam 43 is achieved by adjusting the stretching multiple of the pulse stretcher 51 (note: here the energy chirp is related to two factors: 1, the initial pulse length of the ultrafast laser, 2, the stretching multiple after stretching the laser), and the pulse spacing of the first laser beam 42 and the second laser beam 43 is achieved by adjusting the adjustable delay 54.
As shown in fig. 3, the axicon 6 is located between the pulse stretching and beam splitting system 5 and the electron beam modulating system 2, and the axicon 6 is configured to receive the beat frequency laser beam 44 output by the pulse stretching and beam splitting system 5, so that the wave vectors of the emitted beat frequency laser beam 44 are rotationally symmetrically distributed on the conical surface of a cone and have the same included angle with the initial propagation direction of the femto-second laser 4, and therefore the emitted waves on the conical surface are superimposed in a coherent manner, so as to form the bessel terahertz laser. The initial propagation direction of the femtosecond laser 4 refers to the propagation direction when the femtosecond laser 4 is incident on the axicon 6, i.e., the upward arrow direction in fig. 1 and the rightward direction in fig. 3.
The ability to generate a bessel beam is required to satisfy: firstly, the beat frequency laser beam 44 which is required to be incident is a plane wave or can be similar to the plane wave, the femto-second laser 4 meets the requirement, and the beat frequency laser beam 44 after beat frequency can also meet the requirement; secondly, the radius of the bottom surface of the axicon 6 needs to be larger than the spot size of the beat frequency laser beam 44, and the laser size is in the order of hundreds of micrometers and is generally far smaller than the radius of the bottom surface; thirdly, the beat frequency laser beam 44 is perpendicularly incident on the axicon 6, and the straight line between the spot center of the axicon 6 and the vertex of the axicon 6 should be perpendicular to the incident plane of the axicon 6 as much as possible.
In this embodiment, the bottom radius of the axicon 6 is set to 3 to 30 mm, the base angle is set to 0.5 to 5 degrees, the thickness is set to 3 to 10 mm, and the specific size can be processed according to the actual requirement.
The optical field expression of the formed Bessel terahertz laser is as follows:
Wherein A is the amplitude of Bessel terahertz laser, k z、kρ is a longitudinal wave vector and a transverse wave vector respectively, J o is a zero-order Bessel function, z is a height axis coordinate taking the propagation direction of Bessel terahertz laser as a height axis in a cylindrical coordinate system, and ρ is the radial distance from an observation point to the height axis in the cylindrical coordinate system.
The maximum diffraction-free transmission distance z max of the Bessel terahertz laser is as follows:
where ω is the beam waist radius of the beat frequency laser beam 44, γ is the base angle of the axicon 6, and n is the refractive index of the axicon 6.
Therefore, the non-diffraction transmission distance can be adjusted by changing the refractive index and base angle of the axicon 6 and the beam waist radius of the terahertz beat laser 54, and a larger non-diffraction transmission distance can generate terahertz radiation more efficiently.
The measurement of the beam waist radius of the beat frequency laser beam 44 is the same as that of the conventional laser, and can be measured by a scanning pinhole method, a scanning knife edge method and the like, and the beam waist radius can be adjusted by adding and adjusting a focusing lens, and the value of the beam waist radius of the beat frequency laser beam 44 ranges from 50 to 300 micrometers. The base angle γ is a constant value for the determined axicon, specifically, the base angle of the axicon 6 is set to a value ranging from 0.5 to 5 degrees as described above in this embodiment, and a smaller base angle is selected for a larger diffraction-free distance, which means a larger processing difficulty. The refractive index n is determined by the material of the lens, and is typically 1.45 to 1.5 for ordinary optical glass, and a material having a smaller refractive index tends to be selected for a larger non-diffraction distance.
As shown in fig. 4, the laser electron beam modulation system 2 includes a laser injection system 21, a modulating undulator 22 having a length of 0.5m, and a dispersion section 23, which are sequentially arranged on a first optical axis.
Thus, the laser electron beam modulation system 2 is configured to perform energy modulation on the transverse and longitudinal distribution of the electron beam in the modulating undulator 22 using Bessel terahertz laser, form transverse Bessel modulation and longitudinal terahertz energy modulation in the electron beam, and convert the energy-modulated electron beam into a longitudinal terahertz density-modulated electron beam with transverse Bessel modulation.
The laser injection system 21 is a device that receives the bessel terahertz laser light, and includes two first deflection magnets 211 on a first optical axis, a laser mirror 212 between the two first deflection magnets 211, and two second deflection magnets 213 offset from the first optical axis, the laser mirror 212 being for reflecting the bessel terahertz laser light onto the first optical axis. Since the direction of the laser entering the modulating undulator 22 is the same as the propagation direction of the electron beam, the laser injection system 21 is required to temporarily deflect the electron beam horizontally from the original propagation direction by the first deflection magnet 211 and the second deflection magnet 213, and a laser mirror 212 is inserted in the space of the original propagation direction to reflect the bessel terahertz laser to the original propagation direction of the electron beam (i.e. on the first optical axis), after which the laser injection system 21 restores the propagation direction of the electron beam to the original propagation direction again, and at this time, the bessel terahertz laser and the electron beam can enter the modulating undulator 22 together to interact.
In this embodiment, the period of the modulating undulator 22 is set to 8 cm and the magnetic gap can be continuously adjustable between 10 mm and 40 mm.
In other embodiments, the period of the modulating undulator 22 is set to 3 cm when the linac 1 is set to a low energy accelerator and the period of the modulating undulator 22 is set to 8 cm when the linac 1 is set to a high energy accelerator.
Thereby, the laser electron beam modulation system 2 receives the electron beam from the linac 1 and the bessel terahertz laser light from the axicon 6, respectively, so that the bessel terahertz laser light enters the modulation undulator 22 to interact with the electron beam through the laser injection system 21. In this process, the transverse and longitudinal (time) positions of the electron beam and the Bessel terahertz laser in the modulating undulator 22 should coincide as much as possible, thereby forming both transverse Bessel modulation and longitudinal terahertz energy modulation in the electron beam. The modulating undulator 22 is operative to modulate the transverse and longitudinal distribution of the electron beam by interaction of the electron beam with the Bessel terahertz laser, thereby forming transverse Bessel modulation and longitudinal terahertz energy modulation in the electron beam.
The dispersive section 23 is arranged to convert the longitudinal energy modulation of the electron beam into a longitudinal terahertz density modulated electron beam with transverse Bessel modulation. The energy-modulated electron beam is processed through the dispersion section, and the intensity of the dispersion section is adjusted, so that the electron beam can form density modulation in the longitudinal direction while maintaining transverse Bessel modulation, the energy modulation can be converted into the density modulation, and a modulation signal can be amplified.
The invention can change the energy chirp and the pulse interval of the double laser pulse by adjusting the pulse stretcher 51 and the adjustable delayer 54, thereby adjusting the frequency of the electron beam density modulation. By changing axicon 6 of different parameters (base angle, refractive index) and adjusting the beam waist radius of beat laser beam 44, the diffraction-free transmission distance of the bessel terahertz laser can be optimized. Since the relativistic electron beam is used in the modulation process, the space charge effect has less influence on the terahertz structure, which ensures the feasibility of generating and maintaining the transverse Bessel modulation and the longitudinal terahertz modulation.
Referring again to fig. 1, the terahertz radiation section 3 is configured to receive an electron beam with transverse bezier modulation and longitudinal terahertz modulation output by the laser electron beam modulation system 2, where the magnetic field strength of the terahertz radiation section 3 is adjusted such that the electron beam generates bezier terahertz radiation. The principle of the generation of the efficient Bessel terahertz radiation is as follows: when the electron beam with transverse Bessel modulation and longitudinal terahertz modulation is periodically moved in the terahertz radiation section 3 (i.e., the undulator), and the resonance wavelength of the electron beam coincides with the longitudinal terahertz modulation wavelength, bessel terahertz radiation will be rapidly generated and will be gradually intensified during the movement.
The terahertz radiation section 3 is 2 radiation undulators with a length of 5 meters, and in this embodiment, the undulator period of the terahertz radiation section 3 is 20 cm. In other embodiments, the undulator period is set to 6 cm when the linac is a low energy accelerator and 20 cm when the linac is a high energy accelerator.
The resonance wavelength can be adjusted by adjusting the magnetic field strength of the terahertz radiation section 3. The relationship between the magnetic field intensity of the terahertz radiation section 3 and the resonance wavelength of the electron beam is shown in the following formula, and the resonance wavelength of the electron beam is:
Wherein lambda n is the resonance wavelength; k is a dimensionless parameter of the undulator and is proportional to the magnetic field strength B (in Tesla T) of the terahertz radiation section 3, with a specific relationship of K=0.934 lambda u [ cm ] Bt; gamma is a relativistic factor, n is the harmonic number, and lambda u is the period of the undulator.
Because the Bessel terahertz light field generated by radiation has the characteristics of diffraction-free transmission and self-repairability of the Bessel light field, diffraction and diffusion in the light field amplifying process can be greatly reduced, most of terahertz light can continuously interact with electron beams, the efficiency of terahertz radiation is further improved, rapid amplifying saturation is realized, and finally coherent Bessel terahertz radiation with continuously adjustable and high pulse energy can be generated. The terahertz light finally generated by the invention still has the characteristics of Bessel light beams, so that the transmission distance of the terahertz light can be increased, and the loss in the terahertz transmission process is reduced.
Based on the high-efficiency Bessel terahertz radiation generating device, the high-efficiency Bessel terahertz radiation generating method comprises the following steps of:
Step S1: providing a high efficiency bessel terahertz radiation generating apparatus as described above;
Wherein, the linear accelerator 1 is utilized to generate relativistic electron beams with energy of 0.5GeV to 1.5GeV which are continuously adjustable and pulse length of 200 femtoseconds to 2 picoseconds which are continuously adjustable; generating laser pulses with continuously adjustable pulse length between 30 femtoseconds and 200 femtoseconds by using the femtosecond laser 4 of the Gao Xiaotai Hz generating device; the femtosecond laser generates a beat frequency laser beam 44 with a terahertz signal through the pulse stretching and beam splitting system 5, and the frequency of the terahertz signal can be adjusted by adjusting the pulse interval and the energy chirp of the double-pulse laser in the pulse stretching and beam splitting system 5. The beat frequency laser beam 44 can generate Bessel terahertz laser light through the axicon 6. As described above, by changing the axicon 6 of different parameters (base angle, refractive index) and adjusting the beam waist radius of the beat laser beam 44, the diffraction-free transmission distance of the bessel terahertz laser can be optimized.
Step S2: the pulse widening and beam splitting system 5 and the axicon 6 form Bessel terahertz laser by using the femtosecond laser 4, the Bessel terahertz laser and the electron beam provided by the linear accelerator 1 are injected into the laser electron beam modulation system 2 together, so that the laser electron beam modulation system 2 utilizes the Bessel terahertz laser to perform energy modulation on the transverse and longitudinal distribution of the electron beam, and then the electron beam with transverse Bessel modulated longitudinal terahertz density modulation is generated;
The energy-modulated electron beam is processed through the dispersion section, and the intensity of the dispersion section is adjusted, so that the electron beam maintains transverse Bessel modulation and forms density modulation in the longitudinal direction.
Step S3: passing the electron beam with transverse Bessel modulation and longitudinal terahertz density modulation through a terahertz radiation section 3, and adjusting the magnetic field intensity of the terahertz radiation section 3 so that the electron beam generates Bessel terahertz radiation having a diffraction-free characteristic.
Therefore, the high-efficiency amplification and the rapid saturation can be realized, and finally, the coherent Bessel terahertz radiation with continuously adjustable and high pulse energy is generated. Because the Bessel beam has no diffractiveness, the method can improve the amplification efficiency in the free electron laser amplification process, and finally realize the rapid saturation; in addition, the Bessel terahertz radiation which is finally saturated and output still has the diffraction-free characteristic, so that the terahertz beam can be prevented from being rapidly diffused, and the transmission distance of the terahertz beam is increased.
The foregoing description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and various modifications can be made to the above-described embodiment of the present invention. All simple, equivalent changes and modifications made in accordance with the claims and the specification of the present application fall within the scope of the patent claims. The present invention is not described in detail in the conventional art.
Claims (10)
1. The high-efficiency Bessel terahertz radiation generating device is characterized by comprising a linear accelerator, a laser electron beam modulation system, a terahertz radiation section, a pulse widening and beam splitting system and an axicon, wherein the linear accelerator, the laser electron beam modulation system and the terahertz radiation section are sequentially arranged on a first axis;
The pulse widening and beam splitting system and the axicon are used for forming Bessel terahertz laser; the laser electron beam modulation system is configured to perform energy modulation on an electron beam provided by the linear accelerator by using Bessel terahertz laser, and then generate a longitudinal terahertz density modulated electron beam with transverse Bessel modulation; the magnetic field strength of the terahertz radiation section is adjusted so that the electron beam generates bessel terahertz radiation.
2. The efficient bessel terahertz radiation generating apparatus according to claim 1, wherein the pulse stretching and beam splitting system includes a pulse stretcher, a beam splitting sheet, an adjustable retarder, and a first mirror arranged in this order on a third axis, and a second mirror and a beam combining sheet downstream of the beam splitting sheet and arranged on a fourth axis;
the pulse stretching and beam splitting system stretches the femtosecond laser by using a pulse stretcher, and divides the femtosecond laser into a first laser beam and a second laser beam by using a beam splitting piece; the first laser beam passes through the adjustable delay device and the first reflecting mirror and then reaches the beam combining piece, and the second laser beam passes through the second reflecting mirror and then reaches the beam combining piece; the beam combining sheet combines the first laser beam and the second laser beam into beat frequency laser beams;
the axicon is configured to receive the beat frequency laser beam such that wave vectors of the beat frequency laser beam are rotationally symmetrically distributed on a conical surface of a cone and have the same included angle as an initial propagation direction of the femtosecond laser to form the Bessel terahertz laser.
3. The efficient bessel terahertz radiation generating apparatus according to claim 2, wherein the bottom surface radius of the axicon is set to 3 to 30 mm, the base angle is set to 0.5 to 5 degrees, and the thickness is set to 3 to 10 mm.
4. The efficient bessel terahertz radiation generating apparatus of claim 1, wherein the laser electron beam modulation system includes a modulation undulator and a dispersion section configured to modulate a transverse-longitudinal distribution of the electron beam in the modulation undulator with bessel terahertz laser light to form transverse bessel modulation and longitudinal terahertz energy modulation in the electron beam, and to convert the longitudinal energy modulation of the electron beam into a longitudinal terahertz density modulated electron beam with transverse bessel modulation with the dispersion section.
5. The high efficiency Bessel terahertz radiation generating apparatus of claim 4, wherein the laser electron beam modulation system further comprises a laser injection system comprising two first deflection magnets on a first optical axis and a laser mirror between the two first deflection magnets and two second deflection magnets offset from the first optical axis, the laser mirror being configured to reflect the Bessel terahertz laser light onto the first optical axis.
6. The efficient bessel terahertz radiation generating apparatus according to claim 4, wherein the linac employs a photocathode electron gun and includes S-band, X-band, and C-band; the linear accelerator is a low-energy accelerator with energy of 50 to 150 megaelectron volts or a high-energy accelerator with energy of 0.5GeV to 1.5GeV which is continuously adjustable; the cluster length of the linac can be adjusted by adjusting the phase of the X-band.
7. The high efficiency bessel terahertz radiation generating apparatus according to claim 6, wherein the period of the modulating undulator is 3cm when the linac is a low-energy accelerator, and the period of the modulating undulator is 8 cm when the linac is a high-energy accelerator.
8. The efficient bessel terahertz radiation generating apparatus as in claim 6, wherein the terahertz radiation section is 2 radiation undulators with a length of 5 meters; the undulator period was set to 6 cm when the linac was a low energy accelerator and to 20 cm when the linac was a high energy accelerator.
9. The apparatus of claim 1, wherein the femtosecond laser is an ultrafast laser pulse with a pulse length of 30 to 200 femtoseconds, a pulse energy of 50 to 1 millijoule, and a wavelength of 800 nanometers.
10. A method for generating efficient bessel terahertz radiation, comprising:
Step S1: providing a high-efficiency bessel terahertz radiation generating apparatus as set forth in one of claims 1-9;
Step S2: the pulse widening and beam splitting system and the axicon form Bezier terahertz laser by using femtosecond laser, the Bezier terahertz laser and an electron beam provided by a linear accelerator are injected into a laser electron beam modulation system together, so that the laser electron beam modulation system utilizes the Bezier terahertz laser to carry out energy modulation on the transverse and longitudinal distribution of the electron beam, and then a longitudinal terahertz density modulated electron beam with transverse Bezier modulation is generated;
step S3: passing the electron beam with transverse Bessel modulation and longitudinal terahertz density modulation through a terahertz radiation section, and adjusting the magnetic field intensity of the terahertz radiation section to enable the electron beam to generate Bessel terahertz radiation, wherein the Bessel terahertz radiation has a diffraction-free characteristic.
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