A method for implementing cylindrical grating coherent smith-purcell radiation enhancement

Abstract

The application discloses a kind of methods for realizing cylindrical grating coherent Smith-Purcell radiation enhancement, Smith-Purcell radiation is a kind of incoherent spontaneous radiation, the power of radiation is very low, not easy to detect and application, to realize high intensity Smith-Purcell radiation, the application proposes based on quasi-continuous domain bound state enhancement method.First, the electron bunch string is generated using the electron gun;Then, make the electron bunch string pass through the surface of cylindrical metal grating, the repetition frequency of electron bunch string is used as the resonance frequency of quasi-continuous domain bound state, to realize the enhancement of coherent Smith-Purcell super-radiation, and finally the electron enters the collecting electrode.The cylindrical grating coherent Smith-Purcell radiation enhancement method proposed in the application is simple to operate, and has obvious effect, easy integration, low cost and other characteristics.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for realizing cylindrical grating coherent Smith-Purcell radiation enhancement, and belongs to the field of BACKGROUND

[0002] Terahertz waves refer to electromagnetic waves with a frequency in the range of 0.1 to 10 THz (wavelength of 30 um to 3 mm), which are located between microwaves and infrared radiation, and have many unique properties different from other electromagnetic waves, such as strong penetration and low electron energy. With the development of science and technology, by utilizing these unique advantages, terahertz technology has shown broad application prospects in many important fields such as wireless communication and military radar, and has brought far-reaching influence on technological innovation and national reform. Terahertz radiation sources are the key to promoting the development of terahertz technology, however, the main problem in the research is the lack of compact, adjustable and high-power terahertz radiation sources, which limits the practical application of terahertz waves. At present, terahertz sources are mainly realized based on electronic and photonic technologies, among which vacuum electronic devices based on Smith-Purcell effect have very good development prospects. Smith-Purcell radiation is a kind of incoherent spontaneous radiation, and the radiation power is relatively low, which is not easy to detect and apply, which greatly limits the application of Smith-Purcell radiation sources in terahertz science and technology. SUMMARY

[0003] Technical problem: In view of the above prior art, a method for realizing cylindrical grating coherent Smith-Purcell radiation enhancement is proposed, which solves the problem of low incoherent Smith-Purcell radiation power and realizes high-intensity coherent Smith-Purcell super radiation, so as to provide a Smith-Purcell radiation source with simple structure and high efficiency.

[0004] Technical scheme: The present application is a method for realizing cylindrical grating coherent Smith-Purcell radiation enhancement, which is based on a structure comprising a cathode, a cylindrical metal grating and a collector; the cathode and the collector are placed on the same horizontal plane opposite to each other, and the cylindrical metal grating is placed between the cathode and the collector and has a height lower than the cathode.

[0005] The cathode generates a ring-shaped periodic electron cluster string, which sweeps over the top of the cylindrical metal grating and finally enters the collector; the cylindrical metal grating generates quasi-continuous domain bound states with a resonant frequency consistent with the repetition frequency of the ring-shaped periodic electron cluster string, thereby realizing the enhancement of coherent Smith-Purcell super radiation.

[0006] The cylindrical metal grating is driven by the ring-shaped periodic electron cluster string, and coherent Smith-Purcell radiation is generated through super radiation effect.

[0007] The enhancement of the coherent Smith-Purcell super radiation is achieved by using the cathode to generate a ring-shaped periodic electron bunch string, which passes through the surface of a cylindrical metal grating to generate coherent Smith-Purcell super radiation; the quasi-continuous domain bound state and the continuous domain bound state are generated by using the cylindrical metal grating, wherein the resonance frequency of the quasi-continuous domain bound state and the continuous domain bound state is consistent with the frequency of the periodic electron bunch string, and finally the enhancement of the coherent Smith-Purcell super radiation is realized.

[0008] The enhancement method mainly comprises the following steps:

[0009] Step 1. Electron gun, i.e. cathode, generates electron beam: The electron gun generates, accelerates and focuses a high-energy-density electron beam, and the generated electron beam is controllable and has certain energy, beam current, speed and angle;

[0010] Step 2. Based on super radiation: The continuous electron beam drives the cylindrical metal grating to further form an electron bunch string, and a super radiation effect is generated, which is a kind of coherent radiation and has a high power output at a fixed frequency, which is much larger than the output power of ordinary non-coherent Smith-Purcell radiation;

[0011] Step 3. Quasi-continuous domain bound state enhances Smith-Purcell radiation: In the Smith-Purcell radiation of the cylindrical metal grating, there are continuous domain bound states, and the high-Q resonance near the continuous domain bound states is a quasi-continuous domain bound state, which radiates energy to the outside, thereby providing an enhancement mechanism for the coherent Smith-Purcell super radiation;

[0012] Step 4. Generate high-efficiency coherent Smith-Purcell radiation: Use the periodic electron bunch string to drive the cylindrical metal grating supporting the quasi-continuous domain bound state, so that the resonance frequency of the quasi-continuous domain bound state of the cylindrical metal grating matches the repetition frequency of the periodic electron bunch string, and high-intensity coherent Smith-Purcell super radiation is realized; the matching refers to that the resonance frequency of the quasi-continuous domain bound state of the cylindrical metal grating is consistent with the repetition frequency or the high-order harmonic of the periodic electron bunch string, and the quasi-continuous domain bound state is used to enhance the Smith-Purcell super radiation intensity on the repetition frequency or the high-order harmonic of the periodic electron bunch string.

[0013] Wherein,

[0014] The step 1 comprises:

[0015] 1a, the electron gun and the collecting electrode are placed on the same horizontal plane, and the cylindrical metal grating is placed between the electron gun and the collecting electrode, and the height is lower than that of the electron gun;

[0016] 1b, the electron gun generates a ring-shaped periodic electron bunch string, the electron bunch string sweeps over the top of the cylindrical metal grating, and finally enters the collecting electrode;

[0017] 1c. The electron gun generates a periodic electron string with a repetition frequency of w under the drive of a laser pulse with a repetition frequency of w. The shape and structure of the electron string should remain stable during the movement. If the electron string diverges due to space charge force, the effect of beam-wave interaction will be greatly reduced. In order to stabilize the shape of the electron string, a uniform static magnetic field parallel to the direction of movement of the electron string is added to constrain the electron string.

[0018] 1d. The cylindrical metal grating avoids the problems of loss, heat dissipation, and charge accumulation that occur with dielectric materials in high-power devices.

[0019] Step 2 includes:

[0020] 2a. The ordinary incoherent Smith-Passel radiation refers to the electromagnetic wave radiated at a certain angle above a periodic grating when a charged particle passes through the surface of the grating, following the relation λ=d(1 / β-cosθ) / |n|, where d is the period of the grating, λ is the wavelength of the Smith-Passel radiation, β is the ratio of the electron velocity v to the light speed c, θ is the angle between the direction of electron motion and the direction of Smith-Passel radiation, and n is the order of the Smith-Passel radiation.

[0021] 2b. The superradiation effect refers to the fact that when the electron string (4) passes through the surface of the cylindrical metal grating (2), if the repetition frequency of the electron string or its higher harmonics are located in the radiation continuum, coherent Smith-Passel radiation can be excited at the repetition frequency of the upper electron string or its higher harmonics, and its intensity is much greater than that of incoherent Smith-Passel radiation.

[0022] Step 3 includes:

[0023] 3a. The continuous domain bound state, as a resonance with an infinite mass factor, is a bound state located in the continuous domain, which enhances the interaction between light and matter;

[0024] 3b. The quasi-continuous bound state, as a high-quality factor resonance located near the continuous bound state, radiates energy outward to enhance the intensity of Smith-Passel radiation.

[0025] Beneficial effects: The technical solution adopted in this invention has the following advantages:

[0026] 1. By adjusting and optimizing the grating structure parameters and electronic parameters, Smith-Passel radiation of the ideal frequency can be obtained;

[0027] 2. The generation of strongly coherent Smith-Passel radiation using the quasi-continuous domain bound state resonance is helpful for the development of terahertz science and technology.

[0028] 3. The structure can be extended to planar gratings and various metamaterial structures, providing a basis for the design of other structures. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The system schematic diagram of the present application is shown in Fig. 1.

[0030] Figure 2 The continuous domain bound state of the present application is shown in Fig. 2.

[0031] Figure 3 The E above the grating of the present application is shown in Fig. 3. z Time domain simulation diagram;

[0032] Figure 4 The radiation field E of the present application is shown in Fig. 4. z

[0033] The structure includes a cathode 1, a cylindrical metal grating 2, a collector 3, and a string of electron bunches 4. DETAILED DESCRIPTION

[0034] The specific embodiments of the present application are further described below in conjunction with the accompanying drawings.

[0035] Figure 1 The system schematic diagram of the present application is shown in Fig. 1. The structure mainly includes a cathode, a cylindrical metal grating, and a collector. The cathode and the collector are placed on the same horizontal plane, and the cylindrical metal grating is placed between the cathode and the collector and has a height lower than that of the cathode. The cathode generates a string of annular periodic electron bunches, which pass above the cylindrical metal grating and finally enter the collector. The cylindrical metal grating generates quasi-continuous domain bound states with a resonant frequency consistent with the repetition frequency of the string of annular periodic electron bunches, thereby realizing the enhancement of coherent Smith-Purcell super radiation. The cylindrical metal grating is driven by the string of annular periodic electron bunches and generates coherent Smith-Purcell radiation through super radiation effect.

[0036] The enhancement of the coherent Smith-Purcell super radiation is achieved by generating a string of annular periodic electron bunches from the cathode, making the string of annular periodic electron bunches pass through the surface of the cylindrical metal grating, and generating quasi-continuous domain bound states and continuous domain bound states from the cylindrical metal grating. The resonant frequency of the quasi-continuous domain bound states is consistent with the repetition frequency of the string of annular periodic electron bunches, and finally the enhancement of the coherent Smith-Purcell super radiation is realized. Under the constraint of an external magnetic field, the high-intensity coherent Smith-Purcell radiation is finally generated above the grating. The detailed process is as follows:

[0037] Figure 2 ​For the schematic diagram of the continuous domain bound state of the application, mainly the waveguide array theory based on the field matching method is used for analysis, the structure is studied according to the region division, the expressions of the incident field of the electron and the field of each region are listed, then the coupling coefficients between each mode are solved by using the boundary conditions of each region, and the continuous domain bound state existing in the Smith-Purcell radiation is found. Mainly include the following contents:

[0038] a, the waveguide array theory based on the field matching method is to regard a period of grating as a waveguide structure, all gratings are regarded as a periodic waveguide array, the transverse magnetic wave propagating in the waveguide array is considered, the dispersion relation is obtained through complex mathematical operation according to the boundary conditions of the tangential field component of the waveguide array and the continuity conditions of the grating surface.

[0039] b, the simulation result of the obtained dispersion relation is as shown in Figure 2 , the abscissa is fd / v, the ordinate is fd / c, and the color represents the efficiency factor of the Smith-Purcell radiation. Here, the grating structure parameters take fixed values: period d=602.7um, thickness t=3d, duty cycle η=0.1, and outer radius 6d. The two dashed lines at the bottom of the figure represent the cutoff conditions of the-1 order diffraction wave, and the dashed line at the top right of the figure represents the cutoff condition of the-2 order diffraction wave. The area above the two dashed lines at the bottom represents the radiation continuous domain, which can produce outward Smith-Purcell radiation. The oblique straight line represents the electron beam, corresponding to β=0.585. By observing the color curve in the dashed line range, the line width will disappear at some points, which is the existence of the continuous domain bound state, and the electron beam line intersects with the strong resonance near the continuous domain bound state (i.e. quasi-continuous domain bound state).

[0040] Figure 3 For the E z time domain simulation diagram above the grating of the application, the simulation is based on the three-dimensional electromagnetic simulation software CST particle studio, mainly based on Figure 2 results, the frequency of the periodic electron cluster string is set, the cylindrical metal grating is driven, the distance between the grating and the electron cluster string is set as d / 10, the number of grating periods is 41, and high-intensity coherent Smith-Purcell super radiation is obtained. Mainly include the following contents:

[0041] a, the β of the electron cluster string is set to the corresponding value 0.585 in the dispersion curve diagram, and the structure parameters of the grating are set to the corresponding values in the dispersion curve diagram.

[0042] b, in the simulation, the E z size of the Smith-Purcell radiation above the middle of the grating is compared under two different electron cluster string repetition frequencies (f b1 =299.2GHz, f b2 =289.2GHz). b1Corresponding quasi-continuous domain super-radiation of bound state enhancement, f b3 Corresponding ordinary super-radiation of far quasi-continuous domain bound state, compared with quasi-continuous domain bound state, the Smith-Purcell super-radiation has a significant enhancement effect, and high-efficiency coherent Smith-Purcell radiation is realized.

[0043] Figure 4 The radiation field E of the electron group string repetition frequency f b1 z The radiation angle shown in the simulation result is consistent with the Smith-Purcell radiation formula of lambda=d(1 / beta-cos theta) / |n|, wherein lambda=c / f b1 , d and beta are the values described above, and the-1 order radiation field is observed, and n=-1.

[0044] Compared with the prior art, the cylindrical grating coherent Smith-Purcell radiation enhancement method provided by the present application is based on the quasi-continuous domain bound state enhancement Smith-Purcell radiation mechanism, the system structure is simpler, the miniaturization of the free electron laser system is easier to realize, the high-intensity coherent Smith-Purcell super-radiation is realized, a simple structure and high-efficiency Smith-Purcell radiation source is provided, which is helpful for the development of terahertz science and technology, and has important significance for improving production efficiency and promoting enterprise development.

[0045] The above is the basic principle and main features of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principle of the present application, and any modifications and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application. The protection scope of the present application is defined by the appended claims and their equivalents.​

Claims

1. A method of achieving cylindrical grating coherent Smith-Purcell radiation enhancement, characterized by, The method is based on a structure comprising a cathode (1), a cylindrical metal grating (2), and a collector (3); the cathode (1) and the collector (3) are placed on the same horizontal plane, and the cylindrical metal grating (2) is placed between the cathode (1) and the collector (3) and is lower than the cathode (1); The cathode (1) generates a circular periodic electron cluster string (4), which passes over the cylindrical metal grating (2) and finally enters the collector (3); the cylindrical metal grating (2) generates quasi-continuous domain bound states with a resonant frequency consistent with the repetition frequency of the circular periodic electron cluster string (4), thereby enhancing coherent Smith-Purcell super radiation; The cylindrical metal grating (2) is driven by the circular periodic electron cluster string (4) and generates coherent Smith-Purcell radiation through super radiation effect; The enhancement of the coherent Smith-Purcell super radiation is achieved by generating a circular periodic electron cluster string (4) with the cathode (1), making it pass through the surface of the cylindrical metal grating (2), and generating coherent Smith-Purcell super radiation; and by generating quasi-continuous domain bound states and continuous domain bound states with the cylindrical metal grating (2), wherein the resonant frequency of the quasi-continuous domain bound states is consistent with the repetition frequency of the periodic electron cluster string, thereby finally achieving the enhancement of the coherent Smith-Purcell super radiation.

2. The method of claim 1, wherein the cylindrical grating is implemented by a plurality of cylindrical lenses. The enhancement method mainly comprises the following steps: Step 1. Electron gun, i.e., cathode (1), generates electron beam: The electron gun generates, accelerates, and focuses a high-energy-density electron beam, which is controllable and has a certain energy, beam current, speed, and angle; Step 2. Based on super radiation: Continuous electron beam drives the cylindrical metal grating (2) to further form an electron cluster string, thereby generating a super radiation effect, which is a kind of coherent radiation with high power output at a fixed frequency, far greater than the output power of ordinary incoherent Smith-Purcell radiation; Step 3. Quasi-continuous domain bound state enhances Smith-Purcell radiation: In the Smith-Purcell radiation of the cylindrical metal grating, there are continuous domain bound states, and the high-Q resonance near the continuous domain bound states is quasi-continuous domain bound states, which will radiate energy to the outside, thereby providing an enhancement mechanism for coherent Smith-Purcell super radiation; Step 4. Generate high-efficiency coherent Smith-Purcell radiation: Use the periodic electron cluster string to drive the cylindrical metal grating supporting quasi-continuous domain bound states, so that the resonant frequency of the quasi-continuous domain bound states of the cylindrical metal grating matches the repetition frequency of the periodic electron cluster string, thereby realizing high-intensity coherent Smith-Purcell super radiation; the matching refers to the fact that the resonant frequency of the quasi-continuous domain bound states of the cylindrical metal grating matches the repetition frequency of the periodic electron cluster string or its high-order harmonic, and the quasi-continuous domain bound states are used to enhance the Smith-Purcell super radiation intensity at the repetition frequency of the periodic electron cluster string or its high-order harmonic.

3. The method of claim 2, wherein the cylindrical grating is implemented by a plurality of cylindrical lenses. The step 1 comprises: 1a. The electron gun and the collector are placed on the same horizontal plane, and the cylindrical metal grating is placed between the electron gun and the collector and is lower than the electron gun; 1b、 the electron gun generates a circular periodic electron bunch train, which sweeps over the cylindrical metal grating and finally enters the collector; 1c、 the electron gun generates a periodic electron bunch train with a repetition frequency of w under the driving of laser pulses with a repetition frequency of w, the shape and structure of the electron bunch train should remain stable during the movement, if the electron bunch train diverges due to space charge force, the effect of beam-wave interaction will be greatly reduced, in order to keep the shape of the electron bunch train stable, a uniform static magnetic field parallel to the direction of the electron bunch train movement is applied to constrain the electron bunch train; 1d、 the cylindrical metal grating avoids the problems of loss, heat dissipation and charge accumulation of dielectric materials in high-power devices.

4. The method of claim 2, wherein the cylindrical grating is implemented by a plurality of cylindrical lenses. The step 2 comprises: 2a、 the ordinary incoherent Smith-Purcell radiation refers to the electromagnetic wave radiated at a certain angle above the grating when the charged particles pass through the surface of the periodic grating, which complies with the relationship λ = d(1 / β-cosθ) / |n|, wherein d is the period of the grating, λ is the wavelength of the Smith-Purcell radiation, β is the ratio of the electron movement speed v to the speed of light c, θ is the included angle between the electron movement direction and the direction of the Smith-Purcell radiation, and n is the order of the Smith-Purcell radiation; 2b、 the super-radiation effect refers to that when the electron bunch train (4) passes through the surface of the cylindrical metal grating (2), if the repetition frequency of the electron bunch train or its high-order harmonic is located in the radiation continuous domain, the coherent Smith-Purcell radiation can be excited at the repetition frequency of the electron bunch train or its high-order harmonic, and the intensity is much larger than that of the incoherent Smith-Purcell radiation.

5. The method of claim 2, wherein the cylindrical grating is implemented by a plurality of cylindrical lenses. The step 3 comprises: 3a、 the continuous domain bound state, as a kind of harmonic with infinite quality factor, is a bound state located in the continuous domain, which enhances the interaction between light and matter; 3b、 the quasi-continuous domain bound state, as a kind of high-quality factor resonance located near the continuous domain bound state, radiates energy outward, which is used to improve the intensity of the Smith-Purcell radiation.

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