Method and device for generating higher harmonics on circular polarization axis based on undulator radiation

Abstract

The invention discloses a method and device for generating on-axis higher harmonics of circularly polarized radiation based on an undulator, and belongs to the field of synchrotron radiation and undulators. The method comprises the steps of 1, arranging a plurality of electromagnetic coils between a magnetic block of the circular polarization undulator and an electron beam vacuum tube, and 2, when the circular polarization undulator works, introducing currents with the same magnitude into all the electromagnetic coils, and enabling the polarities of the currents in the transversely adjacent electromagnetic coils to be different, a periodic additional magnetic field is generated on a central shaft of the circular polarization undulator through the electromagnetic effect of each electromagnetic coil, and higher harmonic radiation can be received on the shaft. According to the undulator, the flux of the obtainable circularly polarized radiation higher harmonics can be greatly enhanced on a shaft, and the higher harmonics of a certain specific order can be enhanced in a targeted manner, so that the coverage range of circularly polarized radiation photon energy of the undulator is greatly expanded under the condition that subsequent beam line transformation is not carried out, and the undulator has a wide application prospect. And the user experiment efficiency and the overall performance of the device are improved.

Description

Technical Field

[0001] The present invention relates to the field of synchrotron radiation and undulator technology, and in particular to a method and device for generating high-order harmonics on a circular polarization axis based on undulator radiation. Background Art

[0002] In the bent iron of high-energy electron synchrotron accelerators and particle storage rings, electrons move at relativistic speeds, and the direction of their acceleration points to the center of the circle, which is perpendicular to the direction of the speed. At this time, the electron beam emits synchrotron radiation (bent iron radiation) along the tangent direction of its movement, losing energy. For particle colliders, synchrotron radiation is harmful, causing particle energy loss and limiting the maximum energy they can reach. Among common particles, the synchrotron radiation energy loss of electrons is the most obvious. As researchers further explore synchrotron radiation, synchrotron radiation light sources that use electrons as accelerated particles and are dedicated to producing synchrotron radiation have been born.

[0003] In modern synchrotron radiation sources, radiation is generated by wigglers or undulators. An undulator is composed of multiple groups of short-period dipole magnets that are periodically arranged and have alternating polarities. Compared with bent-iron radiation, the undulator radiation spectrum is narrower and more intense, and the radiation beam is concentrated within a very small angle, which greatly improves the radiation brightness. In typical third- and fourth-generation synchrotron radiation storage rings, undulators can achieve a photon energy coverage range of more than three orders of magnitude, providing a powerful means of detecting the microscopic world.

[0004] Among them, circularly polarized radiation has great demand and important applications in synchrotron radiation sources, especially in the soft X-ray band. However, the fundamental wave flux of circularly polarized radiation generated by all undulators is distributed on the axis, while the flux distribution of high-order harmonic radiation is an off-axis ring, with almost no flux distribution on the axis. This is due to the phase relationship between the motion trajectory of electrons in the undulator and the high-order harmonic radiation. In the undulator, electrons are subjected to lateral forces in the alternating magnetic field and will make spiral or sinusoidal fluctuations in a plane perpendicular to the forward direction. When electrons generate radiation, the fundamental frequency radiation is concentrated close to the axis, while the high-order harmonics are emitted in a larger radiation angle range. This radiation distribution means that the fundamental frequency radiation will be mainly emitted along the axis of the electron beam, and the energy is concentrated near the axis. As the number of harmonics increases, the high-order harmonic radiation will deviate from the axis, that is, the high-order harmonics tend to be emitted outside the axis. Therefore, when a general beam line receives synchrotron radiation from the axis, for circularly polarized radiation, only the fundamental wave radiation can be received, which greatly limits the photon energy range of circularly polarized radiation.

[0005] For some important experimental stations (such as magnetic material research, soft X-ray spectroscopy, etc.), a wider photon energy range is required, usually including circularly polarized fundamental waves and higher harmonics. For example, for the Hefei Advanced Light Source, the electron energy is 2.2GeV. An undulator with a period of 44mm working in circular polarization mode has an adjustable photon energy range of only about 300 to 800eV for its fundamental radiation. If high-order harmonic radiation can be received at the same time, the adjustable photon energy range can be expanded to more than 300 to 2000eV, which can greatly expand the user service capabilities of related experimental stations. Therefore, moving the main light flux distribution of the high-order harmonic radiation of the circularly polarized undulator from off-axis to on-axis can greatly increase the circularly polarized radiation photon energy range that the undulator can cover (more than twice) without changing the subsequent beamline design, and ensure a considerable photon flux. Therefore, the development of a method based on an undulator to generate on-axis higher-order harmonics of circularly polarized radiation has important application needs and practical significance.

[0006] The current method of using circularly polarized high-order harmonics is to receive off-axis high-order harmonics. This method faces problems such as low flux and complex subsequent beamline design.

[0007] In view of this, the present invention is proposed. Summary of the invention

[0008] The purpose of the present invention is to provide a method and device for generating high-order harmonics on the circular polarization axis based on the radiation of an undulator, which can control the magnetic field of the circular polarization undulator, generate high-order harmonics on the axis based on the radiation of the circular polarization undulator, and realize modulation of the radiation spectrum of the circular polarization undulator. It has the characteristics of easy implementation, adjustable structure, covering a wide range of working photon energy, and easy disassembly and assembly, thereby solving the above-mentioned technical problems existing in the prior art.

[0009] The objective of the present invention is achieved through the following technical solutions:

[0010] A method for generating high-order harmonics on a circular polarization axis based on undulator radiation, comprising:

[0011] Step 1, a plurality of electromagnetic coils are arranged between the magnetic block of the circular polarization undulator and the electron beam vacuum tube, wherein each electromagnetic coil is respectively located between the lower surface of the upper magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator and between the upper surface of the lower magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator;

[0012] Step 2, when the circularly polarized undulator is working, the same current is passed through all the electromagnetic coils, and the polarities of the currents in the laterally adjacent electromagnetic coils are made different. The electromagnetic effect of each electromagnetic coil generates a periodic additional magnetic field on the central axis of the circularly polarized undulator, which modulates the magnetic field of the circularly polarized undulator, and converts the high-order harmonic flux radiated by the circularly polarized undulator into an on-axis distribution, so that the high-order harmonic radiation can be received on the axis.

[0013] A device for generating high-order harmonics on a circular polarization axis based on undulator radiation, used to implement the method for generating high-order harmonics on a circular polarization axis based on undulator radiation according to the present invention, comprising:

[0014] A circular polarization undulator and a plurality of electromagnetic coils; wherein,

[0015] A plurality of electromagnetic coils are arranged between the magnetic block of the circular polarization undulator and the electron beam vacuum tube, and each electromagnetic coil is respectively located between the lower surface of the upper magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator and between the upper surface of the lower magnetic block of the circular polarization undulator and the lower surface of the flat vacuum tube of the circular polarization undulator;

[0016] The same current can be passed through all electromagnetic coils, and the current polarities in the laterally adjacent electromagnetic coils are different, thus generating a periodic additional magnetic field on the central axis of the circularly polarized undulator, which modulates the magnetic field of the circularly polarized undulator, and converts the high-order harmonic flux radiated by the circularly polarized undulator into an on-axis distribution, so that the high-order harmonic radiation can be received on the axis.

[0017] Compared with the prior art, the method and device provided by the present invention for generating high-order harmonics on a circular polarization axis based on undulator radiation have the following beneficial effects:

[0018] By setting multiple electromagnetic coils at specific positions in the circularly polarized undulator, the electromagnetic coils are used to generate weak periodic modulated magnetic fields, thereby realizing the regulation of the magnetic field of the circularly polarized undulator. Based on the generation of on-axis high-order harmonics based on the radiation of the circularly polarized undulator, it is only necessary to change the current passed through the coil to attach a weak magnetic field to the undulator that can generate enough fundamental radiation energy to move. It can be installed as an additional part on the existing undulator, and the original energy range of the undulator can be matched in a targeted manner to achieve full coverage. Compared with the conventional method of adding magnetic blocks, it has the advantages of less difficulty, simple structure, and simpler magnetic field integral correction. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0020] Figure 1 A schematic diagram of the structure of a device for generating high-order harmonics on a circularly polarized axis based on undulator radiation provided in an embodiment of the present invention.

[0021] Figure 2 In the method for generating high-order harmonics on a circularly polarized axis based on undulator radiation provided in an embodiment of the present invention, a schematic diagram of a spectrum of a simulated single-sided electromagnetic coil when it is switched on and off is provided, wherein the horizontal axis is the photon energy and the vertical axis is the photon flux received on the axis; wherein (a) is a schematic diagram of a spectrum of a simulated single-sided electromagnetic coil when it is turned on; and (b) is a schematic diagram of a spectrum of a simulated single-sided electromagnetic coil when it is turned off.

[0022] Figure 3 In the method for generating high-order harmonics on the circularly polarized axis based on undulator radiation provided in an embodiment of the present invention, a schematic diagram of the spectrum of the simulated double-sided electromagnetic coils when they are switched on and off is provided, wherein the horizontal axis is the photon energy and the vertical axis is the photon flux received on the axis; wherein (a) is a schematic diagram of the spectrum of the simulated double-sided electromagnetic coils when they are opened; and (b) is a schematic diagram of the spectrum of the simulated double-sided electromagnetic coils when they are closed.

[0023] Figure 4 In the method for generating high-order harmonics on the circular polarization axis based on undulator radiation provided in an embodiment of the present invention, a schematic diagram of the light spot when the electromagnetic coil is turned on is simulated, and the light spot shapes under different working conditions are given; wherein (a) is a schematic diagram of the light spot distribution corresponding to the first harmonic when a single-sided coil is used; (b) is a schematic diagram of the light spot distribution corresponding to the second harmonic when a single-sided coil is used; (c) is a schematic diagram of the light spot distribution corresponding to the first harmonic when a double-sided coil is used; (d) is a schematic diagram of the light spot distribution corresponding to the second harmonic when a double-sided coil is used. DETAILED DESCRIPTION

[0024] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in combination with the specific content of the present invention; it is obvious that the described embodiments are only part of the embodiments of the present invention, not all of the embodiments, which does not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the protection scope of the present invention.

[0025] First, the terms that may be used in this article are explained as follows:

[0026] The term “and / or” means that either or both of them can be realized at the same time. For example, X and / or Y means both “X” or “Y” and “X and Y”.

[0027] The terms "include", "comprises", "contains", "has" or other descriptions with similar semantics should be interpreted as non-exclusive inclusion. For example, including certain technical feature elements (such as raw materials, components, ingredients, carriers, dosage forms, materials, dimensions, parts, components, mechanisms, devices, steps, procedures, methods, reaction conditions, processing conditions, parameters, algorithms, signals, data, products or products, etc.) should be interpreted as including not only certain technical feature elements explicitly listed, but also other technical feature elements known in the art that are not explicitly listed.

[0028] The term "consisting of..." means excluding any technical feature elements not explicitly listed. If this term is used in a claim, it will make the claim closed, so that it does not contain technical feature elements other than the technical feature elements explicitly listed, except for the conventional impurities related to them. If this term only appears in a clause of a claim, it only limits the elements explicitly listed in the clause, and the elements recorded in other clauses are not excluded from the overall claim.

[0029] Unless otherwise specified or limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this article can be understood according to specific circumstances.

[0030] When concentration, temperature, pressure, size or other parameters are expressed in the form of a numerical range, the numerical range should be understood to specifically disclose all ranges formed by the pairing of any upper limit, lower limit, and preferred value in the numerical range, regardless of whether the range is explicitly stated; for example, if a numerical range of "2 to 8" is stated, the numerical range should be interpreted as including ranges such as "2 to 7", "2 to 6", "5 to 7", "3 to 4 and 6 to 7", "3 to 5 and 7", "2 and 5 to 7", etc. Unless otherwise specified, the numerical ranges stated herein include both their end values ​​and all integers and fractions within the numerical range.

[0031] The orientation or position relationship indicated by terms such as "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc. are based on the orientation or position relationship shown in the drawings and are only for the convenience and simplification of description, and do not explicitly or implicitly indicate that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation of this document.

[0032] The scheme provided by the present invention is described in detail below. The contents not described in detail in the embodiments of the present invention belong to the prior art known to professionals in the field. If the specific conditions are not specified in the embodiments of the present invention, they are carried out according to the conventional conditions in the field or the conditions recommended by the manufacturer. If the manufacturers are not specified in the reagents or instruments used in the embodiments of the present invention, they are all conventional products that can be purchased commercially.

[0033] See also Figure 1 , an embodiment of the present invention provides a method for generating high-order harmonics on a circular polarization axis based on undulator radiation, comprising:

[0034] Step 1, a plurality of electromagnetic coils are arranged between the magnetic block of the circular polarization undulator and the electron beam vacuum tube, wherein each electromagnetic coil is respectively located between the lower surface of the upper magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator and between the upper surface of the lower magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator;

[0035] Step 2, when the circularly polarized undulator is working, the same current is passed through all the electromagnetic coils, and the polarities of the currents in the laterally adjacent electromagnetic coils are made different. The electromagnetic effect of each electromagnetic coil generates a periodic additional magnetic field on the central axis of the circularly polarized undulator, which modulates the magnetic field of the circularly polarized undulator, and converts the high-order harmonic flux radiated by the circularly polarized undulator into an on-axis distribution, so that the high-order harmonic radiation can be received on the axis.

[0036] Preferably, in step 1 of the above method, a group of electromagnetic coils are respectively arranged on the upper surface and the lower surface of the electron beam vacuum tube of the circular polarization undulator along the advancing direction of the electron beam.

[0037] Preferably, in the above method, all electromagnetic coils have the same number of turns and shape, and the shape of each electromagnetic coil is circular or racetrack-shaped, and the number of layers is an even number.

[0038] The period of each electromagnetic coil is an integer multiple of the period of the undulator, preferably 6 times.

[0039] Preferably, in the above method, the current density supplied to each electromagnetic coil is 0.1 to 3 amperes per square millimeter. In this way, the current density does not exceed 3 amperes per square millimeter, water cooling is not required, and the volume and energy consumption will not be increased.

[0040] like Figure 1 As shown, an embodiment of the present invention further provides a device for generating high-order harmonics on a circular polarization axis based on undulator radiation, which is used to implement the above-mentioned method for generating high-order harmonics on a circular polarization axis based on undulator radiation, comprising:

[0041] A circular polarization undulator and a plurality of electromagnetic coils; wherein,

[0042] A plurality of electromagnetic coils are arranged between the magnetic block of the circular polarization undulator and the electron beam vacuum tube, and each electromagnetic coil is respectively located between the lower surface of the upper magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator and between the upper surface of the lower magnetic block of the circular polarization undulator and the lower surface of the flat vacuum tube of the circular polarization undulator;

[0043] The same current can be passed through all electromagnetic coils, and the polarities of the currents in the laterally adjacent electromagnetic coils are different, thus generating a periodic additional magnetic field on the central axis of the circularly polarized undulator that modulates the magnetic field of the circularly polarized undulator, thereby converting the high-order harmonic flux radiated by the circularly polarized undulator into an on-axis distribution, so that the high-order harmonic radiation can be received on the axis.

[0044] Preferably, in the above device, a group of electromagnetic coils are respectively arranged on the upper surface and the lower surface of the electron beam vacuum tube of the circular polarization undulator along the advancing direction of the electron beam.

[0045] Preferably, in the above device, all electromagnetic coils have the same number of turns and shape, and each electromagnetic coil is circular or racetrack-shaped, with an even number of layers.

[0046] Preferably, in the above device, the period of each electromagnetic coil is an integer multiple of the period of the undulator.

[0047] Preferably, the above-mentioned device also includes: a monochromator, whose energy is set to be the same as the energy of the higher harmonics corresponding to the radiation of the circularly polarized undulator.

[0048] In summary, the method and device of the embodiments of the present invention, by setting multiple electromagnetic coils at specific positions in the circularly polarized undulator, use the electromagnetic coils to generate a weak periodic modulated magnetic field, thereby realizing the regulation of the magnetic field of the circularly polarized undulator. Based on the generation of on-axis high-order harmonics based on the radiation of the circularly polarized undulator, it is only necessary to change the current passed through the coil to add a weak magnetic field to the undulator that can generate enough fundamental radiation energy to move. It can be installed as an additional part on the existing undulator, and the original energy range of the undulator can be matched in a targeted manner to achieve full coverage. Compared with the conventional method of adding magnetic blocks, it has the advantages of less difficulty, simple structure, and simpler magnetic field integral correction.

[0049] In order to more clearly demonstrate the technical solution and technical effects provided by the present invention, the solution provided by the embodiment of the present invention is described in detail with specific embodiments below.

[0050] Example 1

[0051] This embodiment provides a method for generating on-axis high-order harmonics based on circularly polarized undulator radiation, which can achieve the generation of on-axis high-order harmonics in circularly polarized undulator radiation by superimposing an additional modulated magnetic field on the undulator magnetic field.

[0052] The theoretical basis of the present invention is explained below. First, the influence of the modulated magnetic field on the spectral structure is analyzed.

[0053] The circularly polarized undulator magnetic field is expressed as:

[0054]

[0055] Here, B x and B y Respectively represent the horizontal magnetic field and the vertical magnetic field of the undulator; B xo and B yo They represent the horizontal magnetic field and the vertical magnetic field when there is no modulation. For the circularly polarized undulator, B xo =B yo , is the magnetic induction intensity factor, representing the intensity relationship between the modulating magnetic field and the undulator magnetic field, B coil is the magnetic induction intensity of the coil; represents the relationship between the period of the undulator magnetic field and the period of the modulation magnetic field; λ u is the undulator period length; λ coil is the coil period length; Z is the particle forward direction.

[0056] The motion of particles in the undulator is as follows:

[0057]

[0058] Here we use Alternatively, γ is the electronic Lorentz constant, K x and K y are the undulator magnetic field strength parameters in two directions; β x and β y Represent the speed of the particle in the horizontal and vertical directions respectively; C is the speed of light in vacuum, λ 0 is the wavelength of fundamental radiation.

[0059] The average speed of electrons in the undulator along the axis of the undulator is β z It can be expressed as:

[0060]

[0061] The overall average velocity of the particles is used here for:

[0062]

[0063] Directly introducing the circular polarization undulator condition, the average velocity of the particle in the axial direction of the undulator can be written as:

[0064]

[0065] The position of the particle can also be calculated:

[0066]

[0067]

[0068] The intensity spectrum of synchrotron radiation emitted by an electron bending at a normalized speed β in the direction n to the observation point (radiation energy d in unit frequency interval dω, unit solid angle dΩ) 2 1) is:

[0069]

[0070] Analyzing the paraxial situation, we can get the spectral characteristics. The spectrum is discrete, and the main intensity is located at:

[0071]

[0072] n is the harmonic order, a=0, 1, 2...;

[0073] The flux density can be expressed as:

[0074]

[0075] J ais an a-order Bessel function. It can be found that the flux on the axis is no longer 0, and the parameter Y determines its flux density.

[0076]

[0077] In the radiation of a traditional circularly polarized undulator, only the fundamental wave radiation has a non-zero flux on the axis, and the higher harmonics are all off-axis, and the flux on the axis is zero. The method of this embodiment introduces an additional magnetic field to change the undulator radiation spectrum, so that the higher harmonic flux on the axis is non-zero, and the flux on the axis can be calculated.

[0078] Adding the coil magnetic field will reduce the fundamental radiation energy to a certain extent. Considering that the first harmonic of this method appears at When designing, consideration should be given to using a larger electromagnetic coil and a smaller excitation current to achieve multiples of the fundamental wave energy.

[0079] Specifically, the method of generating on-axis high-order harmonics based on circularly polarized undulator radiation in this embodiment is to introduce a periodic modulated magnetic field in only one direction, which is relatively easy to implement. A group of electromagnetic coils are arranged between the lower surface of the upper magnetic block of the undulator and the upper surface of the flat vacuum tube in the undulator, and between the upper surface of the lower magnetic block of the undulator and the lower surface of the flat vacuum tube in the undulator, and the electromagnetic coils are arranged in the same way along the direction of electron beam advance. All electromagnetic coils have the same number of turns and shape, but the current polarity in adjacent electromagnetic coils is different. When the same current is passed through all electromagnetic coils as above, the electromagnetic coil will generate a periodic additional magnetic field on the central axis of the undulator, which modulates the magnetic field of the undulator. The distribution of the high-order harmonic flux radiated by the circularly polarized undulator changes significantly, and it is no longer off-axis, but becomes an on-axis distribution. In this way, high-order harmonic radiation can be received on the axis. This method can also suppress fundamental radiation to a certain extent and enhance high-order harmonic radiation.

[0080] The conventional method of adding magnetic blocks is difficult, complex in structure, complex in magnetic field integral correction, and narrow in working energy. The method of the present invention uses electromagnetic coils to generate weak periodic modulation magnetic fields. It only needs to change the current passed into the electromagnetic coils to add a weak magnetic field that can generate enough fundamental wave radiation energy to move to the undulator. It can be installed as an additional device on the existing undulator, and can be targeted to match the original energy range of the undulator to achieve full coverage.

[0081] Example 2

[0082] This embodiment provides a device for generating high-order harmonics on a circularly polarized radiation axis based on an undulator, such as Figure 1 As shown, it includes: a circular polarization undulator 1, an electron beam 2, a flat vacuum tube 3 and a plurality of electromagnetic coils 4; wherein,

[0083] The circular polarization undulator is a conventional APPLE-II type undulator that can operate in a circular polarization mode;

[0084] The electromagnetic coils are placed between the flat vacuum tube and the undulator magnetic block. The coil magnetic field is used to modulate the undulator magnetic field with a small amplitude. When generating circularly polarized high-order harmonic radiation on the axis, first, the monochromator is set to have the same energy as the high-order harmonic energy corresponding to the undulator radiation, and then the coil is turned on.

[0085] The method of the present invention realizes the generation of circularly polarized high-order harmonic radiation on the axis by placing a coil between the undulator and the vacuum tube. In addition, optimization can be performed for a specific harmonic, and the optimization method has been reflected in the previous formula. According to the structural characteristics of the undulator, when the output radiation energy is low, the magnetic induction intensity of the undulator is high, and the requirements for the magnetic induction intensity generated by the coil are relatively high. At this time, the method of increasing the coil period, such as selecting three times the undulator period, can be adopted to reduce the engineering difficulty. The space occupied by the coil, the minimum magnetic block gap of the undulator and the thickness of the vacuum tube need to be carefully considered. Adding the coil magnetic field will reduce the fundamental radiation energy to a certain extent. Considering that the harmonic number of this scheme is also reduced to a certain extent, when designing, it should be considered to use a larger coil and a smaller excitation current.

[0086] Example 3

[0087] This embodiment provides a method for generating high-order harmonics on a circularly polarized radiation axis based on an undulator, using Figure 1The structure shown takes the APPLE-II type undulator as an example. The flat vacuum tube in the undulator is made of copper or stainless steel, and the height of the flat vacuum tube is 10 mm. The undulator period length is selected to be 50 mm, the number of undulator periods is selected to be 36, the total length of the undulator is 1.8 meters, and the gap between the upper and lower magnetic arrays is 25 mm. The fundamental wave energy of the undulator is about 440 electron volts. The electromagnetic coil is generally circular or runway-shaped, as long as it can generate a periodic field. The number of coil layers is preferably an even number of layers. The introduction and extraction of the coil current can be achieved at the edge of the coil. The cross-sectional size of the coil is relatively flexible. For example, 1mm×4mm and 2mm×2mm can be used. The total ampere-turns can be kept unchanged, but the requirements of the undulator gap must be met. If the undulator gap is very small, the coil can choose a cross-sectional size of 1mm×4mm. In this embodiment, the cross-sectional area of ​​the electromagnetic coil is selected to be 2 mm × 2 mm, each layer of electromagnetic coil has 37 turns, and there are 2 layers of single-sided electromagnetic coils. Each group of electromagnetic coils is composed of two electromagnetic coils on both sides that are respectively close to the upper and lower surfaces of the vacuum tube, including a total of 4 layers of coils. The radius of a single group of electromagnetic coils is 75 mm, the current in the coil is 10 amperes, and the coil period is 300 mm, which is 6 times the undulator period. Each cycle of coils contains two groups of electromagnetic coils with opposite current directions, a total of 6 cycles of coils, and the ampere-turns of a single group of electromagnetic coils are 1480. The monochromator passes energy as required to set the fundamental wave, first harmonic energy or second harmonic energy, and the bandwidth is one thousandth of the photon energy. The electron beam parameters are: energy of 2.2 GeV, current of 500 mA, and beam emittance of 100 picometers·radians. In the simulation, the single-sided coil scheme is achieved by following Figure 1 The results of the modeled, double-sided coils are calculated by directly generating the magnetic field.

[0088] Figure 2 The influence of the opening and closing state of the electromagnetic coil on the output spectrum of the undulator in the above scheme obtained by simulation is given. The receiving angle is selected as 0.05 milliradian × 0.05 milliradian, and the coil adopts a single-sided coil. It can be observed that at the fundamental wave, after the coil is turned on, the undulator radiation flux is reduced to 58.5% when the coil is turned off. After the coil is turned on, at the first harmonic, the harmonic order is Subharmonic, the second harmonic is Subharmonics. Compared to the conventional APPLE-II undulator, which has almost no flux of high-order harmonics on its axis, the flux of high-order harmonics increases by four orders of magnitude after the coil is turned on.

[0089] Figure 3The influence of the coil opening and closing state on the output spectrum of the undulator in the above scheme obtained by simulation is given. The receiving angle is selected as 0.05 milliradian × 0.05 milliradian, and the coil adopts a double-sided coil. It can be observed that at the fundamental wave, after the coil is opened, the undulator radiation flux is reduced to 27% when the coil is closed. After the coil is opened, at the first harmonic, the harmonic order is Subharmonic, the second harmonic is Subharmonics. Compared to the original APPLE-II undulator, which has almost no flux on the axis, after the coil is turned on, the received high-order harmonic flux is increased by four orders of magnitude. Compared with the single-sided coil, the high-order harmonic flux is further improved and the fundamental wave energy is further reduced. In particular The subharmonic flux is increased by two times compared to the single-sided coil.

[0090] Figure 4 The spot shapes under different working conditions are given. Figure 4 (a) is the spot distribution corresponding to the first harmonic when a single-sided coil is used. Figure 4 (b) is the spot distribution corresponding to the second harmonic when a single-sided coil is used. Figure 4 (c) is the spot distribution corresponding to the first harmonic when using double-sided coils. Figure 4 (d) in the figure is the spot distribution corresponding to the second harmonic when using a double-sided coil. A single-sided coil can significantly increase the flux of high-order harmonics at a small receiving angle, but the spot is not regular enough. When using a double-sided coil, the spot of the high-order harmonic becomes centrally symmetrical, and the distribution is similar to a Gaussian distribution, and the shape is extremely similar to the fundamental wave.

[0091] Without affecting the conventional functions of the undulator and without increasing the complexity of the system, the present invention can achieve a significant enhancement of the available circularly polarized radiation high-order harmonic flux on the axis, and can achieve targeted enhancement of a certain order of high-order harmonics, thereby achieving a significant expansion of the energy coverage range of the circularly polarized radiation photons of the undulator without subsequent beam line modification, thereby improving the efficiency of user experiments and the overall performance of the device.

[0092] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims. The information disclosed in the background technology section of this article is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or in any form that the information constitutes prior art known to those skilled in the art.

Claims

1. A method for generating high-order harmonics on a circular polarization axis based on undulator radiation, characterized in that: include: Step 1, a plurality of electromagnetic coils are arranged between the magnetic block of the circular polarization undulator and the electron beam vacuum tube, wherein each electromagnetic coil is respectively located between the lower surface of the upper magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator and between the upper surface of the lower magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator; Step 2, when the circularly polarized undulator is working, the same current is passed through all the electromagnetic coils, and the polarities of the currents in the laterally adjacent electromagnetic coils are made different. The electromagnetic effect of each electromagnetic coil generates a periodic additional magnetic field on the central axis of the circularly polarized undulator, which modulates the magnetic field of the circularly polarized undulator, and converts the high-order harmonic flux radiated by the circularly polarized undulator into an on-axis distribution, so that the high-order harmonic radiation can be received on the axis.

2. The method for generating high-order harmonics on a circularly polarized axis based on undulator radiation according to claim 1, characterized in that: In the step 1, a group of electromagnetic coils are respectively arranged on the upper surface and the lower surface of the electron beam vacuum tube of the circular polarization undulator along the advancing direction of the electron beam.

3. The method for generating high-order harmonics on a circularly polarized axis based on undulator radiation according to claim 1, characterized in that: All electromagnetic coils have the same number of turns and shape. The shape of each electromagnetic coil is circular or racetrack-shaped, and the number of layers is an even number.

4. The method for generating high-order harmonics on a circularly polarized axis based on undulator radiation according to any one of claims 1 to 3, characterized in that: The period of each electromagnetic coil is an integer multiple of the period of the undulator.

5. The method for generating high-order harmonics on a circular polarization axis based on undulator radiation according to claim 4, characterized in that: The period of each electromagnetic coil is 6 times the period of the undulator; The current density supplied to each electromagnetic coil is 0.1 to 3 amperes per square millimeter.

6. A device for generating high-order harmonics on a circular polarization axis based on undulator radiation, characterized in that: The method for generating high-order harmonics on a circularly polarized axis based on undulator radiation as described in any one of claims 1 to 5 comprises: A circular polarization undulator and a plurality of electromagnetic coils; wherein, A plurality of electromagnetic coils are arranged between the magnetic block of the circular polarization undulator and the electron beam vacuum tube, and each electromagnetic coil is respectively located between the lower surface of the upper magnetic block of the circular polarization undulator and the upper surface of the flat vacuum tube of the circular polarization undulator and between the upper surface of the lower magnetic block of the circular polarization undulator and the lower surface of the flat vacuum tube of the circular polarization undulator; The same current can be passed through all electromagnetic coils, and the current polarities in the laterally adjacent electromagnetic coils are different, thus generating a periodic additional magnetic field on the central axis of the circularly polarized undulator, which modulates the magnetic field of the circularly polarized undulator, and converts the high-order harmonic flux radiated by the circularly polarized undulator into an on-axis distribution, so that the high-order harmonic radiation can be received on the axis.

7. The device for generating high-order harmonics on a circular polarization axis based on undulator radiation according to claim 6, characterized in that: A group of electromagnetic coils are respectively arranged on the upper surface and the lower surface of the electron beam vacuum tube of the circular polarization undulator along the advancing direction of the electron beam.

8. The device for generating high-order harmonics on a circularly polarized axis based on undulator radiation according to claim 6 or 7, characterized in that: The number of turns and shape of all electromagnetic coils are the same. The shape of each electromagnetic coil is circular or racetrack-shaped, and the number of layers is an even number.

9. The device for generating high-order harmonics on a circularly polarized axis based on undulator radiation according to claim 6 or 7, characterized in that: The period of each electromagnetic coil is an integer multiple of the period of the undulator.

10. The device for generating high-order harmonics on a circular polarization axis based on undulator radiation according to claim 6 or 7, characterized in that: Also includes: The monochromator, through energy is set to be the same as the energy of the corresponding high harmonics of the circularly polarized undulator radiation.

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