Method and device for generating high-order harmonics on a circularly polarized axis based on undulator radiation

Abstract

The present invention discloses a method and device for generating on-axis high-order harmonics of circularly polarized radiation based on an undulator, belonging to the field of synchrotron radiation and undulators. The method comprises: step 1, arranging a plurality of electromagnetic coils between the magnetic block of the circularly polarized undulator and the electron beam vacuum tube; step 2, when the circularly polarized undulator is in operation, passing currents of the same magnitude into all the electromagnetic coils, and making the current polarities of the laterally adjacent electromagnetic coils different, so that a periodic additional magnetic field is generated on the central axis of the circularly polarized undulator by the electromagnetic effect of each electromagnetic coil, and high-order harmonic radiation can be received on the axis. The present invention can achieve a significant enhancement of the obtainable on-axis circularly polarized radiation high-order harmonic flux, and can achieve targeted enhancement of a specific 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 beamline modification, thereby improving the efficiency of user experiments and the overall performance of the device.

Description

Technical Field

[0001] The present invention relates to the technical field of synchrotron radiation and undulator technology, and in particular to a method and device for generating high-order harmonics on a circularly polarized 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 their acceleration direction points to the center of the circle, perpendicular to the direction of velocity. 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 are born that use electrons as accelerated particles and are specifically used to produce synchrotron radiation.

[0003] In modern synchrotron radiation sources, radiation is generated by wigglers or undulators. Undulators consist of multiple sets of short-period dipole magnets arranged periodically with alternating polarity. Compared to bent-iron radiation, undulator radiation has a narrower and more intense spectrum, and the radiation beam is concentrated within a very small angle, significantly enhancing the radiation brightness. In typical third- and fourth-generation synchrotron radiation storage rings, undulators can achieve photon energy coverage exceeding three orders of magnitude, providing a powerful means of probing the microscopic world.

[0004] Circularly polarized radiation is in great demand and has important applications in synchrotron radiation sources, particularly in the soft X-ray band. However, all currently available undulators generate circularly polarized radiation with a fundamental frequency flux distribution on-axis, while the higher harmonic radiation flux distribution is an off-axis ring, with virtually no on-axis flux distribution. This is due to the phase relationship between the electron trajectory in the undulator and the higher harmonic radiation. In an undulator, electrons are subjected to transverse forces in an alternating magnetic field, causing them to oscillate in spiral or sinusoidal patterns in a plane perpendicular to their direction of travel. When electrons generate radiation, the fundamental frequency is concentrated near the axis, while the higher harmonics are emitted over a wider range of radiation angles. This radiation distribution means that the fundamental frequency radiation is primarily emitted along the electron beam axis, with energy concentrated near the axis. As the harmonic order increases, the higher harmonic radiation deviates from the axis, meaning that higher harmonics tend to be emitted off-axis. Therefore, when a typical beamline receives synchrotron radiation on-axis, it only receives the fundamental frequency radiation, significantly limiting the photon energy range of the circularly polarized radiation.

[0005] For some important experimental stations (such as magnetic materials 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 2.2 GeV. An undulator with a period of 44 mm operating in circular polarization mode has an adjustable photon energy range of only about 300 to 800 eV 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 2000 eV, which can greatly expand the user service capabilities of the relevant 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 the undulator to generate on-axis high-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 circularly polarized undulator axis based on the radiation of the circularly polarized undulator, which can regulate the magnetic field of the circularly polarized undulator, generate high-order harmonics on the axis based on the radiation of the circularly polarized undulator, and realize the modulation of the radiation spectrum of the circularly polarized undulator. It has the characteristics of easy implementation, adjustable structure, wide coverage of working photon energy range, and easy disassembly and assembly, thereby solving the above-mentioned technical problems existing in the prior art.

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

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

[0011] Step 1: Disposing a plurality of electromagnetic coils 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 polarity of the current in the laterally adjacent electromagnetic coils is 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 circularly polarized axis based on undulator radiation, for implementing the method for generating high-order harmonics on a circularly polarized axis based on undulator radiation described in the present invention, comprising:

[0014] A circularly polarized undulator and a plurality of electromagnetic coils; wherein,

[0015] Multiple 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 polarity in the laterally adjacent electromagnetic coils is different, 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.

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

[0018] By setting up multiple electromagnetic coils at specific positions in the circularly polarized undulator, the electromagnetic coils are used 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. This can be installed as an additional part on the existing undulator, and the original energy range of the undulator can be targetedly matched to achieve full coverage. Compared with the conventional method of adding magnetic blocks, this method 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 following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any 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 the 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 the spectrum of the simulated single-sided electromagnetic coil when it is turned on; and (b) is a schematic diagram of the spectrum of the 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 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 the spectrum of the simulated double-sided electromagnetic coil when it is turned on; and (b) is a schematic diagram of the spectrum of the simulated double-sided electromagnetic coil when it is turned off.

[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 simulated is given, and the light spot shapes under different working states are given; among them, (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 conjunction with the specific content of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments, and do not constitute a limitation of the present invention. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0025] First, the following terms may be used in this article:

[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 includes both “X” or “Y” and “X and Y”.

[0027] The terms "include," "comprises," "contains," "has," or other similar expressions should be interpreted as non-exclusive. For example, "including certain technical features (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 manufactured articles, etc.) should be interpreted as including not only the technical features explicitly listed, but also other technical features known in the art that are not explicitly listed.

[0028] The term "consisting of" excludes any technical features not explicitly listed. If used in a claim, this term renders the claim closed, excluding any technical features other than those explicitly listed, except for conventional impurities associated with them. If this term appears only in a clause of a claim, it limits only the elements explicitly listed in that clause; elements listed in other clauses are not excluded from the claim as a whole.

[0029] Unless otherwise specified or limited, the terms "mounted," "connected," "connect," and "fixed" should be interpreted broadly. For example, they can refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediary; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in this document based on 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, or preferred value within 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 of "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 endpoints and all integers and fractions within the numerical range.

[0031] The terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying 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 operate in a specific orientation, and therefore should not be understood as a limitation to this document.

[0032] The scheme provided by the present invention is described in detail below. The contents not described in detail in the examples of the present invention belong to the prior art known to professionals in this field. If specific conditions are not specified in the examples of the present invention, they are carried out according to conventional conditions in the field or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used in the examples of the present invention is not specified, 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 circularly polarized axis based on undulator radiation, comprising:

[0034] Step 1: Disposing a plurality of electromagnetic coils 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 polarity of the current in the laterally adjacent electromagnetic coils is 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 provided on the upper surface and the lower surface of the electron beam vacuum tube of the circular polarization undulator along the forward 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 undulator period, 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. Such a current density not exceeding 3 amperes per square millimeter does not require water cooling and does not increase the volume and energy consumption.

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

[0041] A circularly polarized undulator and a plurality of electromagnetic coils; wherein,

[0042] Multiple 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 polarity of the current in the laterally adjacent electromagnetic coils is different, which generates a periodic additional magnetic field on the central axis of the circularly polarized undulator, modulating the magnetic field of the circularly polarized undulator, and 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 provided on the upper surface and the lower surface of the electron beam vacuum tube of the circular polarization undulator along the forward direction of the electron beam.

[0045] Preferably, in the above device, all electromagnetic coils have the same number of turns and shape, and the shape of 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 further comprises: a monochromator, the energy of which 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 apparatus of the embodiments of the present invention achieve control of the magnetic field of the circularly polarized undulator by setting multiple electromagnetic coils at specific positions in the circularly polarized undulator and utilizing the electromagnetic coils to generate a weak periodic modulated magnetic field. Based on the generation of on-axis high-order harmonics radiated by 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 sufficient fundamental radiation energy to move. This can be installed as an additional component on the existing undulator, and can perform targeted matching of the original energy range of the undulator to achieve full coverage. Compared with the conventional method of adding magnetic blocks, this method 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 reference to 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 by 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 of the undulator 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 moving direction.

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

[0057]

[0058] Here we use Alternatively, γ is the electron 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, and λ0 is the wavelength of the fundamental radiation.

[0059] The average velocity of electrons in the undulator along the axial direction 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 circularly polarized 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 in bending motion with a normalized velocity β in the direction n to the observation point (radiation energy d in unit solid angle dΩ and unit frequency interval dω) 2 1) is:

[0069]

[0070] Analyzing the paraxial case, 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 conventional circularly polarized undulators, only the fundamental radiation has a non-zero on-axis flux; all higher harmonics are off-axis and have zero on-axis flux. This embodiment introduces an additional magnetic field to alter the undulator's radiation spectrum, making the on-axis flux of higher harmonics non-zero and calculable.

[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 the electromagnetic coil to multiple the fundamental wave energy, the use of a larger electromagnetic coil and a smaller excitation current should be considered.

[0079] Specifically, the method of generating on-axis higher harmonics based on circularly polarized undulator radiation in this embodiment introduces a periodically modulated magnetic field in only one direction, making its implementation relatively simple. A set of electromagnetic coils is placed between the lower surface of the upper magnetic block of the undulator and the upper surface of the flat vacuum tube within the undulator, and between the upper surface of the lower magnetic block of the undulator and the lower surface of the flat vacuum tube within the undulator. These coils are arranged identically along the direction of electron beam travel. All electromagnetic coils have the same number of turns and shape, but the current polarity in adjacent coils differs. When the same current is passed through all electromagnetic coils in this manner, they generate a periodic additional magnetic field on the central axis of the undulator, modulating the undulator's magnetic field. The flux distribution of the higher harmonics radiated by the circularly polarized undulator changes significantly, no longer being off-axis but on-axis. This allows for on-axis reception of the higher harmonic radiation. This method also suppresses fundamental radiation to a certain extent, enhancing higher harmonic radiation.

[0080] Conventional methods involving additional magnetic blocks are difficult, complex, and require complex magnetic field integral correction, resulting in a narrow operating energy range. However, the present invention utilizes electromagnetic coils to generate a weak periodically modulated magnetic field. Simply by varying the current flowing through the electromagnetic coils, a weak magnetic field sufficient to shift the fundamental radiation energy can be added to the undulator. This can be installed as an add-on to existing undulators, enabling targeted matching of the undulator's existing energy range to achieve full coverage.

[0081] Example 2

[0082] This embodiment provides a device for generating high-order harmonics on the circularly polarized radiation axis based on an undulator. 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 magnet. The coil magnetic field is used to slightly modulate the undulator magnetic field. To generate circularly polarized high-harmonic radiation on the axis, the monochromator is first set to the same energy as the corresponding high-harmonic radiation from the undulator, and then the coils are turned on.

[0085] The method of the present invention achieves 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 engineering difficulty can be reduced by increasing the coil period, such as selecting three times the undulator period. 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 order of this scheme is also reduced to a certain extent, the design should consider using a larger coil and a smaller excitation current.

[0086] Example 3

[0087] This embodiment provides a method for generating circularly polarized radiation on-axis high harmonics based on an undulator, using the following method: Figure 1The structure shown uses an APPLE-II type undulator as an example. The flat vacuum tube within the undulator is made of copper or stainless steel and is 10 mm tall. The undulator period length is 50 mm, the number of periods is 36, the total length 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 approximately 440 electron volts. The electromagnetic coil is generally circular or racetrack-shaped, as long as it can generate a periodic field. An even number of coil layers is preferred, and current can be introduced and removed at the coil edges. The coil cross-sectional dimensions are flexible, for example, 1 mm × 4 mm and 2 mm × 2 mm are both acceptable, as long as the total ampere-turns remain constant. However, the undulator gap requirements must be met. If the undulator gap is very small, a 1 mm × 4 mm cross-sectional dimension can be used. 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 close to the upper and lower surfaces of the vacuum tube, respectively, 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 is set to the fundamental wave, first harmonic energy or second harmonic energy as needed, 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 double-sided coils obtained by modeling are calculated by directly generating the magnetic field.

[0088] Figure 2 The simulated results show the effect of the electromagnetic coil's open and closed state on the undulator's output spectrum. The receiving angle is selected to be 0.05 milliradians × 0.05 milliradians, and the coil is a single-sided coil. It can be observed that at the fundamental wave, after the coil is opened, the undulator's radiation flux is reduced to 58.5% of the value 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 conventional APPLE-II undulator, which has almost no high-order harmonic flux on its axis, the high-order harmonic flux increases by four orders of magnitude after the coil is turned on.

[0089] Figure 3The simulated results show the effect of the coil opening and closing state on the undulator output spectrum in the above scheme. The receiving angle is selected as 0.05 milliradians × 0.05 milliradians, and the coil is 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% of the coil closing state. 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 opened, 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 a double-sided coil. Figure 4 Figure (d) shows the spot distribution corresponding to the second harmonic when using a double-sided coil. A single-sided coil significantly increases the flux of higher harmonics at small acceptance angles, but the spot is less regular. Using a double-sided coil, the higher harmonic spot becomes centrally symmetrical, with a Gaussian distribution and a shape very 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 significantly enhance 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 beamline modification, thereby improving the efficiency of user experiments and the overall performance of the device.

[0092] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection 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 any form of implication that the information constitutes prior art already known to those skilled in the art.

Claims

1. A method for generating high-order harmonics on a circularly polarized axis based on undulator radiation, characterized in that: include: Step 1: Disposing a plurality of electromagnetic coils 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 polarity of the current in the laterally adjacent electromagnetic coils is 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 is respectively arranged on the upper surface and the lower surface of the electron beam vacuum tube of the circular polarization undulator along the forward 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 circularly polarized 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 circularly polarized 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 circularly polarized 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, 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 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 polarity in the laterally adjacent electromagnetic coils is different, 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.

7. The device for generating high-order harmonics on a circularly polarized 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 forward 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: 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.

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 circularly polarized axis based on undulator radiation according to claim 6 or 7, characterized in that: Also includes: The energy of the monochromator is set to be the same as the energy of the corresponding high harmonic of the circularly polarized undulator radiation.

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