Pulse electron gun and uniform electron beam generation method

Abstract

The invention provides a pulse electron gun and a uniform electron beam generation method, and the pulse electron gun comprises a cathode which is used for emitting electrons; the femtosecond laser is used for generating ultra-short pulse laser, and the cathode absorbs the ultra-short pulse laser to excite and form a pulse electron beam; the anode acceleration structure is used for accelerating the pulse electron beam; and the focusing shaping structure is used for carrying out collimation shaping on the accelerated pulse electron beam to obtain a uniform pulse electron beam. The focusing shaping structure is arranged in the pulse electron gun, the transverse electron density distribution of the pulse electron beam is homogenized, beam bunch expansion caused by the space charge effect can be effectively restrained, and the problems that in the prior art, a radio frequency bunching device is large in size and large in size are solved. The equipment cost is high; and the time jitter may be caused by using a radio frequency compression technology.

Description

Technical Field

[0001] This application belongs to the field of electron microscopy imaging technology, and in particular relates to a pulsed electron gun and a method for generating a uniform electron beam. Background Technology

[0002] Electron microscopy is an advanced microscopy technique that uses the interaction between an electron beam and matter to obtain high-resolution images. With the spatial resolution of modern electron microscopy reaching the sub-angstrom level, developing electron microscopy with ultra-high temporal resolution to directly observe various ultrafast dynamic processes has become an urgent need and an important direction for scientific research. The ultrafast electron gun, which generates ultrashort pulsed electron beams, is the core component for achieving ultra-high temporal resolution electron microscopy. However, due to the space charge effect, electrons are continuously subjected to Coulomb repulsion after being excited to form a pulsed electron beam, causing the pulsed electron beam to broaden. The space charge effect refers to the physical phenomenon where charged particles, when concentrated in a local area, experience distortions in their trajectory, energy distribution, and density distribution due to Coulomb repulsion. Coulomb repulsion refers to the repulsion between like charges. To achieve higher temporal resolution, it is necessary to effectively compress the length of the pulsed electron beam during its generation.

[0003] Current pulsed electron beam compression mainly relies on radio frequency (RF) technology, which applies energy chirping to the electron beam using RF signals at a frequency of around 3 GHz, thereby compressing the electrons during transmission. However, RF pulse compression technology has the following technical problems: RF devices are bulky and costly, making them difficult to apply to electron microscopy systems; RF compression technology inherently suffers from timing jitter, affecting the system's time resolution. Summary of the Invention

[0004] The purpose of this application is to provide a pulsed electron gun and a method for generating a uniform pulsed electron beam, in order to solve the problems of time jitter caused by using radio frequency compression technology to compress the pulsed electron beam in the prior art, as well as the large size and high cost of radio frequency focusing devices, which make them difficult to apply.

[0005] To address the aforementioned technical problems, in a first aspect, the present invention provides a pulsed electron gun, the pulsed electron gun comprising:

[0006] Cathode, used to emit electrons;

[0007] A femtosecond laser is used to generate ultrashort pulse lasers, wherein the cathode absorbs the ultrashort pulse lasers and is excited to form a pulsed electron beam;

[0008] An anode acceleration structure is used to accelerate the pulsed electron beam;

[0009] Focusing shaping structures are used to homogenize, collimate, and shape the accelerated pulsed electron beam.

[0010] In one embodiment, the focusing shaping structure includes:

[0011] At least two shaping structures are used to homogenize and shape the accelerated pulsed electron beam;

[0012] A focusing structure is used to strongly focus the pulsed electron beam output by the shaping structure to obtain a uniform pulsed electron beam.

[0013] In one embodiment, the focusing shaping structure includes:

[0014] A focusing structure is used to strongly focus the accelerated pulsed electron beam;

[0015] At least two shaping structures are used to homogenize and shape the pulsed electron beam output by the focusing structure.

[0016] In one embodiment, the shaping structure is an octagonal magnet, and the focusing structure is a magnetic focusing coil.

[0017] In one embodiment, the pulsed electron gun is further provided with a conduit, and the shaping structure and the focusing structure are respectively disposed on the conduit, the conduit being used to transmit the pulsed electron beam.

[0018] In one embodiment, the anode acceleration structure includes a first electrode and a second electrode, an electrostatic field is provided between the cathode and the first electrode, and the first electrode is located between the second electrode and the cathode.

[0019] In one embodiment, the first electrode is radially disposed on the inner sidewall of the pulsed electron gun, and the first electrode has at least one inclined surface, the extension line of the inclined surface making an angle of 50° to 60° with the direction of motion of the pulsed electron beam.

[0020] In one embodiment, at least one third electrode is provided between the first electrode and the second electrode.

[0021] In one embodiment, the pulsed electron gun further includes a fourth electrode disposed around the cathode to suppress electron emission outside the cathode's emitting end.

[0022] In one embodiment, the distance between the cathode and the anode acceleration structure is 4-12 mm, and the length of the anode acceleration structure is 12-25 mm.

[0023] In one embodiment, the distance between the cathode and the anode acceleration structure is less than 1 mm, and the length of the anode acceleration structure is less than 15 mm.

[0024] In one embodiment, the pulsed electron gun is further provided with an aperture assembly and a frequency multiplier. The aperture assembly is located between the anode acceleration structure and the focusing shaping structure. The aperture assembly is used to filter out stray electrons in the pulsed electron beam. The frequency multiplier is used to control the wavelength of the ultrashort pulse laser irradiating the cathode surface.

[0025] Secondly, the present invention provides a method for generating a uniform pulsed electron beam, applied to the aforementioned pulsed electron gun, the method comprising:

[0026] Electrons emitted from the cathode are irradiated with an ultrashort pulse laser, and the electrons absorb the ultrashort pulse laser to form a pulsed electron beam;

[0027] The pulsed electron beam is accelerated to obtain an accelerated pulsed electron beam;

[0028] A nonlinear magnetic field is used to homogenize, collimate, and shape the transverse electron density of the accelerated pulsed electron beam to generate a uniform pulsed electron beam.

[0029] As described above, the pulsed electron gun and uniform pulsed electron beam generation method of this application have the following beneficial effects:

[0030] This application, by setting a focusing and shaping structure inside the pulse electron gun, homogenizes the lateral electron density distribution of the pulse electron beam, effectively suppressing the bunch expansion caused by the space charge effect. This solves the problems of large size and high cost of existing radio frequency focusing devices, as well as the time jitter problem that may be caused by the use of radio frequency compression technology. Attached Figure Description

[0031] Figure 1 The diagram shown is a schematic diagram of the principle of a pulse electron gun provided as an exemplary embodiment of this application.

[0032] Figure 2 The diagram shown is a schematic diagram of the principle structure of a pulse electron gun provided as another exemplary embodiment of this application.

[0033] Figure 3 The diagram shown is a schematic diagram of the principle structure of a pulse electron gun provided as another exemplary embodiment of this application.

[0034] Figure 4 The diagram shown is a cross-sectional structural schematic of an octagonal magnet according to an embodiment of this application.

[0035] Figure 5 The diagram shown is a schematic representation of the electron density distribution of the original pulsed electron beam along the lateral direction in an embodiment of this application.

[0036] Figure 6The diagram shows the electron density distribution of the homogenized pulsed electron beam in the lateral direction according to an embodiment of this application.

[0037] Figure 7 The diagram shows the distribution of space charge force within the original pulsed electron beam in an embodiment of this application.

[0038] Figure 8 The diagram shows the distribution of space charge force of the pulsed electron beam after homogenization according to an embodiment of this application.

[0039] Figure 9 The diagram shown is a schematic representation of the phase space before the pulsed electron beam transmission after homogenization, according to an embodiment of this application.

[0040] Figure 10 The diagram shown is a phase space schematic of the pulsed electron beam after homogenization and transmission for 0.1 meters according to an embodiment of this application.

[0041] Figure 11 The diagram shown is a schematic representation of the Gaussian distribution of the original pulsed electron beam propagation phase space in an embodiment of this application.

[0042] Figure 12 The diagram shown is a phase space schematic of the original pulsed electron beam with a Gaussian distribution after transmission for 0.1 meters, as described in an embodiment of this application.

[0043] Figure 13 The diagram shown is a flowchart of a uniform pulsed electron beam generation method provided in another exemplary embodiment of this application.

[0044] Instruction manual illustrations and reference numerals:

[0045] 1. Cathode; 2. Femtosecond laser; 3. Anode acceleration structure; 31. First electrode; 32. Second electrode; 33. Third electrode; 4. Focusing and shaping structure; 41. Shaping structure; 42. Focusing structure; 5. Aperture assembly; 51. First aperture; 52. Second aperture; 53. Third aperture; 6. Frequency doubler; 7. Fourth electrode. Detailed Implementation

[0046] The following specific examples illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification. This application can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this application. It should be noted that, unless otherwise specified, the following embodiments and features in the embodiments can be combined with each other.

[0047] It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of this application. Therefore, the drawings only show the components related to this application and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0048] Ultrashort pulse lasers refer to lasers with pulse widths ranging from picoseconds (10⁻¹² seconds) to femtoseconds (10⁻¹² seconds). 5 Laser technology on the order of seconds (seconds) has extremely high peak power and ultra-fast time resolution.

[0049] Radio frequency (RF) compression technology is a key technology for compressing ultrashort pulse electron beams (femtosecond / picosecond scale). Its core lies in utilizing the longitudinal periodic electric field generated by the RF cavity to shorten the pulse length through a process of "energy modulation + dispersion compression." Energy modulation refers to the process where, when the pulse electron beam enters the RF cavity, the earlier arriving pulse electron beam encounters a deceleration phase (where the electric field direction is opposite to the beam's direction of motion), gaining negative energy (i.e., deceleration); the later arriving pulse electron beam encounters an acceleration phase (where the electric field direction is the same as the beam's direction of motion), gaining positive energy (i.e., acceleration). Dispersion compression refers to the process where, after energy modulation, the pulse electron beam enters the dispersive element. Higher-energy pulse electron beams (later arriving pulse electron beams) have smaller trajectory curvature and travel shorter paths, while lower-energy pulse electron beams (earlier arriving pulse electron beams) have larger trajectory curvature and travel longer paths. Ultimately, the later arriving pulse electron beam catches up with the earlier arriving pulse electron beam, and the pulse length is compressed.

[0050] The core requirement of radio frequency (RF) compression is that the arrival time of the pulsed electron beam must be strictly synchronized with a specific phase of the RF electric field. Specifically, the deceleration phase is given to the earlier-arriving pulsed electron beam, and the acceleration phase is given to the later-arriving pulsed electron beam. If there are even slight fluctuations in the arrival time of the pulsed electron beam, known as time jitter, the phase of the RF electric field encountered by the pulsed electron beam will deviate from the ideal value, thus deteriorating the compression effect. The cause of time jitter may include fluctuations in the arrival time of ultrashort pulsed lasers due to the stability of the light source, directly leading to jitter in the emission time of the pulsed electron beam.

[0051] The sub-angstrom level signifies that imaging technology can resolve details smaller than 0.5 angstroms (i.e., 50 picometers).

[0052] Depending on the electron beam energy range, electron microscopy encompasses various types, from low-energy electron microscopy (0~100 eV) to medium- and high-energy transmission electron microscopy (50~300 keV). Low-energy electron microscopy is mainly used for surface structure analysis, utilizing the elastic backscattering of electrons to obtain real-space or diffraction-space images of the surface. Medium- and high-energy electron microscopy, on the other hand, can achieve transmission imaging and diffraction analysis of the internal microstructure of materials, and is widely used in research fields such as materials science, biology, and nanoscience.

[0053] Researchers have raised a demand for high-resolution measurements with time-resolved capabilities, hoping to capture ultrafast dynamic processes on picosecond to femtosecond timescales, including: transient behavior of surface-adsorbed molecules, carrier-lattice interactions in two-dimensional materials, structural evolution during phase transitions, intermediate states in catalytic reactions, and conformational changes in biomolecules. These ultrafast dynamic processes often determine the fundamental properties and functions of materials, and are of great scientific significance for understanding catalytic mechanisms, optimizing the performance of energy materials, developing novel electronic devices, and exploring the molecular mechanisms of life processes.

[0054] To address the aforementioned technical problems, the present application provides a pulsed electron gun according to the following embodiments, such as... Figure 1 As shown, the pulsed electron gun includes: a cathode 1 for emitting electrons; a femtosecond laser 2 for generating ultrashort pulse lasers, the emitted electrons absorbing the ultrashort pulse lasers to form a pulsed electron beam; an anode accelerating structure 3 for accelerating the pulsed electron beam to form a pulsed electron beam; and a focusing and shaping structure 4 for homogenizing, collimating, and shaping the accelerated pulsed electron beam.

[0055] When the ultrashort pulse laser generated by the femtosecond laser 2 is frequency multiplied by the frequency multiplier 6 and reaches the cathode 1, the electrons on the surface of the cathode 1 absorb the energy of one or more photons and are excited to a high energy level to form a pulsed electron beam. The pulsed electron beam is accelerated to the highest energy by passing through the first electrode 31 and the second electrode 32 in sequence. The highest energy can be controlled in the range of 10-300keV according to the actual working conditions.

[0056] This embodiment uses a focusing and shaping structure within the pulsed electron gun to homogenize the lateral electron density distribution of the pulsed electron beam. This effectively suppresses the bundle expansion caused by the space charge effect, solving the problems of large size and high cost of existing radio frequency devices, as well as the timing jitter that may be caused by the use of radio frequency compression technology.

[0057] In some embodiments, such as Figures 1 to 3 As shown, the pulsed electron gun is also provided with an aperture assembly 5, which is located between the anode acceleration structure 3 and the focusing shaping structure 4. The aperture assembly 5 is used to filter stray electrons in the pulsed electron beam.

[0058] In some embodiments, the aperture assembly 5 includes an aperture plate and a driving device (not shown). The aperture plate has at least three aperture holes with different aperture sizes. The driving device is driven to move the aperture plate by inserting and pulling it out, so that the aperture holes with different aperture sizes on the aperture plate can be selectively aligned with the electron optical axis, thereby modulating the lateral size of the pulsed electron beam in the range of 40μm to 200nm.

[0059] For example, a first aperture 51, a second aperture 52, and a third aperture 53 are evenly distributed around the center of the aperture plate. The aperture of the first aperture 51 can be set to 40 μm, the aperture of the second aperture 52 to 100 μm, and the aperture of the third aperture 53 to 200 nm. The driving device drives the aperture plate to rotate around its central axis, thereby switching the first aperture 51, the second aperture 52, or the third aperture 53 to a working position aligned with the path of the pulsed electron beam, thereby realizing the selection and modulation of the lateral size of the pulsed electron beam.

[0060] Assuming Figures 1 to 3 The aperture size for the pulsed electron beam to pass through is 40 μm, which is the first aperture 51. In actual production applications, the lateral dimension of the pulsed electron beam is approximately equal to the aperture diameter (this is common knowledge to those skilled in the art and will not be described in detail here). Therefore, the lateral dimension of the pulsed electron beam passing through the first aperture 51 is approximately 40 μm.

[0061] This embodiment sets up an aperture assembly 5, which uses the aperture in the aperture assembly 5 to limit the lateral expansion of the electron beam, allowing only pulsed electron beams within the aperture range to pass through. This controls the lateral size of the pulsed electron beam, solving the problem of lateral diffusion of the pulsed electron beam caused by the divergence angle and space charge effect of the field emission source, which prevents it from being focused into a small beam spot and affects the imaging resolution.

[0062] In some embodiments, such as Figures 1 to 3 As shown, the cathode 1 is a needle-tip field emission cathode, which can be a cold field emission type or a Schottky field emission type. For a charged conductor tip, the electric field strength on its surface is inversely proportional to the radius of curvature of the tip, so that under the same voltage conditions, the local electric field strength of the electrons emitted in this embodiment is stronger.

[0063] In some embodiments, such as Figures 1 to 3 As shown, when the needle-shaped field emission cathode is a cold field emission type, the tip radius of the needle-shaped field emission cathode is typically within 200 nanometers, and the electric field strength on the tip surface does not exceed 5 × 10⁻⁶. 9At V / m, the energy barrier is lowered. When a pulsed laser irradiates the cathode tip, electrons absorb photon energy and escape from the cathode surface, forming an electron beam. Cold field emission has high electron beam brightness and good monochromaticity, making it the preferred electron source for high-performance electron microscopy systems.

[0064] The distance between the cathode and the anode acceleration structure 3 can be set to 4~12 mm, and the length of the anode acceleration structure 3 can be set to 12~25 mm.

[0065] In a preferred embodiment, the distance between the needle-shaped field emission cathode and the anode acceleration structure 3 is set to 6.5 mm, and the length of the anode acceleration structure 3 is set to 20 mm. This can prevent the high-voltage breakdown caused by the distance between the needle-shaped field emission cathode and the anode acceleration structure 3 being too short, and can also prevent the accumulation of a large space charge effect caused by the distance between the needle-shaped field emission cathode and the anode acceleration structure 3 being too long, thus avoiding space waste.

[0066] In some embodiments, such as Figures 1 to 3 As shown, when the tip-shaped field emission cathode is a Schottky field emission type, the distance between the tip-shaped field emission cathode and the anode accelerating structure 3 is less than 1 mm, and the length of the anode accelerating structure 3 is less than 15 mm. Due to the manufacturing process and working environment of Schottky emission cathodes, the diameter of its cathode tip is usually less than 1 micrometer, and the size of its corresponding excited electron beam is also larger. Therefore, the working distance between its cathode and anode is generally much smaller than that of the cold field emission scheme, in order to minimize the increase in beam size caused by element aberrations and space charge effects in the accelerating optical path.

[0067] In this embodiment, the distance between the needle-shaped field emission cathode and the anode acceleration structure 3 is less than 1 mm, and the length of the anode acceleration structure 3 is less than 15 mm.

[0068] In some embodiments, such as Figures 1 to 3 As shown, the anode acceleration structure 3 includes a first electrode 31 and a second electrode 32. An electrostatic field is provided between the cathode 1 and the first electrode 31, and an electrostatic field is provided between the first electrode 31 and the second electrode 32. The first electrode 31 is located between the second electrode 32 and the cathode 1.

[0069] Specifically, the first electrode 31 is the negative electrode, the second electrode 32 is grounded, the potential of the first electrode 31 is lower than that of the cathode, and the potential of the second electrode 32 is lower than that of the first electrode 31.

[0070] In this embodiment, an electrostatic field is set between the cathode 1 and the first electrode 31 to reduce the energy barrier on the tip surface of the needle-type field emission cathode.

[0071] In this embodiment, a first electrode 31 and a second electrode 32 are preferably provided. The first electrode 31 is radially symmetrically arranged on the inner wall of the pulse electron gun, and the second electrode 32 is radially symmetrically installed on the inner wall of the pulse electron gun, so that the pulse electron beam can generate a lateral focusing force during acceleration and effectively control the aberrations in the pulse electron beam's motion path.

[0072] In another possible embodiment, the first electrode 31 and the second electrode 32 may be asymmetrically disposed on the inner wall of the pulse electron gun.

[0073] For example, the first electrode 31 and the second electrode 32 can be annular, conical, or frustum-shaped, etc., and are not limited thereto.

[0074] In this embodiment, by setting the first electrode 31 radially symmetrically on the inner wall of the pulse electron gun and the second electrode 32 radially symmetrically mounted on the inner wall of the pulse electron gun, the pulse electron beam can generate a lateral focusing force during acceleration, thereby making the difference in focusing force between off-axis emitted electrons and on-axis emitted electrons smaller, thus significantly reducing spherical aberration and preventing astigmatism that may be caused by electrode asymmetry.

[0075] In some embodiments, such as Figures 1 to 3 As shown, the pulsed electron gun is also equipped with a frequency multiplier 6, which is used to control the wavelength of the ultrashort pulse laser irradiating the cathode surface.

[0076] In this embodiment, the wavelength of the ultrashort pulse laser is controlled by setting a frequency multiplier 6, thereby ensuring that the energy of one or more photons absorbed by the electrons in the cathode 1 is greater than the potential barrier, and a transition occurs to form a pulsed electron beam.

[0077] In some embodiments, such as Figures 1 to 3 As shown, the first electrode 31 is radially disposed on the inner sidewall of the pulse electron gun. The first electrode 31 has at least one inclined surface. The angle between the extension line of the inclined surface and the direction of motion of the pulse electron beam is 50°~60°, wherein the direction of motion of the pulse electron beam is emitted along the axial direction of the pulse electron gun.

[0078] In this embodiment, the first electrode 31 is symmetrically arranged radially on the inner wall of the pulsed electron gun, and the second electrode 32 is symmetrically installed radially on the inner wall of the pulsed electron gun. The angle between the extension line of the inclined surface of the first electrode 31 and the direction of motion of the pulsed electron beam is 50°~60°, which can not only effectively focus or converge the electron beam and offset some of the beam divergence caused by the charge repulsion effect, but also guide the electron beam to move along the predetermined path more effectively. At the same time, it can more effectively control the aberrations in the movement path of the pulsed electron beam.

[0079] In some embodiments, such as Figures 1 to 3As shown, the focusing and shaping structure 4 includes: at least two shaping structures 41 for homogenizing and shaping the accelerated pulsed electron beam; and a focusing structure 42 for strongly focusing the pulsed electron beam output by the shaping structure 41.

[0080] In some embodiments, at least two focusing structures 42 are provided.

[0081] This embodiment, by setting at least two shaping structures, can better achieve the homogenization of the pulsed electron beam in the lateral direction, effectively suppress the space charge effect of the pulsed electron beam during transmission, and improve the spatial and temporal resolution of the electron microscopy imaging system.

[0082] In some embodiments, the shaping structure 41 is an octagonal magnet and the focusing structure 42 is a magnetic focusing coil.

[0083] This embodiment achieves homogenization of the pulsed electron beam in the lateral direction by setting an octagonal magnet, effectively suppressing the space charge effect of the pulsed electron beam during transmission, and improving the spatial and temporal resolution of the electron microscopy imaging system.

[0084] The octagonal magnet used in this embodiment is significantly smaller than that of the radio frequency device and is easy to install, solving the problem that radio frequency devices in the prior art are bulky and difficult to apply.

[0085] Specifically, the magnetic induction intensity of the octagonal magnet and the magnetic focusing coil can be adjusted according to the energy of the transmitted pulsed electron beam.

[0086] For example, when the energy of the transmitted pulsed electron beam is 15keV, the magnetic induction intensity of the octagonal magnet should not exceed 0.1T, and the magnetic induction intensity of the magnetic focusing coil should not exceed 0.3T.

[0087] In some embodiments, the octagonal magnet can be replaced with a quadrupole magnet, a hexagonal magnet, or even a combined multipole magnet, etc., as needed, and will not be elaborated on here.

[0088] In some embodiments, such as Figures 1 to 3 As shown, the pulsed electron gun also has a pipe, on which the shaping structure 41 and the focusing structure 42 are respectively mounted. The pipe is used to transmit the pulsed electron beam.

[0089] In this embodiment, the shaping structure 41 and the focusing structure 42 are directly mounted on the pipe, so that the shaping structure 41 and the focusing structure 42 can better homogenize and focus the pulsed electron beam. At the same time, by setting up the pipe, the movement speed of the electron beam can be prevented from being affected by other impurities during the movement process, and the homogenization, focusing and collimation process of the pulsed electron beam can be disrupted.

[0090] In this embodiment, the symmetrically arranged magnetic poles within the shaping structure 41 generate a nonlinear magnetic field that applies a nonlinear Lorentz force to the off-axis electrons in the pulsed electron beam. This adjusts the trajectory of the pulsed electron beam, dispersing electrons in high-density regions and concentrating electrons in low-density regions, thereby achieving uniform lateral electron density. The magnetic focal length coil of the focusing structure 42 achieves strong focusing of the pulsed electron beam through the radial focusing force of the axisymmetric axial magnetic field.

[0091] In some embodiments, the conduit is a vacuum conduit. This embodiment uses a pulsed electron beam to transmit the pulsed electron beam, reducing interference from gas molecules. Under non-vacuum conditions, nitrogen, oxygen, and water molecules in the air collide with electrons, causing energy loss or changes in direction of the electron beam, affecting processing accuracy. The vacuum conduit removes gas molecules, reducing the probability of collisions and allowing the electron beam to reach the processing target more accurately. Simultaneously, when electrons move in a vacuum, they will not deviate from their path due to gas molecule obstruction, ensuring the linearity and energy stability of the electron beam.

[0092] In some possible embodiments, the focusing and shaping structure 4 includes: a focusing structure 42 for strongly focusing the accelerated pulsed electron beam; and at least two shaping structures 41 for homogenizing and shaping the pulsed electron beam output from the focusing structure 42.

[0093] In some embodiments, such as Figure 2 As shown, at least one third electrode 33 is provided between the first electrode 31 and the second electrode 32.

[0094] This embodiment can prevent high voltage breakdown that may occur when the pulsed electron gun accelerates the pulsed electron beam to a value greater than 300 keV by providing at least one third electrode 33 between the first electrode 31 and the second electrode 32.

[0095] Specifically, the third electrode 33 is the negative electrode, and its potential is between the potential of the first electrode 31 and the potential of the second electrode 32. In another possible embodiment, the third electrode 33 may also be grounded.

[0096] In another possible embodiment, the third electrode 33 may be disposed symmetrically or asymmetrically on the inner wall of the pulse electron gun.

[0097] In some embodiments, such as Figure 2 As shown, the third electrode 33 is symmetrically disposed on the inner wall of the pulsed electron gun. The cross-section of the third electrode 33 is conical, and the conical surfaces on both sides of the third electrode 33 form an angle of 50° to 60° with the direction of motion of the pulsed electron beam. In a preferred embodiment of this example, the conical surfaces on both sides of the third electrode 33 are set to form an angle of 54° with the direction of motion of the pulsed electron beam.

[0098] In this embodiment, the third electrode 33 is symmetrically disposed on the inner wall of the pulsed electron gun. Setting the conical surfaces on both sides of the third electrode 33 at an angle of 50° to 60° with the direction of motion of the pulsed electron beam effectively focuses or converges the electron beam, offsetting some of the beam divergence caused by charge repulsion. This allows the electric field direction to more effectively guide the electron beam along a predetermined path, while also more effectively controlling aberrations in the pulsed electron beam's path. Furthermore, setting the conical surfaces on both sides of the third electrode 33 at an angle of 54° with the direction of motion of the pulsed electron beam achieves the most effective focusing or converging effect on the electron beam.

[0099] In some embodiments, such as Figure 3 As shown, when the needle-shaped field emission cathode is a Schottky field emission type, the pulse electron gun also includes a fourth electrode 7, which is disposed around the cathode 1 to suppress electron emission outside the emission end of the cathode 1.

[0100] Specifically, the fourth electrode is negative and its potential is lower than that of the cathode 1.

[0101] In some embodiments, the fourth electrode 7 can be a suppression electrode.

[0102] In some embodiments, such as Figure 4 The diagram shows the cross-sectional structure of an octagonal magnet, which consists of four pairs of oppositely polarized magnetic poles symmetrically distributed around a pulsed electron beam, forming a closed high-order magnetic field. By utilizing the nonlinear focusing force of the magnetic field in the octagonal magnet, the transverse electron density of the particles can be homogenized, reducing the space charge force in the pulsed electron beam's core region.

[0103] In some embodiments, such as Figure 5 This diagram illustrates a Gaussian distribution of the electron density projection along the transverse direction (x or y) of the original pulsed electron beam. The original pulsed electron beam is the electron beam that has not been homogenized by an octagonal magnet. The space charge force of the Gaussian-distributed original pulsed electron beam exhibits a non-linear distribution along the transverse direction, typically reaching its maximum at approximately 1.5 to 2 times the root-mean-square radius of the original pulsed electron beam.

[0104] like Figure 6 This diagram illustrates a uniform electron density distribution in the transverse direction of a pulsed electron beam after homogenization with an octagonal magnet. From... Figure 5 and Figure 6 It can be seen that the electron density of the homogenized pulsed electron beam is significantly lower than that of the Gaussian distributed pulsed electron beam.

[0105] In some embodiments, such as Figure 7A schematic diagram of the space charge force distribution within the original pulsed electron beam is shown, such as... Figure 8 A schematic diagram of the space charge force distribution of the homogenized pulsed electron beam is shown. The space charge force within the beam nucleus region exhibits a linear distribution in the transverse direction, and decreases significantly after leaving the nucleus region. (Figure...) Indicates standard deviation, It represents the space charge force.

[0106] In some embodiments, such as Figures 9 to 10 The diagrams show the lateral and longitudinal phase space distributions of the homogenized pulsed electron beam before and after propagation for 0.1 meters in the electron optical path. (See diagram for example.) Figures 11 to 12 The diagrams showing the phase space distribution of the Gaussian-distributed original pulsed electron beam before and after propagation for 0.1 meters in the electron optical path are presented. The vertical axes px and pz represent momentum, respectively.

[0107] from Figures 9 to 12 It can be seen that the homogenized pulsed electron beam can effectively reduce pulsed electron beam distortion and the generation of stray particles during transmission, and can effectively improve the quality of the pulsed electron beam.

[0108] When the ultrashort pulse laser generated by the femtosecond laser 2 is frequency multiplied by the frequency multiplier 6 and reaches the cathode 1, the electrons on the surface of the cathode 1 absorb the energy of one or more photons and are excited to a high energy level to form a pulsed electron beam. The pulsed electron beam is accelerated to the highest energy by passing through the first electrode 31 and the second electrode 32 in sequence. The highest energy can be controlled within the range of 10~300keV according to the actual working conditions. The accelerated pulsed electron beam is filtered out by the aperture assembly 5 to remove stray electrons in the pulsed electron beam. The filtered pulsed electron beam is homogenized, collimated and shaped by the focusing and shaping structure 4 to effectively suppress the bundle expansion caused by the space charge effect. This solves the problems of large size and high cost of radio frequency devices in the prior art, as well as the time jitter problem that may be caused by the use of radio frequency compression technology.

[0109] Based on the above working principle, this embodiment provides a method for generating a uniform pulsed electron beam, such as... Figure 13 As shown, the method includes:

[0110] Step 101: Irradiate the electrons emitted from the cathode with an ultrashort pulse laser. The electrons absorb the ultrashort pulse laser to form a pulsed electron beam.

[0111] Step 102: Accelerate the pulsed electron beam to obtain an accelerated pulsed electron beam;

[0112] Step 103: Use a nonlinear magnetic field to homogenize, collimate and shape the transverse electron density of the accelerated pulsed electron beam to generate a uniform pulsed electron beam.

[0113] This embodiment effectively suppresses the bundle expansion caused by the space charge effect by homogenizing the lateral electron density distribution of the pulsed electron beam. It solves the problems of large size and difficulty in application of radio frequency devices in the prior art, high equipment cost caused by low radio frequency signal frequency and the need for high magnetic field strength, as well as the time jitter problem that may be caused by using radio frequency technology to compress the pulsed electron beam.

[0114] The descriptions of the processes or structures corresponding to the above figures each have their own emphasis. For parts of a process or structure that are not described in detail, please refer to the relevant descriptions of other processes or structures.

[0115] The above embodiments are merely illustrative of the principles and effects of this application and are not intended to limit this application. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of this application. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in this application should still be covered by the claims of this application.

Claims

1. A pulsed electron gun, characterized in that, The pulse electron gun includes: Cathode, used to emit electrons; A femtosecond laser is used to generate ultrashort pulse lasers, wherein the cathode absorbs the ultrashort pulse lasers and is excited to form a pulsed electron beam; An anode acceleration structure is used to accelerate the pulsed electron beam; The focusing shaping structure is used to homogenize, collimate, and shape the accelerated pulsed electron beam to obtain a uniform pulsed electron beam.

2. The pulse electron gun according to claim 1, characterized in that, The focused shaping structure includes: At least two shaping structures are used to homogenize and shape the accelerated pulsed electron beam; A focusing structure is used to strongly focus the pulsed electron beam output by the shaping structure.

3. The pulse electron gun according to claim 1, characterized in that, The focused shaping structure includes: A focusing structure is used to strongly focus the accelerated pulsed electron beam; At least two shaping structures are used to homogenize and shape the pulsed electron beam output by the focusing structure.

4. The pulse electron gun according to claim 2 or 3, characterized in that, The shaping structure is an octagonal magnet, and the focusing structure is a magnetic focusing coil.

5. The pulse electron gun according to claim 2 or 3, characterized in that, The pulsed electron gun is also provided with a conduit, and the shaping structure and the focusing structure are respectively disposed on the conduit. The conduit is used to transmit the pulsed electron beam.

6. The pulse electron gun according to claim 1, characterized in that, The anode acceleration structure includes a first electrode and a second electrode. An electrostatic field is provided between the cathode and the first electrode, and between the first electrode and the second electrode. The first electrode is located between the second electrode and the cathode.

7. The pulse electron gun according to claim 6, characterized in that, The first electrode is radially disposed on the inner sidewall of the pulsed electron gun. The first electrode and the second electrode each have at least one inclined surface, and the angle between the extension line of the inclined surface and the direction of motion of the pulsed electron beam is 50° to 60°.

8. The pulse electron gun according to claim 6 or 7, characterized in that, At least one third electrode is provided between the first electrode and the second electrode.

9. The pulse electron gun according to claim 1, characterized in that, The pulsed electron gun also includes a fourth electrode, which is disposed around the cathode to suppress electron emission outside the cathode's emitting end.

10. The pulse electron gun according to claim 1, characterized in that, The distance between the cathode and the anode acceleration structure is 4-12 mm, and the length of the anode acceleration structure is 12-25 mm.

11. The pulse electron gun according to claim 1, characterized in that, The distance between the cathode and the anode acceleration structure is less than 1 mm, and the length of the anode acceleration structure is less than 15 mm.

12. The pulse electron gun according to claim 1, characterized in that, The pulsed electron gun is also equipped with an aperture assembly and a frequency multiplier. The aperture assembly is located between the anode acceleration structure and the focusing shaping structure. The aperture assembly is used to filter out stray electrons in the pulsed electron beam. The frequency multiplier is used to control the wavelength of the ultrashort pulse laser irradiating the cathode surface.

13. A method for generating a uniform pulsed electron beam, applied to the pulsed electron gun according to any one of claims 1 to 12, the method comprising: Electrons emitted from the cathode are irradiated with an ultrashort pulse laser, and the electrons absorb the ultrashort pulse laser to form a pulsed electron beam; The pulsed electron beam is accelerated to obtain an accelerated pulsed electron beam; A nonlinear magnetic field is used to homogenize, collimate, and shape the transverse electron density of the accelerated pulsed electron beam to generate a uniform pulsed electron beam.

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