A DC ultrafast electron gun with electrostatic single lens

Abstract

This invention provides a DC photocathode ultrafast electron gun with an electrostatic single lens, comprising a cathode, an anode, an electrostatic single lens electrode plate, a ceramic support, a single lens support ring, and a single lens bracket. The anode is a disc with a central through-hole and smooth surfaces. The single lens electrode plate is a double-layered disc structure with smooth surfaces, comprising a circular top disc and a circular protruding bottom disc. The diameter of the circular top disc is larger than that of the circular protruding bottom disc, resulting in a stepped structure. The electrostatic single lens electrode plate has a central through-hole, and the circular edges on both sides of the through-hole are rounded. The single lens bracket is a bimetallic disc connection structure, with a smooth upper surface on the top disc and a central through-hole. The single lens support ring is a double-ring structure, comprising a top ring and a bottom ring. The inner diameter of the bottom ring is smaller than that of the top ring, thus creating a stepped structure internally. The support ring has a through-hole on its side.

Description

Technical Field

[0001] The invention belongs to the field of ultrafast electron diffraction, and in particular relates to a direct current photocathode ultrafast electron gun focusing by an electrostatic single lens. Background Art

[0002] Ultrafast electron diffraction (UED) is a technology that uses ultrashort pulse lasers to achieve time-resolved pump detection. Its principle is to use a spectrometer to divide ultrashort pulse lasers into two beams, one beam is used to excite the cathode of the electron gun to generate probe electrons, and the other beam pumps the reaction area. By controlling the optical path difference between the two laser beams, the probe electrons can advance or delay the detection of the reaction area, thereby achieving time-resolved measurement. Ultrafast electron diffraction technology has made great progress, and transmission diffraction, surface reflection diffraction, shadow and schlieren imaging, ultrafast transmission electron microscopy, ultrafast scanning electron microscopy and other schemes have gradually emerged, and the requirements for detection systems have gradually increased. Among them, the shadow and schlieren imaging method has been applied to plasma dynamics diagnosis to a certain extent, and is expected to be applied to the diagnosis of laser inertial confinement fusion. The DC photocathode electron gun is the probe electron source of ultrafast electron diffraction technology and is the core part of this technology.

[0003] Due to the repulsive force between electrons, the probe electron pulse will be broadened in the process of drifting to the sample, and the radial radius will increase, resulting in reduced spatial resolution. Magnetic lenses can focus electron pulses, but they will cause the electrons in the electron pulses to rotate perpendicular to the propagation direction, producing a spiral path, which is not conducive to shadow and schlieren imaging in ultrafast electron diffraction technology. Electrostatic lenses use the generated electrostatic field to focus the electron pulses, and no spiral path will be generated. Magnetic lenses and electrostatic lenses can be used to focus together to improve the performance of ultrafast electron diffraction systems. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a DC photocathode ultrafast electron gun with an electrostatic single lens, including a cathode, an anode, and a single lens electrode plate determined by the results of an electric field and particle simulation program; and for fixing and installing the above-mentioned components, it also includes a ceramic support, a single lens support ring, and a single lens bracket.

[0005] The anode is a circular disk with smooth surfaces on both sides, and a first through hole with a diameter of 0.2 mm is opened in the center of the disk; the single-lens electrode plate is a double-layer disk structure, including a circular top disk and a circular protruding bottom disk, the surfaces of the circular top disk and the circular protruding bottom disk are both smoothed, and the diameter of the circular top disk is larger than that of the circular protruding bottom disk, thereby forming a first step structure on the back side of the single-lens electrode plate, the single-lens electrode plate has an opening on the side, and a second through hole with a diameter of 4 mm is opened in the center of the single-lens electrode plate, and the circular edges on both sides of the second through hole are chamfered, and the chamfer radius is 2 mm.

[0006] According to the simulation results of the electronic pulse computer program, the electric field is required to be symmetrically distributed. The electron gun has a special installation structure to ensure coaxial symmetry, including a ceramic support for fixing the cathode, a single lens support ring for fixing the electrostatic single lens electrode plate, and a single lens bracket for fixing the above-mentioned anode, single lens electrode plate and single lens support ring.

[0007] The single lens support is a double metal disk connection structure, the upper surface of the top disk of the single lens support is smoothed, and a third through hole with a diameter of 0.2 mm is opened in the center of the top disk. The support ring is a double ring structure, which includes a top ring and a bottom ring. The inner diameter of the bottom ring is smaller than that of the top ring, thereby generating a second step structure inside the support ring, and a through hole is opened on the side of the support ring.

[0008] According to a DC ultrafast electron gun with an electrostatic single lens of the present invention, preferably, the anode disk has a diameter of 80 mm and a thickness of 1 mm, the single lens circular top disk has a diameter of 70 mm and a thickness of 2 mm, and the single lens circular protruding bottom disk has a diameter of 60 mm and a thickness of 2 mm. The top ring of the support ring has an outer diameter of 80 mm and an inner diameter of 70 mm, and the bottom ring has an outer diameter of 80 mm and an inner diameter of 60 mm. The outer diameters of the top disk and the bottom disk of the single lens support are both 80 mm.

[0009] According to a DC ultrafast electron gun with an electrostatic single lens of the present invention, preferably, after installation, the back side of the anode disk is 4 mm away from the surface of the circular top disk of the single lens, and the circular protruding bottom disk surface of the single lens is 4 mm away from the upper surface of the top disk of the single lens bracket.

[0010] The electron gun fixing method of the present invention ensures uniform and symmetrical distribution of the electric field, ensures the realization of electron pulse acceleration and convergence, reduces the size of the device, and facilitates transplantation to other devices. The electrostatic single lens can be used in conjunction with the magnetic lens for focusing, and the adjustment of spatial resolution becomes more flexible, expanding the experimental function. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The embodiments of the present invention are further described below with reference to the accompanying drawings, in which:

[0012] Figure 1 It is a three-dimensional view of the internal electron gun structure after the peripheral cavity is cut in the X and Y directions along the central axis Z shown in the figure and one quarter is removed;

[0013] Figure 2 A three-dimensional view of the disassembled structure of an electron gun according to an embodiment of the present invention;

[0014] Figure 3 A three-dimensional view of an electrostatic single lens electrode plate and a supporting ring thereof of an electron gun according to an embodiment of the present invention;

[0015] Figure 4 A three-dimensional view of an electrostatic single lens support for an electron gun according to an embodiment of the present invention;

[0016] Figure 5 This is a simulation diagram of the potential distribution of an electrostatic single lens;

[0017] Figure 6 It is the particle simulation result of the focusing effect of the probe electron pulse under the single lens of the present invention. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail by specific embodiments in conjunction with the accompanying drawings, wherein the same reference numerals represent the same components in each of the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0019] To facilitate drawing description, Figure 1 A coordinate system is drawn in which the Z axis coincides with the central axis of the electron gun.

[0020] Figure 1 The figure is a structural view of the electron gun after a quarter of the electron gun peripheral vacuum chamber 9 is cut along the central axis Z in the X and Y directions. Figure 2 The electron gun is a three-dimensional view of the disassembled structure. The electron gun mainly includes a cathode 1, an anode 2, a cathode fixing ceramic support 3, a single lens support ring 4, an electrostatic single lens electrode plate 5, an electrostatic single lens bracket 6, a cathode vacuum electrode 7, a single lens vacuum electrode 8, an electron gun vacuum chamber 9, a single lens support flange 10, and a cathode support flange 11. The cathode 1 is connected to an external negative high-voltage power supply through the vacuum electrode 7, and the electrostatic single lens electrode plate 5 is connected to an external focusing control power supply through the vacuum electrode 8. The incident laser 13 is incident on the cathode through the laser window 12 to excite the probe electrons.

[0021] The cathode 1 of the electron gun has an axisymmetric structure, and is a hollow metal cylinder as a whole. The top is a bell-like structure. The center of the top of the bell structure is a circular bottom surface, and the circular bottom surface and the hollow cylindrical side surface are arc-shaped connecting surfaces. A light hole 1.1 is opened at the center of the bottom of the cathode, and four through holes 1.2 are distributed next to the light hole 1.1.

[0022] The anode 2 is a disc with both sides polished smooth. The disc has a diameter of 80 mm and a thickness of 1 mm. A through hole 2.1 (the first through hole) with a diameter of 0.2 mm is opened in the center.

[0023] The ceramic support 3 has a through hole 3.1 in the center, which is a laser channel, and four through holes 3.2 are distributed next to the through hole 3.1. The cathode is fixed to the ceramic support 3 through the hole 1.2, and the ceramic support 3 is fixed to the supporting flange 11 through the hole 3.2. The flange 11 has a light hole 11.1 in the center.

[0024] Figure 3 This is a three-dimensional view of the electrostatic single lens electrode plate 5 and its support ring 4. The electrode plate 5 includes a circular top plate 5.4 and a circular protruding bottom plate 5.3. The top plate 5.4 has an outer diameter of 70 mm and a thickness of 2 mm. The bottom plate 5.3 has an outer diameter of 60 mm and a thickness of 2 mm. The entire electrode plate 5 is processed from a whole piece of metal, and the disc surfaces on both sides are polished smooth. The top plate 5.4 has a larger diameter than the bottom plate 5.3, resulting in a step structure 5.6 (the first step structure) behind the electrode plate 5. An electrode connection hole 5.5 is opened on the side of the electrode plate 5, and a through hole 5.1 is opened in the center. The through hole 5.1 has a diameter of 4 mm, and the edges on both sides are rounded chamfered 5.2 ( Figure 5 The chamfer radius is 2mm.

[0025] The support ring 4 is a double-ring structure, including a top ring 4.1 and a bottom ring 4.2. To ensure straightness, the entire support ring 4 is machined from a whole piece of ceramic. The top ring 4.1 has an outer diameter of 80 mm and an inner diameter of 70 mm. The bottom ring 4.2 has an outer diameter of 80 mm and an inner diameter of 60 mm. Since the inner diameter of the bottom ring 4.2 is smaller than that of the top ring 4.1, a step structure 4.3 (the second step structure) is generated. A through hole 4.4 is opened on the side of the support ring 4. During installation, the electrode plate 5 is placed inside the support ring 4, and the step structure 5.6 and the step structure 4.3 are embedded in each other. The electrode connection hole 5.5 is aligned with the through hole 4.4, and the electrode wire of the vacuum electrode 8 is inserted into the two holes, so that the electrode 8 is connected to the electrode plate 5, and the electrode plate 5 and the support ring 4 are fixed.

[0026] Figure 4 The figure is a three-dimensional view of a single lens support 6. The support 6 is a bimetallic disk connection structure. The diameter of the disks on both sides is 80 mm. The upper surface of the top disk needs to be smoothed. A through hole 6.1 (the third through hole) with a diameter of 0.2 mm is opened in the center of the top disk. Four threaded holes 6.2 are opened on the outside of the through hole 6.1 for fixing the support ring 4. A through hole 6.4 is opened in the center of the bottom disk of the support 6 for the electronic pulse channel. Four through holes 6.3 are opened around the through hole 6.4. Bolts can be used to fix the support 6 to the support flange 10 through the through holes 6.3. A through hole 10.1 is opened in the center of the support flange 10. A flange knife edge is opened on the outer back side thereof for connecting with other devices.

[0027] In order to further optimize the surface electric field distribution and ensure surface smoothness, the surfaces of the cathode 1, anode 2, single lens electrode plate 5, and single lens bracket 6 are subjected to mechanical rough polishing, electrolytic polishing, manual fine polishing, and high vacuum cleaning to achieve surface smoothness of micrometer level.

[0028] Electron gun assembly: cathode 1 is fixed to ceramic support 3 through through hole 1.2, ceramic support 3 is fixed to support flange 11 through through hole 3.2, flange 11 is installed to the side of chamber 9 close to incident laser 13, and laser window 12 is installed to the outside of flange 11. Anode 2 is fixed to the surface of top ring 4.1 of support ring 4, and single lens electrode plate 5 is placed on step 4.3 inside support ring 4. Support ring 4 is fixed to the top disk of bracket 6, bracket 6 is fixed to support flange 10, and support flange 10 is installed to the other side of chamber 9. Anode 2, single lens electrode plate 5, and top disk of bracket 6 together constitute the electrostatic single lens of electron gun, wherein the back of anode 2 is 4 mm away from the surface of top disk 5.4 of electrode plate 5, and the surface of bottom disk 5.3 is 4 mm away from the top side surface of top disk of bracket 6.

[0029] Working of electron gun: laser 13 enters the chamber through the chamber laser window 12 and through hole 11.1, then enters the cathode through through hole 3.1 of the ceramic support and cathode through hole 1.1, exciting the metal film on the circular top surface of the cathode to generate electron pulses. The generated electron pulses are accelerated by the electric field between cathode 1 and anode 2, and then pass through the anode center through hole 2.1, electrode plate through hole 5.1, and bracket through hole 6.1. The electric field between anode 2, electrode plate 5, and the top disk of bracket 6 converges the electron pulses. Next, the electron pulses pass through through hole 6.4 and finally leave the chamber through through hole 10.1.

[0030] Figure 5 This is a simulation diagram of the potential distribution of an electrostatic single lens, where the anode potential is 0V, the potential of the disk on the top of the bracket 6 is 0V, and the electrostatic single lens electrode plate 5 is -10000V. The dimensions of the relevant parts have been explained above.

[0031] Figure 6 In order to use the particle simulation program to simulate the electron pulse in this design, the horizontal axis Distance is the distance from the cathode surface in meters, and the vertical axis Radius is the electron pulse radius. The initial radius of the electron pulse is 50 microns, and the initial radial diffusion velocity is set to 0. The acceleration voltage between the cathode and the anode is -100kV. Figure 6The straight dotted line AA' is used to mark the approximate position of the electrostatic single lens. In the electron radius variation curve in the figure, the solid curve is marked with V=-15000, N=100000, which means that the simulation parameters used in this result are -15000V voltage on the single lens electrode 5, and 100000 electrons in the electron pulse. Similarly, the dotted line curve is marked with V=-10000, N=10000, indicating that the simulation parameters used in this result are -10000V voltage on the single lens electrode plate 5, and 10000 electrons in the electron pulse. It can be seen from the figure that the electrostatic single lens in the present invention has a converging effect on the electron pulse.

[0032] It should be clarified here that the size and configuration mentioned in the present invention are only suitable for certain application scenarios, and are not necessarily the optimal configuration for applications in other situations. Therefore, if the electron gun is to be designed and applied, it is necessary to analyze the specific situation and use electric field and particle program simulation to obtain the design parameters that are most suitable for the application scenario. The size and configuration mentioned in the present invention can be used as a reference for related scientific research and design applications.

[0033] Although the present invention has been described through preferred embodiments, the present invention is not limited to the embodiments described herein but includes various changes and modifications that may be made without departing from the scope of the present invention.

Claims

1. A DC ultrafast electron gun with an electrostatic single lens, characterized in that: The invention comprises a cathode (1), an anode (2), a cathode fixing ceramic support (3), a single lens support ring (4), an electrostatic single lens electrode plate (5), an electrostatic single lens support (6), a cathode vacuum electrode (7), a single lens vacuum electrode (8), an electron gun vacuum chamber (9), a single lens support flange (10), and a cathode support flange (11); an incident laser (13) is incident on the cathode through a laser window (12); the cathode (1) has an axisymmetric hollow cylindrical structure, the anode (2) is a disk with smooth surfaces on both sides, the electrostatic single lens electrode plate (5) has a first step structure (5.6), the single lens support ring (4) has a second step structure (4.3), the electrostatic single lens support (6) is fixed on the single lens support flange (10), the cathode (1) is fixed on the cathode fixing ceramic support (3), the cathode fixing ceramic support (3) is fixed on the cathode support flange (11), and the cathode support flange (11) is installed to a side of the electron gun vacuum chamber (9) close to the incident laser (13); the anode (2) is fixed to the surface of the single lens support ring (4), the electrostatic single lens electrode plate (5) is placed on the second step structure (4.3) of the single lens support ring (4), the first step structure (5.6) and the second step structure (4.3) are embedded in each other, the single lens support ring (4) is fixed to the electrostatic single lens bracket (6), and the single lens support flange (10) is installed to the other side of the electron gun vacuum chamber (9).

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