Ultrafast electron diffraction system and diffraction imaging method based on 300kV direct current acceleration and radio frequency compression

Abstract

The invention discloses an ultrafast electron diffraction system based on 300kV direct current acceleration and radio frequency compression and a diffraction imaging method, and solves the problems that the time resolution of the current keV ultrafast electron diffraction system is insufficient due to the space charge effect, and the time-varying electric field generated by a radio frequency compression cavity is used for carrying out time compression on the beam bunch length of an electronic pulse; and the electronic pulse energy is low. According to the invention, femtosecond laser is split, one beam of light generates an ultrafast electronic pulse by using an ultrafast electron generation module based on 300kV direct current acceleration, the energy of the ultrafast electronic pulse is relatively high, and then the pulse width of the ultrafast electronic pulse is compressed by using a radio frequency compression module; the pulse broadening caused by the space charge effect of the ultrafast electronic pulse is effectively inhibited, the time resolution below hundred femtoseconds can be realized, meanwhile, the charge quantity of a single beam of ultrafast electronic pulse reaches 16fC (105e-), and the method can be used for ultrafast kinetic research of irreversible materials.

Description

Technical Field

[0001] The invention relates to an ultrafast electron diffraction system, and in particular to an ultrafast electron diffraction system based on 300 kV direct current acceleration and radio frequency compression and a diffraction imaging method. Background Art

[0002] With the emergence of femtosecond lasers in the 1980s and Ahmed Zewail's application of electron diffraction technology to the field of femtosecond chemistry in the 1990s, ultrafast electron diffraction (UED) with femtosecond time resolution has been widely used in ultrafast dynamics research such as phase change and chemical reaction kinetics, coupling of electronic and nuclear degrees of freedom in materials and molecules, and correlation between the structure, function and previously hidden decay or non-equilibrium states of matter.

[0003] Ultrafast electron diffraction technology has been developed to be divided into keV UED and MeV UED. Both UED technologies have different advantages and limitations. KeV UED has a longer de Broglie wavelength. It has the advantages of less radiation damage to materials, relatively compact overall structure, and low cost. However, the obvious space charge effect in the electron beam line limits the improvement of its time resolution. Therefore, a variety of technical means have been proposed to improve the time resolution of keV UED. Typical keVUED technologies include:

[0004] The paper Design of ultrabright 270keV DC photoelectron gun for ultrafast electron diffraction. Yang J, Yu Y, Qi Y, et al. AIP Advances, 2020, 10(8):-. DOI: 10.1063 / 5.0012248. discloses a 270kV ultrafast electron diffraction technology based on three-stage electrostatic acceleration. The concept of multi-stage acceleration in electron microscopes was used to design a three-stage accelerated 270kV high-voltage DC electron gun. However, the distortion of the electric field in the central pinhole area of ​​multiple accelerating electrodes changes the coherence of the electron beam, resulting in a short coherence length of the electron pulse. The entire ultrafast electron diffraction system adopts an ultra-compact structure, and the position of the sample is only 2mm away from the exit of the electron gun. However, the space charge effect causes its time resolution to reach only 105fs@10 4 e - , and does not have the ability of single-shot imaging.

[0005] In the paper Electron source concept for single-shot sub-100fs electron diffraction in the 100keV range. T. van Oudheusden, EF de Jong, et al. Journal of Applied Physics, 102, 093501 (2007); doi: 10.1063 / 1.2801027., it was first proposed to use the time-varying electric field generated by the radio frequency compression cavity to time compress the bunch length of the electron pulse, so that the time resolution of the 100keV UED system can reach below sub-100 femtoseconds and have the ability of single-shot imaging. However, the electron pulse energy is low, and the preparation of detectable samples is difficult. At the same time, the early time synchronization technology limits the improvement of the time resolution of the system. Summary of the invention

[0006] The purpose of the present invention is to solve the problem that the current keV ultrafast electron diffraction system has insufficient time resolution due to the space charge effect, and the electron pulse energy is low when the time-varying electric field generated by the radio frequency compression cavity is used to time compress the bunch length of the electron pulse, and to provide an ultrafast electron diffraction system and a diffraction imaging method based on 300kV DC acceleration and radio frequency compression.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] An ultrafast electron diffraction system based on 300 kV DC acceleration and radio frequency compression, comprising a femtosecond laser, wherein the femtosecond laser is used to emit femtosecond laser; the system is special in that it also comprises:

[0009] A first beam splitter is arranged on the optical path of the femtosecond laser and is used to split the femtosecond laser into two beams, namely a first beam and a second beam;

[0010] A first optical path adjustment component and an ultrafast electron generation module based on 300kV DC acceleration are sequentially arranged on the optical path where the first light beam is located, which are used to adjust the direction of the first light beam and generate ultrafast electron pulses; the output end of the ultrafast electron generation module is connected to a radio frequency compression module, which is used to compress the ultrafast electron pulse width; the output end of the radio frequency compression module is connected to a sample chamber, a sample stage is arranged in the sample chamber, a metastable lattice is arranged on the sample stage, and the ultrafast electron pulse after pulse width compression is incident on the metastable lattice;

[0011] A second beam splitter is arranged on the optical path where the second light beam is located, and is used to split the second light beam into a trigger light signal and a pump light signal; a radio frequency signal source is arranged on the optical path where the trigger light signal is located, and the radio frequency signal source is connected to control the radio frequency compression module; an optical time delayer and a second optical path adjustment component are arranged in sequence on the optical path where the pump light signal is located, and are used to delay the pump light and change the direction of the pump light, and synchronously with the ultrafast electron pulse after pulse width compression to be incident on the metastable lattice, so as to generate an electron diffraction beam flow;

[0012] An electron detector is arranged on the sample chamber facing the metastable crystal lattice, and is used for detecting the electron diffraction beam generated by the metastable crystal lattice and displaying the electron diffraction image.

[0013] Furthermore, the ultrafast electron generation module comprises an electron gun chamber with an incident port and an exit port, the exit port of the electron gun chamber is connected to a movable anode, and an anode aperture is provided in the middle of the movable anode, a photocathode is connected in the electron gun chamber through an inverted insulating ceramic, and the photocathode and the movable anode are arranged opposite to each other; a high-voltage direct current power supply with an adjustable range of 100kV-300kV is connected between the photocathode and the movable anode; the exit port of the electron gun chamber is connected to a radio frequency compression module as an output end of the ultrafast electron generation module; the exit light of the first optical path adjustment component is incident on the photocathode through the incident port of the electron gun chamber to generate an electron pulse, a high-voltage electric field is formed between the photocathode and the movable anode through the high-voltage direct current power supply with an adjustable range of 100kV-300kV, the electron pulse is accelerated, and an ultrafast electron pulse is obtained, which passes through the anode aperture in the middle of the movable anode and enters the radio frequency compression module.

[0014] Furthermore, the photocathode adopts a Bruce electrode structure, and the diameter of the anode aperture on the movable anode is 400 μm.

[0015] Furthermore, the movable anode is connected to the electron gun chamber via a moving device, and the distance between the photocathode and the movable anode is 10 mm-30 mm.

[0016] Furthermore, the RF compression module includes a vacuum conduit and a RF compression chamber, one end of the vacuum conduit is connected to the output end of the ultrafast electron generation module, and the other end is connected to the input end of the RF compression chamber, the output end of the RF compression chamber is connected to the chamber, and the signal input end of the RF compression chamber is connected to the RF signal source for inputting a trigger signal of the RF compression chamber.

[0017] Furthermore, the outer wall of the vacuum conduit is also connected to a collimating magnetic lens and a focusing magnetic lens for collimating and focusing the ultrafast electron pulses in the vacuum conduit.

[0018] Furthermore, the first optical path adjustment component includes a first reflector and a first optical focusing lens, the first reflector is used to change the direction of the first light beam to the input end of the ultrafast electron generation module, and the first optical focusing lens is used to focus the first light beam with changed direction and input it to the input end of the ultrafast electron generation module.

[0019] Furthermore, the second optical path adjustment component includes a second reflector, a second optical focusing lens and a third reflector, the third reflector is arranged in the sample chamber, and an optical path through hole is opened on the sample chamber; the pump light after the delay output by the optical time delay device passes through the second reflector, the second optical focusing lens and the third reflector in sequence and is incident on the metastable lattice.

[0020] At the same time, the present invention also provides an ultrafast electron diffraction imaging method based on 300 kV DC acceleration and radio frequency compression, using the above-mentioned ultrafast electron diffraction system based on 300 kV DC acceleration and radio frequency compression, which is special in that it includes the following steps:

[0021] S1, turn on the femtosecond laser, emit femtosecond laser and split it into two beams through the first beam splitter;

[0022] S2, after the first light beam is changed in direction and focused by the first optical path adjustment component, it is input into the ultrafast electron generation module based on 300kV DC acceleration to generate ultrafast electron pulses; and the ultrafast electron pulses are input into the radio frequency compression module;

[0023] At the same time, the second light beam is input into the second beam splitter for splitting to obtain a trigger light signal and a pump light signal. The trigger light signal is input into the radio frequency signal source to trigger the radio frequency compression module to compress the ultrafast electron pulse and then hit the metastable lattice. The pump light signal is delayed by the optical time delay device, and then adjusted by the second optical path adjustment component and hits the metastable lattice.

[0024] S3. Determine whether an electron diffraction image appears in the electron detector. If so, save the electron diffraction image. If not, adjust the delay time of the optical time delay device until an electron diffraction image appears in the electron detector.

[0025] Furthermore, in S2, the ultrafast electron pulse is input into the RF compression module, the ultrafast electron pulse is collimated and focused by the collimating magnetic lens and the focusing magnetic lens, and then the RF compression module is triggered by the triggering light signal input into the RF signal source to compress the ultrafast electron pulse.

[0026] Beneficial effects of the present invention:

[0027] 1. The present invention discloses an ultrafast electron diffraction system based on 300kV DC acceleration and radio frequency compression. The system splits a femtosecond laser beam, and uses an ultrafast electron generation module based on 300kV DC acceleration to generate an ultrafast electron pulse with high energy. The radio frequency compression module is then used to compress the pulse width of the ultrafast electron pulse, which effectively suppresses the pulse broadening caused by the space charge effect of the ultrafast electron pulse, and can achieve a time resolution of less than 100 femtoseconds. At the same time, the charge amount of a single ultrafast electron pulse reaches 16fC (10 5 e - ), which can be used to study the ultrafast dynamics of irreversible materials.

[0028] 2. The present invention discloses an ultrafast electron diffraction system based on 300kV DC acceleration and RF compression. The system controls the delay between pump light and ultrafast electron pulse by adjusting an optical time delay device to detect metastable lattice vibrations at different times, thereby obtaining diffraction information of lattice vibrations at different times.

[0029] 3. The present invention provides an ultrafast electron diffraction system based on 300 kV DC acceleration and radio frequency compression, which adjusts the energy of ultrafast electron pulses by adjusting the distance between the photocathode and the movable anode to adapt to metastable lattices of different thicknesses.

[0030] 4. The present invention discloses an ultrafast electron diffraction imaging method based on 300kV DC acceleration and radio frequency compression. An ultrafast electron generation module based on 300kV DC acceleration is used to generate ultrafast electron pulses, and then a radio frequency compression module is used to compress the pulse width of the ultrafast electron pulses. The ultrafast electron pulses of 16fC can be compressed to 19.8fs (RMS). Considering the time jitter (~50fs) between the radio frequency signal and the laser pulse, the time resolution of the ultrafast electron diffraction system can finally reach an order of magnitude better than 100fs. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a structural schematic diagram of an embodiment of an ultrafast electron diffraction system based on 300kV DC acceleration and radio frequency compression of the present invention;

[0032] Figure 2 It is a schematic diagram of the change in bunch length of ultrafast electron pulses in an embodiment of an ultrafast electron diffraction imaging method based on 300kV DC acceleration and radio frequency compression of the present invention.

[0033] Among them, 1-femtosecond laser, 2-first beam splitter, 3-first reflector, 4-first optical focusing lens, 5-electron gun chamber, 6-inverted insulating ceramic, 7-photocathode, 8-movable anode, 9-collimating magnetic lens, 10-vacuum conduit, 11-focusing magnetic lens, 12-RF compression chamber, 13-sample chamber, 14-sample stage, 15-third reflector, 16-electron detector, 17-second optical focusing lens, 18-second reflector, 19-optical time delay, 20-second beam splitter, 21-RF signal source, 22-ultrafast electron pulse, 23-electron diffraction beam, 24-first light beam, 25-trigger light signal, 26-delayed and redirected pump light. DETAILED DESCRIPTION

[0034] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Example

[0036] In this embodiment, an ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression is used. Figure 1 As shown, it includes a femtosecond laser 1, which is used to emit femtosecond laser; and also includes:

[0037] A first beam splitter 2 is arranged on the optical path of the femtosecond laser and is used to transmit and reflect the femtosecond laser into two beams, namely, a first beam 24 obtained by transmission and a second beam obtained by reflection;

[0038] A first reflector 3, a first optical focusing lens 4, and an electron gun chamber 5 with an incident port and an exit port are sequentially arranged on the optical path where the first light beam 24 is located. A movable anode 8 is connected to the exit port of the electron gun chamber 5 through a moving device. The distance between the photocathode 7 and the movable anode 8 is 10mm-30mm, and the axial displacement adjustment within the range of 10mm-30mm between the positive and negative electrodes can be achieved, so that the working voltage of the electron gun is adjustable within the range of 100kV-300kV; an anode aperture is arranged in the middle of the movable anode 8, and the diameter of the anode aperture on the movable anode 8 is 400μm. The photocathode 7 is connected to the electron gun chamber 5 through an inverted insulating ceramic 6, and the photocathode 7 is arranged opposite to the movable anode 8; a high-voltage DC power supply with an adjustable range of 100kV-300kV is connected between the photocathode 7 and the movable anode 8.

[0039] In this embodiment, the photocathode 7 is a Bruce electrode structure, consisting of a plane area, a curve area and an arc area. The diameter of the entire photocathode 7 is 160 mm, and a negative high voltage of 300 kV is applied. The distance between the photocathode 7 and the movable anode 8 is 30 mm. The movable anode 8 includes an anode electrode and an anode aperture. The diameter of the anode aperture is 400 μm, and the voltage applied to the movable anode 8 is zero.

[0040] In this embodiment, the cathode window of the photocathode 7 adopts a magnesium fluoride substrate and the material is gold.

[0041] The first light beam 24 is reflected by the first reflector 3, then enters the first optical focusing lens 4 for focusing, enters the electron gun chamber 5 through the input port, and hits the cathode window of the photocathode 7, exciting the photocathode 7 to generate an electron pulse. A high-voltage DC power supply with an adjustable range of 100kV-300kV forms a high-voltage electric field between the photocathode 7 and the movable anode 8. The electron pulse is accelerated in the high-voltage electric field and output through the anode aperture to obtain an ultrafast electron pulse 22.

[0042] The output port of the electron gun chamber 5 is connected to the vacuum conduit 10 and the radio frequency compression chamber 12 in sequence. The radio frequency compression chamber 12 is connected to the radio frequency signal source 21. The outer wall of the vacuum conduit 10 is also connected to the collimating magnetic lens 9 and the focusing magnetic lens 11. After the ultrafast electron pulse 22 enters the vacuum conduit 10, it is collimated and focused by the collimating magnetic lens 9 and the focusing magnetic lens 11 and then input into the radio frequency compression chamber 12.

[0043] Among them, the collimating magnetic lens 9 is 60mm away from the end of the vacuum tube 10, has an inner diameter of 30mm, an outer diameter of 55mm, a width of 52mm, and a total number of turns of 1060 turns, and is mainly used for collimating the ultrafast electron pulse 22. The focusing magnetic lens 11 is installed at 9 300mm away from the end of the vacuum tube 10, has an inner diameter of 30mm, an outer diameter of 55mm, a width of 52mm, and a total number of turns of 740 turns, and is mainly used for focusing the electron probe to the metastable lattice.

[0044] A second beam splitter 20 is arranged on the optical path where the second light beam is located, so as to split the second light beam into a trigger light signal 25 and a pump light signal.

[0045] The trigger light signal 25 is input into the RF signal source 21, which triggers the RF signal source 21 to input a 3GHz RF signal into the RF compression chamber 12, forming a time-varying electric field in the RF compression chamber 12, compressing the bunch length of the collimated and focused ultrafast electron pulse 22, outputting an ultrashort ultrafast electron pulse, and outputting it through the RF compression chamber 12 to hit the metastable lattice. The compression cavity in the RF compression chamber 12 uses a 3GHz RF time compression cavity, which can compress an ultrafast electron pulse of 16fC to an ultrafast electron pulse of 19.8fs (RMS), so that the time resolution of the keV electron diffraction system is better than 100fs.

[0046] An optical time delay 19, a second reflector 18, a second optical focusing lens 17 and a third reflector 15 are sequentially arranged on the optical path where the pump light signal is located; the pump light signal is input into the optical time delay 19 for time delay, and the delay time is set according to the diffraction effect. After the delay, the pump light signal is sequentially passed through the second reflector 18 for direction adjustment and the second optical focusing lens 17 for focusing, and finally the light beam is adjusted to the metastable lattice through the third reflector 15 to excite the metastable lattice and generate a diffracted electron beam 23.

[0047] An electron detector 16 is disposed on the sample chamber 13 facing the metastable crystal lattice, for detecting the electron diffraction beam 23 generated by the metastable crystal lattice and displaying an electron diffraction image.

[0048] This embodiment provides an ultrafast electron diffraction imaging method based on 300 kV DC acceleration and radio frequency compression, using an ultrafast electron diffraction system based on 300 kV DC acceleration and radio frequency compression, and includes the following steps:

[0049] S1, turning on the femtosecond laser 1, emitting femtosecond laser and splitting it into two beams through the first beam splitter 2, namely the first beam 24 and the second beam;

[0050] S2, after the first light beam 24 is changed in direction and focused by the first optical path adjustment component, it is input into the ultrafast electron generation module based on 300kV DC acceleration to generate an ultrafast electron pulse 22; and the ultrafast electron pulse 22 is input into the radio frequency compression module; specifically:

[0051] The first light beam 24 is reflected by the first reflector 3, then enters the first optical focusing lens 4 for focusing, enters the electron gun chamber 5 through the input port, and hits the cathode window of the photocathode 7, exciting the photocathode 7 to generate an electron pulse. A high-voltage DC power supply with an adjustable range of 100kV-300kV forms a high-voltage electric field between the photocathode 7 and the movable anode 8. The electron pulse is accelerated in the high-voltage electric field and output through the anode aperture to obtain an ultrafast electron pulse 22. After the ultrafast electron pulse 22 enters the vacuum conduit 10, it is collimated and focused by the collimating magnetic lens 9 and the focusing magnetic lens 11 and then input into the radio frequency compression chamber 12.

[0052] At the same time, the second light beam is input into the second beam splitter 20 for splitting, and a trigger light signal 25 and a pump light signal are obtained. The trigger light signal 25 is input into the radio frequency signal source 21 to trigger the radio frequency compression module to compress the aligned and focused ultrafast electron pulse 22, and hit the metastable lattice; the pump light signal is delayed by the optical time delay 19, and then adjusted by the second optical path adjustment component to obtain a delayed and redirected pump light 26, and hit the metastable lattice; specifically:

[0053] The second light beam is input into the second beam splitter 20 for splitting, and a trigger light signal 25 and a pump light signal are obtained. The trigger light signal 25 is input into the radio frequency signal source 21, and the trigger radio frequency signal source 21 inputs a 3GHz radio frequency signal into the radio frequency compression chamber 12, and a time-varying electric field is formed in the radio frequency compression chamber 12, and the bunch length of the collimated and focused ultrafast electron pulse 22 is compressed, and an ultrashort ultrafast electron pulse is output, and it is output through the radio frequency compression chamber 12 and hits the metastable lattice. An optical time delay 19, a second reflector 18, a second optical focusing lens 17 and a third reflector 15 are sequentially arranged on the optical path where the pump light signal is located; the pump light signal is input into the optical time delay 19 for time delay, and the delay time is set according to the diffraction effect. After the delay, the direction is adjusted by the second reflector 18, and the second optical focusing lens 17 is focused. Finally, the light beam is adjusted to the metastable lattice by the third reflector 15, and the metastable lattice is excited to generate a diffracted electron beam 23.

[0054] S3, judging whether an electron diffraction image appears in the electron detector 16, if so, saving the electron diffraction image, if not, adjusting the delay time of the optical time delay device 19 until an electron diffraction image appears in the electron detector 16.

[0055] In an ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to an embodiment of the present invention, the change of the bunch length of the ultrafast electron pulse is as follows: Figure 2 As shown; when the number of electrons in the ultrafast electron pulse is 1e 5When the phase jitter of the RF compression cavity is taken into account (less than 50 fs), the time resolution of the entire electron diffraction system can be obtained to be less than 100 fs.

[0056] The above description is only a specific embodiment of the present invention, and a comparison of the effects of the specific embodiments and the related comparative examples, but the protection scope of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. An ultrafast electron diffraction system based on 300 kV DC acceleration and radio frequency compression, comprising a femtosecond laser (1), wherein the femtosecond laser (1) is used to emit femtosecond laser; characterized in that: Also includes: A first beam splitter (2) is arranged on the optical path of the femtosecond laser and is used to split the femtosecond laser into two beams, namely a first beam and a second beam; A first optical path adjustment component and an ultrafast electron generation module based on 300 kV DC acceleration are sequentially arranged on the optical path where the first light beam is located, for adjusting the direction of the first light beam and generating ultrafast electron pulses; the output end of the ultrafast electron generation module is connected to a radio frequency compression module for compressing the ultrafast electron pulse width; the output end of the radio frequency compression module is connected to a sample chamber (13); a sample stage (14) is arranged in the sample chamber (13); a metastable lattice is arranged on the sample stage (14); and the ultrafast electron pulse after pulse width compression is incident on the metastable lattice; A second beam splitter (20) is arranged on the optical path where the second light beam is located, and is used to split the second light beam into a trigger light signal and a pump light signal; a radio frequency signal source (21) is arranged on the optical path where the trigger light signal is located, and the radio frequency signal source (21) is connected to control the radio frequency compression module; an optical time delayer (19) and a second optical path adjustment component are arranged in sequence on the optical path where the pump light signal is located, and are used to delay the pump light and change the direction of the pump light, and synchronously irradiate the pump light with the ultrafast electron pulse after pulse width compression onto the metastable lattice, so as to generate an electron diffraction beam flow; An electron detector (16) is arranged on the sample chamber (13) facing the metastable crystal lattice, and is used to detect the electron diffraction beam generated by the metastable crystal lattice and display the electron diffraction image.

2. The ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to claim 1, characterized in that: The ultrafast electron generation module comprises an electron gun chamber (5) with an incident port and an exit port, the exit port of the electron gun chamber (5) is connected to a movable anode (8), and an anode aperture is provided in the middle of the movable anode (8), a photocathode (7) is connected to the electron gun chamber (5) via an inverted insulating ceramic (6), and the photocathode (7) and the movable anode (8) are arranged opposite to each other; a high-voltage direct current power supply adjustable in the range of 100 kV-300 kV is connected between the photocathode (7) and the movable anode (8); the exit port of the electron gun chamber (5) is connected to a radio frequency compression module as an output end of the ultrafast electron generation module; The output light of the first optical path adjustment component is incident on the photocathode (7) through the incident port of the electron gun chamber (5), generating an electron pulse. A high-voltage electric field is formed between the photocathode (7) and the movable anode (8) through a high-voltage DC power supply adjustable in the range of 100kV-300kV, accelerating the electron pulse to obtain an ultrafast electron pulse, which passes through the anode hole in the middle of the movable anode (8) and enters the radio frequency compression module.

3. The ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to claim 2, characterized in that: The photocathode (7) adopts a Bruce electrode structure, and the diameter of the anode aperture on the movable anode (8) is 400 μm.

4. The ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to claim 2, characterized in that: The movable anode (8) is connected to the electron gun chamber (5) via a moving device, and the distance between the photocathode (7) and the movable anode (8) is 10 mm-30 mm.

5. The ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to claim 1, characterized in that: The radio frequency compression module comprises a vacuum conduit (10) and a radio frequency compression chamber (12); one end of the vacuum conduit (10) is connected to the output end of the ultrafast electron generation module, and the other end is connected to the input end of the radio frequency compression chamber (12); the output end of the radio frequency compression chamber (12) is connected to a chamber (13); the signal input end of the radio frequency compression chamber (12) is connected to a radio frequency signal source (21) for inputting a trigger signal of the radio frequency compression chamber (12).

6. The ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to claim 5, characterized in that: The outer wall of the vacuum conduit (10) is also connected to a collimating magnetic lens (9) and a focusing magnetic lens (11) for collimating and focusing ultrafast electron pulses in the vacuum conduit (10).

7. The ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to claim 1, characterized in that: The first optical path adjustment component comprises a first reflector (3) and a first optical focusing lens (4), wherein the first reflector (3) is used to change the direction of the first light beam to the input end of the ultrafast electron generation module, and the first optical focusing lens (4) is used to focus the first light beam whose direction has been changed and input it to the input end of the ultrafast electron generation module.

8. The ultrafast electron diffraction system based on 300 kV DC acceleration and RF compression according to claim 1, characterized in that: The second optical path adjustment component comprises a second reflector (18), a second optical focusing lens (17) and a third reflector (15); the third reflector (15) is arranged in a sample chamber (13), and an optical path through hole is provided in the sample chamber (13); the pump light after delay output by the optical time delay device (19) passes through the second reflector (18), the second optical focusing lens (17) and the third reflector (15) in sequence and is incident on the metastable lattice.

9. An ultrafast electron diffraction imaging method based on 300kV DC acceleration and radio frequency compression, characterized in that: Using an ultrafast electron diffraction system based on 300 kV DC acceleration and radio frequency compression as described in any one of claims 1 to 8, comprising the following steps: S1, turning on a femtosecond laser (1), emitting femtosecond laser light and splitting it into two beams through a first beam splitter (2); S2, after the first light beam is changed in direction and focused by the first optical path adjustment component, it is input into the ultrafast electron generation module based on 300kV DC acceleration to generate ultrafast electron pulses; and the ultrafast electron pulses are input into the radio frequency compression module; At the same time, the second light beam is input into the second beam splitter (20) for splitting to obtain a trigger light signal and a pump light signal. The trigger light signal is input into the radio frequency signal source (21) to trigger the radio frequency compression module to compress the ultrafast electron pulse and then hit the metastable lattice. The pump light signal is delayed by the optical time delay device (19), adjusted by the second optical path adjustment component and hits the metastable lattice. S3, determining whether an electron diffraction image appears on the electron detector (16); if so, saving the electron diffraction image; if not, adjusting the delay time of the optical time delay device (19) until an electron diffraction image appears on the electron detector (16).

10. The ultrafast electron diffraction imaging method based on 300 kV DC acceleration and radio frequency compression according to claim 9, characterized in that: In S2, the ultrafast electron pulse is input into the radio frequency compression module, and the ultrafast electron pulse is collimated and focused by the collimating magnetic lens (9) and the focusing magnetic lens (11), and then the radio frequency compression module is triggered by the triggering light signal input into the radio frequency signal source (21) to compress the ultrafast electron pulse.

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