A high repetition rate ultrafast pulse X-ray source generation system and generation method

Abstract

The present invention relates to a pulse radiation generation system and method, and specifically to a high-repetition-rate ultrafast pulse X-ray source generation system and method. The system addresses the problems of existing ps-level ultrafast pulse X-ray sources, such as high construction cost, large system scale, low repetition frequency, and difficulty in popularizing and applying them in ordinary laboratories, as well as the technical problem of low synchronization time accuracy in pump-probe methods. The high-repetition-rate ultrafast pulse X-ray source generation system comprises a DC high-voltage source, a driving laser, an electron gun, a first correction magnet, an electromagnetic solenoid, a second correction magnet, and a target chamber, wherein a composite target holder is disposed within the target chamber; the DC high-voltage source is connected to the electron gun's photocathode; an electron beam is incident on the composite target holder to generate high-repetition-rate ultrafast pulse X-rays, enabling the system of the present invention to meet the basic requirements for ultrafast pulse X-ray sources for research work such as detector performance calibration and multi-beam pump detection.

Description

Technical Field

[0001] The present invention relates to a pulse radiation generating system and a generating method, and in particular to a high repetition rate ultrafast pulse X-ray source generating system and a generating method. Background Art

[0002] The main feature of ultrafast pulsed X-ray sources is that the duration of the pulsed X-rays is very short, and the time scale of the pulsed X-ray field formed by this is usually below 100ps or even 1ps. The Bremsstrahlung method is the earliest and most widely used pulsed X-ray generation technology, which uses high-speed electrons to bombard metal targets to generate X-rays. Since the pulsed X-ray source achieved by this method has many advantages such as small size, low cost, small focal spot and high intensity, it has been widely used in various fields such as medical imaging, industrial testing, hazardous materials inspection, and material structure characterization. However, for a long time, due to the long pulse time of the electron beam generated by hot cathodes or field emission (cold cathodes), the length of the X-ray pulses generated by target shooting is usually only at the sub-ns level, which cannot meet the application requirements of modern scientific research fields such as time-resolved imaging technology, plasma physics, ultrafast carrier dynamics, and material phase transition process research. Among traditional radiation sources, only pulsed X-rays (with an energy spectrum of 0.1 keV-100 keV) output by laser inertial confinement fusion (ICF) systems have a pulse duration of less than 200 ps. With the advancement of ultrafast laser technology, ultrafast pulsed X-ray sources with picosecond or even femtosecond timescales generated by ultrafast laser beams have emerged. Based on different physical processes, there are four typical ultrafast pulsed X-ray sources:

[0003] (1) Synchrotron radiation and free electron laser systems;

[0004] Synchrotron radiation is electromagnetic radiation produced by relativistic charged particles moving along curved paths under the influence of an electromagnetic field. First discovered at an electron synchrotron, it is also called synchrotron radiation. Its characteristic feature is the production of high-brightness X-ray photon beams spanning the energy spectrum from vacuum ultraviolet (UV) and soft X-rays to hard X-rays. Synchrotron radiation sources can be divided into two types based on accelerator type: one is a synchrotron radiation source based on a circular accelerator, such as the Bessy-1 system in Berlin, Germany, the PLS system in Pohang, South Korea, and the Diamond system in South Oxfordshire, UK; and in China, the Shanghai Synchrotron Radiation Facility, the Hefei Synchrotron Radiation Facility, and the Beijing Synchrotron Radiation Facility. The other is a free electron laser (FEL) system based on an electron linear accelerator. FELs generate coherent X-rays from free electrons (as opposed to electrons bound in atoms or molecules). Unlike the storage rings (which use deflection magnets) and drive methods of third-generation light sources, FELs utilize magnetic inserts such as wigglers and undulators to generate quasi-coherent spontaneous radiation. Due to their extremely high radiation brightness (peak X-ray brightness can reach 10 30 phs / mm 2 mrad 2 s (0.1% BW), also known as the fourth-generation light source. Examples include the PAL-XFEL system in South Korea, the FLASH system in Hamburg, Germany, the LCLS coherent light source at Stanford University in the United States, and the SwissX-FEL at PSI in Switzerland. In China, there are the Dalian Coherent X-ray Source, the Shanghai soft X-ray system, and the Shanghai hard X-ray system under construction.

[0005] Synchrotron radiation sources generally produce continuous spectrum X-rays. Using a monochromator, monochromatic light of a certain wavelength and bandwidth can be extracted to achieve adjustable photon energy. The beam of synchrotron radiation, especially free electron lasers, has very good directionality, a small divergence angle, good stability, and natural polarization characteristics. The pulse time length is very short. The pulse X-ray length of the free electron laser system is generally less than 10ps. In addition, the free electron laser is coherent radiation, and its photon flux, spectral distribution, polarization and angular distribution and other characteristics can all be calculated using formulas. This good calculability allows synchrotron radiation sources to be used as standard light sources to calibrate and scale other light sources and detectors.

[0006] (2) Ultrafast pulsed X-ray source generated by laser driving;

[0007] Laser-driven X-ray sources are generated by the interaction of high-intensity lasers with high-density plasmas. As superhot electrons propagate through the plasma, they emit X-ray pulses encompassing both a continuous spectrum and characteristic spectral lines. These sources primarily include laser-driven K-shell X-ray emission, electron cyclotron (Betatron) emission from acceleration processes, and all-optical inverse Compton scattering sources. While maintaining a source focal spot size on the order of μm, the electron density involved in X-ray excitation is extremely high. Combined with the confinement of the plasma's tens of megaGs self-generated magnetic field, the resulting laser X-ray sources possess three key characteristics: 1) pulse widths ranging from fs to hundreds of nanoseconds on a temporal scale; 2) spatially, focal spot sizes can be as small as a few μm; and 3) precise synchronization with the laser pulse and its associated physical processes, with synchronization accuracy at the femtosecond level (the fastest synchronization accuracy for electronic processes, such as free electron lasers, is in the hundreds of fs level).

[0008] This type of X-ray source mainly includes laser-driven K α There are two types of X-ray sources: characteristic X-ray sources and Betatron radiation sources. Since the hard X-rays generated by laser drive mainly use electrons as the intermediate carrier of energy conversion, the generated X-rays have a continuous spectrum and characteristic radiation of the K shell. Considering the laser absorption efficiency, solid targets and special configuration targets (such as gas cluster targets, etc.) are mainly used to generate K in the hard X-ray energy band. α The main problems in current experimental research are: the photon energy conversion efficiency obtained from the widely used solid target experiment is usually only 10 -5 , and is emitted in the 4π direction, resulting in a low available photon yield; in addition, K α The characteristic line accounts for 10% of the total light intensity, and a large number of photons are in the continuous spectrum of the bremsstrahlung background, resulting in a low signal-to-noise ratio of the quasi-monoenergetic X-rays. In addition, due to the long transport process of hot electrons in the solid target, the temporal width of the pulsed X-rays is widely dispersed, mostly at the ps level. The use of small-sized gas cluster targets and special structure targets can further shorten the length of the pulsed X-ray source (hundreds of fs level) and improve the photon yield (>10 10 ph / J@10 17 W / cm 2 ) and monochromaticity. Different from K α The characteristic X-ray source has a large divergence, and the Betatron radiation that appeared later has the advantage of a small divergence. This ray source is a strong laser that excites the wake field in a low-density plasma. The electrons injected into the ion "cavitation" are accelerated longitudinally and oscillate transversely with the plasma scale as a period (this process is similar to the principle of free electron laser), thereby generating electron cyclotron radiation similar to synchrotron radiation. The domestic physics institute used a 3TW laser to fire an Ar gas cluster target and obtained electron cyclotron radiation X-rays with an energy greater than 2.4keV. The photon yield in this energy spectrum range reached 108 / pulse, the divergence angle is about 10mrad, and the divergence angle of 1mrad can be achieved by using He gas. Therefore, due to its good direction and spatial coherence, K α The peak brightness of the characteristic X-ray source can reach 10 21 phs / mm 2 mrad 2 ·s (0.1% BW), which is comparable to the peak brightness of the Shanghai Synchrotron Radiation Source, and the pulse duration can reach 10 fs, but the energy distribution of this Betatron radiation is broad. In recent years, two other research hotspots are the research on all-optical inverse Compton scattering (Thomson scattering) sources based on laser acceleration and laser high harmonics. These two X-ray sources are different from K α The main characteristic of Betatron radiation is that the pulsed X-rays are quasi-monoenergetic.

[0009] (3) Thomson scattering X-ray source;

[0010] High-power laser pulses focused to the micron scale scatter at a certain angle from a picosecond relativistic energy electron beam, generating ultrashort pulses of X-rays and even gamma rays in the direction of the electron's motion. These pulsed X-ray beams are characterized by quasi-monochromaticity, continuously adjustable energy, ultrahigh brightness, ultrafast pulse duration (sub-ps), low interference background, polarized output, and excellent directionality and coherence. The pulse length of X-rays produced by Thomson scattering depends on the lateral and longitudinal dimensions of the electron and laser beams, as well as the scattering structure.

[0011] Currently, both domestic and international Thomson scattering X-ray systems based on the collision of electron linear accelerators with lasers have been built and are producing beams. It should be noted that as the energy of the electrons increases, the energy of the output X-rays also increases, and the ray source is also called an Inverse Compton Scattering X / gamma ray source. Currently, there are only three Inverse Compton Scattering light sources under construction worldwide: the MEGa-ray system being planned by Lawrence Livermore National Laboratory (LLNL) in the United States, the ELI-NP system in Bucharest, Romania, and the Inverse Compton Scattering Gamma Ray System under construction in China.

[0012] (4) Ultrafast pulse X-ray source generated by ultrafast pulse electron beam targeting;

[0013] The most common method of generating pulsed X-ray sources is to use a pulsed electron beam to hit a metal target through the bremsstrahlung process. With the development of high-energy ultrafast pulsed laser beam technology, the gradually developed photocathode electron gun has made it possible to generate ps-level pulsed electron beams. The most common of these is the photocathode microwave electron gun, which has appeared in the 1980s and has been widely used in various photocathode injectors. Currently, S-band photocathode microwave electron guns usually operate at around 100MV / m, so that the electron beam energy output by accelerators based on this type of electron gun is usually above 10MeV. Therefore, the bremsstrahlung spectrum generated by hitting a target with this type of electron beam is mainly concentrated in the gamma energy band above the MeV level. In addition, the time synchronization accuracy of the pulsed X-rays generated by the microwave electron gun and the laser is relatively low, making it difficult to use as an ultrafast probe.

[0014] All of the aforementioned ps-class ultrafast pulsed X-ray sources share two drawbacks: First, construction costs are extremely high, ranging from tens of millions of RMB to hundreds of millions or even tens of billions. The systems are also very large, with long construction cycles, resulting in significant investment in ancillary facilities. Second, the repetition rate is low, typically below 100 Hz or even 10 Hz. This is primarily due to the radio frequency acceleration used by electron accelerators, which prevents the electron gun from providing high-repetition-rate, high-current electron beams. Furthermore, X-ray sources driven by high-power femtosecond lasers typically have repetition rates below 10 Hz due to the repetition rate limitations of the laser pump source. Furthermore, the system's operating and maintenance costs are also very high, making it difficult to promote and apply in ordinary laboratories.

[0015] Time-resolved techniques, such as the "pump-probe" method, play a crucial role in cutting-edge basic research in ultrafast photonics, carrier dynamics, material phase transitions, and chemical reaction kinetics. This method requires the probe pulse to be sufficiently short and the spatiotemporal synchronization between the probe pulse and the pump pulse to be sufficiently precise. For some tasks requiring imaging, the probe must also possess sufficient spatial resolution. Therefore, an ultrafast pulsed radiation source with high synchronization accuracy is a key element in conducting research on the application of time-resolved "pump-probe" methods. High repetition rates are also beneficial for meeting the needs of various experimental scenarios. Given the demand for ultrafast pulsed X-ray sources in pump-probe technology involving multiple beam combinations, it is imperative to calibrate the time response capabilities of various pulsed radiation field detection systems. Summary of the Invention

[0016] The purpose of the present invention is to address the problems of existing ps-level ultrafast pulse X-ray sources, such as high construction cost, large system scale, low repetition frequency, and difficulty in popularization and application in ordinary laboratories, as well as the technical problem of low synchronization time accuracy in the pump-probe method. The present invention provides a high repetition rate ultrafast pulse X-ray source generation system and generation method, which achieves high repetition frequency (≥1MHz), continuously adjustable X-ray peak energy, high synchronization time accuracy, low construction cost, and small footprint. It can also provide experimental research conditions for ultrafast pulse radiation detection at the picosecond or even femtosecond level, and can meet the basic requirements for ultrafast pulse X-ray sources in research work such as detector performance calibration and multi-beam pump detection.

[0017] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0018] A high repetition rate ultrafast pulse X-ray source system, which is special in that it includes a DC high voltage source, a driving laser, an electron gun, a first correction magnet, an electromagnetic solenoid, a second correction magnet and a target chamber arranged in sequence along the direction of photoelectron emission from the electron gun, wherein a composite target holder is arranged in the target chamber;

[0019] The driving laser is used to generate an ultrashort pulse laser beam;

[0020] A DC high-voltage source is connected to the electron gun's photocathode, which is located in the optical path of the ultrashort pulse laser. The DC high-voltage source is used to apply a negative high voltage to the electron gun's photocathode, causing it to generate ultrafast pulsed photoelectrons. This forms an electrostatic field between the electron gun's cathode and anode, accelerating the photoelectrons. Once the photoelectrons reach a set energy, the electron beam is output from the electron gun's anode.

[0021] The first correction magnet is used to adjust the position of the electron beam so that the electron beam passes through the center of the electromagnetic solenoid; the electromagnetic solenoid is used to perform transverse beam spot focusing on the electron beam;

[0022] The second correction magnet is used to control the emission direction of the electron beam and adjust the target position of the electron beam entering the target chamber;

[0023] The electron beam is incident on the composite target holder to generate high repetition rate ultrafast pulse X-rays.

[0024] Furthermore, it also includes ICT probes;

[0025] The target chamber includes a 1# target chamber and a 2# target chamber which are sequentially arranged along the exit path of the electron beam;

[0026] There are two composite target racks, which are respectively arranged in the 1# target chamber and the 2# target chamber;

[0027] The ICT probe is located between the second correction magnet and the 1# target chamber and is used to measure the charge of the electron beam;

[0028] The electron beam is incident on the composite target holder in the 1# target chamber to generate high-energy spectrum X-rays; or the electron beam passes through the 1# target chamber and is incident on the composite target holder in the 2# target chamber to generate low-energy spectrum X-rays.

[0029] Furthermore, it also includes ICT probes;

[0030] The target chamber includes a 2# target chamber and a 1# target chamber which are sequentially arranged along the exit path of the electron beam;

[0031] There are two composite target racks, which are respectively arranged in the 1# target chamber and the 2# target chamber;

[0032] The ICT probe is located behind the 1# target chamber and is used to measure the charge of the X-rays;

[0033] The electron beam is incident on the composite target holder in the 2# target chamber to generate low-energy spectrum X-rays; or the electron beam passes through the 2# target chamber and is incident on the composite target holder in the 1# target chamber to generate high-energy spectrum X-rays.

[0034] Furthermore, the composite target stand includes a first target position, a second target position and a third target position arranged from top to bottom; the first target position is used to set a bremsstrahlung target, the second target position is used to set a fluorescent screen, and the third target position is an empty target;

[0035] A bremsstrahlung target #1 is provided on the first target position in the 1# target chamber, and the angle between the target surface of the 1# bremsstrahlung target and the electron beam is 45°; a second target position in the 1# target chamber is provided perpendicularly to the first target position;

[0036] A 2# Bremsstrahlung target is arranged on the first target position in the 2# target chamber, and the target surface of the 2# Bremsstrahlung target is perpendicular to the electron beam; the angle between the second target position in the 2# target chamber and the electron beam is 45°.

[0037] Furthermore, the system further comprises a gate valve disposed between the electron gun and the first correction magnet, a first ion pump connected to the electron gun, a second ion pump connected to the gate valve, and an electric target lifting mechanism;

[0038] The electric target lifting mechanism is connected to the two composite target stands respectively and is used to lift the composite target stands;

[0039] The first exit window is provided on the 1# target chamber;

[0040] A second exit window is provided on the 2# target chamber.

[0041] Furthermore, the repetition frequency of the driving laser is ≥100 Hz, the output wavelength is less than 300 nm, the ultraviolet laser pulse length is ≤10 ps, ​​and the ultraviolet laser single pulse energy is ≥10 μJ;

[0042] The photocathode material of the electron gun is Cs2Te or mirror oxygen-free Cu or Mg, and the working vacuum degree of the electron gun chamber is higher than 10 -6 Pa, double-sided anti-reflection coating corresponding to the output wavelength of the driving laser;

[0043] The material of the 1# bremsstrahlung target is metal W or metal Ta;

[0044] The material of the second # bremsstrahlung target is metal Al or metal Mo.

[0045] Furthermore, the laser incident window of the electron gun is made of quartz glass;

[0046] The material of the gate valve is metal, and its vacuum degree is ≤10 -6 Pa;

[0047] The effective length of the first correction magnet is ≥100 mm, the integral field is ≥0.25 Gs·m, and the kicking amount is ≥15 mrad;

[0048] The effective length of the electromagnetic solenoid is ≥100mm, and the average integral field is ≥3698Gs 2 ·m;

[0049] The effective length of the second correction magnet is ≥100 mm, the integral field is ≥0.63 Gs·m, and the kicking amount is ≥35 mrad;

[0050] The material of the 1# target chamber is 316L stainless steel. The flange and window of the 1# target chamber must be sealed with knife edge. The thickness of the 1# bremsstrahlung target must be ≥1mm and the diameter must be ≥2cm.

[0051] The 2# target chamber uses metal Al, metal Au or metal Ag with a thickness of ≤500μm. The target surface of the 2# target chamber is processed by metal polishing process; the thickness of the 2# bremsstrahlung target is ≤500μm;

[0052] The first extraction window and the second extraction window are both Be windows, and both have a thickness of 100 μm to 250 μm.

[0053] At the same time, the present invention also provides a method for generating a high repetition rate ultrafast pulse X-ray source, which is based on the above-mentioned high repetition rate ultrafast pulse X-ray source generation system and has the following characteristics:

[0054] 1) A laser beam is incident on the photocathode of the electron gun to generate ultrafast pulsed photoelectrons. At the same time, a DC high-voltage source is used to apply a negative high voltage to the photocathode of the electron gun, forming an electrostatic electric field between the cathode and anode of the electron gun. The electrostatic electric field accelerates the photoelectrons, so that when the photoelectrons reach the set energy, the electron beam is output from the electron gun anode;

[0055] 2) adjusting the electron beam position using a first correction magnet so that the electron beam passes through the center of the electromagnetic solenoid;

[0056] 3) After focusing the electron beam in a transverse beam spot using an electromagnetic solenoid, a second correction magnet is used to control the electron beam's emission direction and adjust the electron beam's target position;

[0057] 4) Make the electron beam hit the target to produce X-rays.

[0058] Furthermore, step 4) is specifically as follows:

[0059] 4.1. Use the ICT probe to measure the charge of the electron beam and adjust the internal current of the electromagnetic solenoid to focus the electron beam on the No. 1 bremsstrahlung target and perform reflection shooting to generate high-energy spectrum X-rays.

[0060] 4.2. Measure the electron beam focal spot, then focus the electron beam on the No. 2 bremsstrahlung target and perform in-situ transmission target shooting to generate low-energy spectrum X-rays.

[0061] Furthermore, in step 1), the incident angle of the pulsed laser beam incident on the photocathode of the electron gun is ≤30°;

[0062] In step 5.1, the angle between the target surface of the reflective type 1# bremsstrahlung target and the electron beam is 45°;

[0063] In step 5.2, the transmission mode is that the 2# bremsstrahlung target and the electron beam are perpendicularly transmitted.

[0064] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0065] (1) Capable of multi-beam combined application. The high-repetition-rate ultrafast pulse X-ray source generation system of the present invention includes three wavelengths of ultrafast laser beams, three types of ps-level electron beams, and different ps-level X-ray ultrafast pulse radiation beams. The multi-beam output capability can well meet the special experimental requirements of multi-beam combined application research.

[0066] (2) The repetition rate of X-rays is very high. In the high repetition rate ultrafast pulse X-ray source generation system of the present invention, the repetition rate of X-rays is consistent with the repetition rate of the driving laser, and the repetition rate of X-rays is very high and can reach above 79MHz. The electron gun used has a high current intensity characteristic of mA level, so that the pulse X-ray source developed based on it can easily achieve repetition rate pulses at the kHz or even 10MHz level. Moreover, since the photoelectron yield depends on the traditional bremsstrahlung process, it has a very stable photon intensity and is very suitable for carrying out "pump-probe" and other application research work with strict requirements on pulse repetition. Compared with the existing technology, due to the limitations of technical factors such as the current intensity of microwave electron guns and the energy level of ultra-high power (generally >10TW) femtosecond lasers, the repetition rate of currently common ultrafast pulse X-ray sources is basically at the 10Hz level, which to a certain extent affects the demand for many high repetition rate X-ray source applications.

[0067] (3) Synchronization time accuracy. The high repetition rate ultrafast pulse X-ray source generation system of the present invention has a synchronization time accuracy of less than 100 minutes and seconds because the X-rays and lasers are "co-sourced". The synchronization time accuracy makes the time synchronization accuracy between the X-rays and laser beams very high. Its accuracy is also one of the core factors for whether it can be used in the "pump-probe" time resolution technology. Common X-ray sources such as free electron laser systems, Thompson scattering X-ray source systems and synchrotron radiation source systems all use microwave accelerator tubes for acceleration. Such electronic processes make the time synchronization at best at the sub-ps level, which also limits the application scope of these systems in ultrafast physical processes such as semiconductor carrier dynamics. Only ultrafast pulse X-ray sources generated by laser beam targeting can achieve synchronization accuracy at the fs level. However, such systems not only have high construction costs but also generally have poor stability in the output photon yield. They are also not suitable for multi-shot scanning "pump-probe" technology research.

[0068] (4) The focal spot size of the X-ray source is small. The composite target holder design in the target chamber of the high-repetition-rate ultrafast pulse X-ray source generation system of the present invention can realize in-situ target shooting with a small focal spot of the electron beam. At the same time, the system of the present invention can accurately measure the focal spot size of the X-ray source. The focal spot size of the X-ray output from the 1# target chamber can usually be less than 500μm, which can meet the application requirements of X-ray imaging with higher spatial resolution.

[0069] (5) The dual target chamber structure enables the output of X-rays with different characteristic energies. The high repetition rate ultrafast pulse X-ray source generation system of the present invention requires different characteristic energies for ultrafast pulse X-ray diffraction technology. The dual target chambers (i.e., target chamber 1 and target chamber 2) provided in the system of the present invention are equipped with different bremsstrahlung targets. The target materials and the reflection and transmission target shooting methods can be easily switched to output X-rays with different characteristic energies. In addition, the target materials in each target chamber have the function of being replaced to meet different X-ray application requirements.

[0070] (6) Suppressing electron beam dispersion. The high-repetition-rate ultrafast pulse X-ray source generation system of the present invention fully utilizes the advantages of femtosecond laser beam and electrostatic electric field acceleration. The synchronization time accuracy is only related to the DC high-voltage power supply. The design structure is compact and can effectively suppress the dispersion of electron beam over time. It can achieve synchronization time accuracy below 100 fs, which meets the requirements of experimental research on the dynamic processes of common radiation converters such as scintillators and semiconductors.

[0071] (7) Electron beam energy is continuously adjustable. The high repetition rate ultrafast pulse X-ray source generation method of the present invention can easily achieve continuous adjustment of electron beam energy by adjusting the voltage value of the DC high-voltage source, thereby achieving adjustment of the peak energy of the X-ray. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 This is a schematic structural diagram of an embodiment of a high repetition rate ultrafast pulse X-ray source generation system of the present invention;

[0073] Figure 2 Schematic diagram of the structure of the composite target holder in an embodiment of the high repetition rate ultrafast pulse X-ray source generation system of the present invention;

[0074] Figure 3 A schematic diagram of the path of the electron beam targeting in an embodiment of the high repetition rate ultrafast pulse X-ray source generation system of the present invention;

[0075] Figure 4 This is a graph showing the relationship between the average total electron beam current, the single pulse charge, and the accelerating voltage at a power of 0.15 W in an embodiment of the high repetition rate ultrafast pulse X-ray source generation system of the present invention. The square curve represents the average total electron beam current, the triangle curve represents the single pulse charge, and the circle curve represents the net photocurrent. The abscissa represents the accelerating voltage, and the ordinate represents the average electron beam current.

[0076] Figure 5 Schematic diagram of the electron focal spot obtained after focusing by the electromagnetic solenoid in an embodiment of the high repetition rate ultrafast pulse X-ray source generation system of the present invention;

[0077] Figure 6 This is an X-ray waveform diagram obtained by using MCP in an embodiment of the high repetition rate ultrafast pulse X-ray source generation system of the present invention; the abscissa represents time, and the ordinate represents the oscilloscope waveform voltage amplitude;

[0078] Figure 7 This is an X-ray waveform diagram obtained by using MCP in combination with ZnO scintillator in an embodiment of the high repetition rate ultrafast pulse X-ray source generation system of the present invention. The horizontal axis represents time, and the vertical axis represents the oscilloscope waveform voltage amplitude;

[0079] Figure 8This is a schematic diagram of an embodiment of a method for generating a high repetition rate ultrafast pulse X-ray source according to the present invention.

[0080] The accompanying drawings are denoted as follows:

[0081] 1-driving laser, 2-electron gun, 21-DC high-voltage source, 22-first ion pump, 3-gate valve, 31-second ion pump, 4-first calibration magnet, 5-electromagnetic solenoid, 6-second calibration magnet, 7-ICT probe, 8-1# target chamber, 81-first extraction window, 82-1# Bremsstrahlung radiation, 9-2# target chamber, 91-second extraction window, 92-2# Bremsstrahlung radiation, 10-composite target holder. DETAILED DESCRIPTION

[0082] like Figure 1 As shown, the present invention provides a high repetition rate ultrafast pulse X-ray source system for generating a high repetition rate ultrafast pulse X-ray source, including a DC high-voltage source 21, a driving laser 1, an electron gun 2, a gate valve 3 arranged in sequence along the photoelectron emission direction of the electron gun 2, a first correction magnet 4, an electromagnetic solenoid 5, a second correction magnet 6, an ICT probe 7, a first ion pump 22, a second ion pump 82, an electric target lifting mechanism and a target chamber.

[0083] The driving laser 1 is used to generate an ultrashort pulse laser beam; in this embodiment, the repetition frequency of the driving laser 1 is ≥100 Hz, the output wavelength is less than 300 nm, the ultraviolet laser pulse length is ≤10 ps, ​​and the ultraviolet laser single pulse energy is ≥10 μJ.

[0084] The electron gun 2 is used to output an electron beam; the DC high-voltage source 21 is used to apply a high voltage of -150kV to -20kV to the photocathode of the electron gun 2, so that the photocathode generates ultrafast pulsed photoelectrons, and forms an electrostatic electric field between the cathode and anode of the electron gun 2, which is used to accelerate the photoelectrons so that after the photoelectrons reach the set energy, the electron beam is output from the anode of the electron gun 2; preferably, the electron gun 2 adopts an electrostatic acceleration method, and a negative high voltage is applied to the photocathode, and the voltage adjustment range is -150kV to -20kV. The distance between the photocathode and the anode is adjustable, and the replacement of photocathodes of different materials is achieved by using a detachable photocathode head. The first ion pump 22 is connected to the electron gun 2, so that the working vacuum degree of the chamber of the electron gun 2 is higher than 10 -6 Pa.

[0085] In this embodiment, the electron gun 2 performs a two-stage acceleration function. Initial photoelectrons are first electrostatically accelerated by the electron gun 2. After being focused within the electromagnetic solenoid 5, a DC high-voltage source 21 is introduced for secondary acceleration. This DC high voltage is a positive high voltage applied to the cathode of the electron gun 2, which is insulated from the vacuum tube using insulating ceramic. The photocathode material of the electron gun 2 is Cs2Te; in other embodiments, mirror-finished oxygen-free Cu or Mg can also be used. The laser entrance window of the electron gun 2 is made of quartz glass, coated on both sides with an anti-reflection coating of 266 nm or the corresponding output wavelength of the drive laser 1.

[0086] In this embodiment, the material of the gate valve 3 is metal, and the second ion pump 31 is connected to the gate valve 3 to make its vacuum degree ≤10 -6 The first correction magnet 4 is used to adjust the electron beam position; the effective length of the first correction magnet 4 is ≥100 mm, the integral field is ≥0.25 Gs·m, and the kicking amount is ≥15 mrad.

[0087] The electromagnetic solenoid 5 is used to focus the electron beam in the transverse direction; the effective length of the electromagnetic solenoid 5 is ≥100mm, and the average integral field is ≥3698Gs 2 ·m.

[0088] The second correction magnet 6 is used to control the emission direction of the electron beam and adjust the target position of the electron beam. In this embodiment, the effective length of the second correction magnet 6 is ≥100 mm, the integral field is ≥0.63 Gs·m, and the kicking amount is ≥35 mrad.

[0089] The ICT probe 7 is used to measure the charge amount of the electron beam.

[0090] The target chamber is used to generate X-rays of different energies. Figure 2 、 Figure 3As shown, the target chamber includes a 1# target chamber 8 and a 2# target chamber 9 arranged in sequence along the exit path of the electron beam, and two composite target stands 10 and two extraction windows respectively arranged in the 1# target chamber 8 and the 2# target chamber 9; the extraction windows are used to extract the generated high repetition rate ultrafast pulse X-ray beam; the two extraction windows are a first extraction window 81 arranged on the 1# target chamber 8 and a second extraction window 91 arranged on the 2# target chamber 9; the two composite target stands 10 are respectively connected to the electric target lifting mechanism for lifting the composite target stand; the composite target stand 10 includes a first target position, a second target position and a third target position arranged from top to bottom; the first target position is used to set the bremsstrahlung target, the second target position is used to set the fluorescent screen, and the third target position is an empty target for electric The electron beam passes through; a 1# bremsstrahlung target 82 is provided on the first target position 11 in the 1# target chamber 8, and the target surface of the 1# bremsstrahlung target 82 has an angle of 45° with the electron beam; the second target position 12 and the third target position 13 in the 1# target chamber 8 are located on the same plane, and the second target position 12 and the third target position 13 are both arranged perpendicular to the first target position 11; a 2# bremsstrahlung target 92 is provided on the first target position 11 in the 2# target chamber 9, and the 2# bremsstrahlung target 92 has the same structure as the 1# bremsstrahlung target 82; the target surface of the 2# bremsstrahlung target 92 is perpendicular to the electron beam; the second target position 12 and the third target position 13 in the 2# target chamber 9 are located on the same plane, and the target surface of the second target position 12 has an angle of 45° with the electron beam.

[0091] 1# Bremsstrahlung target 82 adopts an electric target lifting mechanism to realize the insertion and removal of the target piece of 1# Bremsstrahlung target 82. When inserted, 1# Bremsstrahlung is fired, and when removed, the electron beam is offset backward to realize the measurement of the electron beam profile or the firing of 2# Bremsstrahlung target 92; the composite chamber of the electron beam profile measurement chamber and 2# target chamber 9 adopts an in-situ firing structure, and the target piece of 2# Bremsstrahlung target 92 is coaxially arranged with the fluorescent screen (i.e., YAG fluorescent screen) in the vertical beam direction, and the normal of the target surface is 45° to the normal of the YAG fluorescent screen, that is, the YAG fluorescent screen is 45° to the incident direction of the electron beam, and the electron beam is vertically incident on the target surface of 2# Bremsstrahlung target 92, and the target surface of 2# Bremsstrahlung target 92 is processed by metal polishing process; 2# target chamber 9 adopts energy spectrum design, and under the premise of mainly adopting transmission firing, it has the basic conditions for side window emission of reflection firing.

[0092] In this embodiment, the material of the 1# target chamber 8 is 316L stainless steel, and the flange and window of the 1# target chamber 8 must be knife-edge sealed; the material of the 1# bremsstrahlung target 82 is metal W or metal Ta, with a thickness ≥1 mm and a diameter ≥2 cm; the 2# target chamber 9 is made of metal Al, and in other embodiments, metal Au or metal Ag can also be used, with a thickness ≤500 μm, and the target surface of the 2# target chamber 9 is processed by a metal polishing process; the material of the 2# bremsstrahlung target 92 is metal Al or metal Mo, with a thickness ≤500 μm; the first extraction window 81 and the second extraction window 91 are both Be windows, with a thickness of 100 μm to 250 μm.

[0093] The position of target chamber 2# and ICT probe 7 can also be interchanged. The length and size of the brackets and pipeline installation of the two are consistent. After target chamber 2# is replaced to the position of ICT probe 7, a reflection target shooting method is adopted to achieve a smaller beam spot and a shorter X-ray pulse.

[0094] The working principle of the above system is:

[0095] The 266nm ultrashort pulse laser beam generated by the driving laser 1 is incident on the photocathode of the electron gun 2 at a relatively small angle (relative to the normal line of the photocathode) to generate ultrafast pulse photoelectrons. The photocathode is applied with negative high voltage by a DC high-voltage power supply, so that an electrostatic electric field is formed between the cathode and anode of the electron gun 2. The electrostatic electric field accelerates the photoelectrons so that after the photoelectrons reach the set energy, the electron beam is output from the anode of the electron gun 2; after the electron beam passes through the gate valve 3, the first correction magnet 4 is used to adjust the position of the electron beam so that the electron beam passes through the center of the electromagnetic solenoid 5, and the electromagnetic solenoid 5 focuses the electron beam on the horizontal beam spot, and then the second correction magnet 6 is used to control the emission direction of the electron beam, thereby adjusting the target position of the electron beam. The charge of the electron beam is measured by the ICT probe 7, and the internal current of the electromagnetic solenoid 5 is adjusted to focus the electron beam on the target position on the 1# Bremsstrahlung target 82 in the 1# target chamber 8 and hit the target to generate X-rays. The 2# target chamber can be used to measure the focal spot of the electron beam, and at the same time, to achieve the purpose of in-situ measurement and in-situ hitting of the 2# Bremsstrahlung target 92 to generate X-rays. In the 1# target chamber 8, the electron beam is incident on the 1# Bremsstrahlung target 82 at 45°, and a high repetition rate ultrafast pulse X-ray beam (ps level) is generated after the X-rays interact with the 1# Bremsstrahlung target 82 perpendicular to the incident direction of the electron beam. A stainless steel cavity is used around the 1# target chamber 8 for X-ray shielding and sealing to ensure the safety of laboratory personnel.

[0096] In this embodiment, a YAG:Nd laser with a central wavelength of 1064nm is frequency quadrupled to obtain a 266nm driving laser beam. The laser beam spot size is approximately 1mm, the repetition frequency is 2MHz, the pulse width is ≤10ps, and the photoelectron distribution is Gaussian circular. Cs2Te is used as the photocathode material of the electron gun 2. A high-voltage DC voltage of ≤-100kV is applied to the cathode of the electron gun 2. The distance between the cathode and anode of the electron gun 2 is 40mm. The length of the electromagnetic solenoid 5 is 200mm, its effective magnetic length is 160mm, and the maximum magnetic field on the axis is 2000Gs. The effective lengths of the first correction magnet 4 and the second correction magnet 6 are both 100mm, and their magnetic field stability is <10 -3 The central magnetic field is 100Gs. The 1# bremsstrahlung target 82 adopts a 2mm thick W target with a target piece size of 20mm. 45° reflection shooting is adopted. The X-ray extraction window on the 1# target chamber 8 adopts a 200μm thick Be window. The charge of the electron beam is obtained by measuring the average current intensity of the electron beam.

[0097] like Figure 4 As shown in the figure, in order to examine the relationship between the accelerating voltage, i.e. the DC high voltage power supply, and the electron beam charge, we first fixed the laser power of the pulsed laser beam at 0.15W and changed the accelerating voltage of the DC high voltage power supply. The measured curves of the total electron beam average current intensity and dark emission current change with voltage are as follows. Figure 5 As can be seen, as the accelerating voltage increases, the dark emission current and the average total electron beam current also increase. At 50 kV, the average total electron beam current reaches a maximum of 180 μA, with a net photocurrent of 116 μA, corresponding to a single-pulse charge of 58 pC. Under these conditions, the target single-pulse charge of 50 pC is achieved.

[0098] like Figure 5 As shown, the electron beam focal spot lateral information is obtained by measuring the YAG screen profile. The electron beam focal spot photograph is obtained at the center of the YAG screen profile measurement cavity. The diameter of the YAG screen is 20 mm. Due to the high average current intensity of the total electron beam, the YAG screen has been cracked. Therefore, the focus position is offset by the first correction magnet 4. The acceleration voltage during measurement is -30 kV. Figure 6As can be seen in the figure, under the focusing action of the electromagnetic solenoid 5, the electron beam focal spot can be focused to a minimum of approximately 2.5 mm, exceeding the design target of 2 mm. This is mainly due to the significant increase in electron beam emittance caused by the oblique incidence of the laser beam. Generally, as the acceleration voltage increases, the electron beam emittance decreases, and the 1# target chamber 8 is closer to the photocathode of the electron gun 2. Therefore, the final electron beam focal spot size obtained during actual target shooting will be less than 2.5 mm. After target shooting with an electron beam at this electron beam focal spot level, an X-ray point source with the same electron beam focal spot size can be generated. This shows that the present invention can easily realize a small focal spot X-ray source, thereby meeting the demand for pulsed X-ray sources for X-ray imaging applications with high spatial resolution requirements.

[0099] For a single pulse charge of 50 pC, the total number of electrons is 3×10 8 / pulse, hitting a 2mm thick W target can produce a conversion efficiency of 1%, and the number of photons in a single pulse X-ray can reach 3×10 6 This intensity can meet the requirements of most applications including X-ray diffraction.

[0100] like Figure 6 As shown in the figure, the X-ray pulse waveform and pulse repetition frequency measurement results were measured using a fast-response microchannel photomultiplier (MCP) in the experiment. The electron beam acceleration voltage was 60kV and the signal amplitude was 1.5V. It can be seen that the waveform basically reflects the extreme time response capability of the MCP. The pulse width is about 330ps and the rising edge is 145ps. Figure 7 As shown, the waveform on the upper side of the figure shows that the X pulse repetition frequency is 2MHz, and corresponds one to one with the waveform of the monitoring signal of the driving laser 1 at the lower side of the figure.

[0101] Based on the above test results, it can be seen that the method of the present invention can easily achieve high-repetition-rate ultrafast pulsed X-ray beam output at the MHz level, with high pulse intensity, which can meet the application requirements of detector time response characteristics research, material ultrafast dynamic characteristics research, X-ray imaging, etc. The typical pulse width of the system of the present invention is generally ≤10ps, and the operating modes are divided into long pulse mode and short pulse mode, focusing on high pulse intensity and fast pulse duration requirements respectively. The X-ray photon energy range is less than 150keV; the maximum single-pulse electron beam charge is generally ≥100pC; the repetition rate is determined by the repetition rate of the driving laser 1 and can be adjusted between 5Hz and 10MHz according to actual needs.

[0102] like Figure 8 As shown, a method for generating a high repetition rate ultrafast pulse X-ray source includes the following steps:

[0103] 1) A laser beam is incident on the photocathode of the electron gun 2 to generate ultrafast pulsed photoelectrons. At the same time, a negative high voltage is applied to the photocathode of the electron gun 2 by a DC high-voltage source 21, forming an electrostatic electric field between the cathode and anode of the electron gun 2. The electrostatic electric field accelerates the photoelectrons, so that when the photoelectrons reach a set energy, an electron beam is output from the anode of the electron gun 2;

[0104] 2) The electron beam position is adjusted using the first correction magnet 4 so that the electron beam passes through the center of the electromagnetic solenoid 5 .

[0105] 3) After the electromagnetic solenoid 5 is used to perform transverse beam spot focusing on the electron beam, the second correction magnet 6 is used to control the emission direction of the electron beam and adjust the target position of the electron beam.

[0106] 4) causing the electron beam to hit the target and generate X-rays; specifically:

[0107] 4.1. Use the ICT probe 7 to measure the charge of the electron beam and adjust the internal current of the electromagnetic solenoid 5 to focus the electron beam on the first bremsstrahlung target 82 and perform reflection shooting to generate high-energy spectrum X-rays.

[0108] 4.2. Measure the electron beam focal spot, then focus the electron beam on the 2# bremsstrahlung target 92 and perform in-situ transmission target shooting to generate low-energy spectrum X-rays.

[0109] In this embodiment, the reflection-type targeting is that the angle between the target surface of the 1# bremsstrahlung target 82 and the electron beam is 45°; the transmission-type targeting is that the target surface of the fluorescent screen and the electron beam are perpendicular to each other.

Claims

1. A high repetition rate ultrafast pulse X-ray source generation system, characterized by: The invention comprises a DC high voltage source (21), a driving laser (1), an electron gun (2), a first correction magnet (4), an electromagnetic solenoid (5), a second correction magnet (6) and a target chamber, wherein a composite target frame (10) is arranged in the target chamber; A gate valve (3), a first ion pump (22) connected to the electron gun (2), a second ion pump (31) connected to the gate valve (3), and an electric target lifting mechanism are provided between the electron gun (2) and the first correction magnet (4); the driving laser (1) is used to generate an ultrashort pulse laser beam; The DC high-voltage source (21) is connected to the photocathode of the electron gun (2); the photocathode of the electron gun (2) is located in the optical path of the ultrashort pulse laser; the DC high-voltage source (21) is used to apply a negative high voltage to the photocathode of the electron gun (2), so that the photocathode generates ultrafast pulse photoelectrons, and forms an electrostatic electric field between the cathode and anode of the electron gun (2) for accelerating the photoelectrons, so that the electron beam is output from the anode of the electron gun (2) after the photoelectrons reach a set energy; The first correction magnet (4) is used to adjust the position of the electron beam so that the electron beam passes through the center of the electromagnetic solenoid (5); the electromagnetic solenoid (5) is used to perform transverse beam spot focusing on the electron beam; The second correction magnet (6) is used to control the emission direction of the electron beam and adjust the target position of the electron beam entering the target chamber; The target chamber comprises a 1# target chamber (8) and a 2# target chamber (9) sequentially arranged along the exit path of the electron beam; there are two composite target racks (10), which are respectively arranged in the 1# target chamber (8) and the 2# target chamber (9); the electron beam is incident on the composite target rack (10) to generate high-repetition-rate ultrafast pulse X-rays; the electric target lifting mechanism is respectively connected to the two composite target racks (10) to lift the composite target rack (10); the 1# target chamber is provided with a first extraction window (81); and the 2# target chamber is provided with a second extraction window (91).

2. A high repetition rate ultrafast pulse X-ray source generation system according to claim 1, characterized in that: Also included is an ICT probe (7) for measuring the charge amount of the electron beam current; The ICT probe (7) is located between the second correction magnet (6) and the first target chamber (8) and is used to measure the charge of the electron beam; The electron beam is incident on the composite target stand (10) in the first target chamber (8) to generate high-energy spectrum X-rays; or the electron beam passes through the first target chamber (8) and is incident on the composite target stand (10) in the second target chamber (9) to generate low-energy spectrum X-rays.

3. The high repetition rate ultrafast pulse X-ray source generation system according to claim 1, characterized in that: Also included is an ICT probe (7) for measuring the charge amount of the electron beam current; The target chamber comprises a 2# target chamber (9) and a 1# target chamber (8) which are sequentially arranged along the exit path of the electron beam; There are two composite target stands (10), which are respectively arranged in the 1# target chamber (8) and the 2# target chamber (9); The ICT probe (7) is located behind the 1# target chamber (8) and is used to measure the charge of the X-rays; The electron beam is incident on the composite target stand (10) in the second target chamber (9) to generate low-energy spectrum X-rays; or the electron beam passes through the second target chamber (9) and is incident on the composite target stand (10) in the first target chamber (8) to generate high-energy spectrum X-rays.

4. A high repetition rate ultrafast pulse X-ray source generation system according to claim 2 or 3, characterized in that: The composite target stand (10) comprises a first target position, a second target position and a third target position arranged from top to bottom; the first target position is used to set a bremsstrahlung target, the second target position is used to set a fluorescent screen, and the third target position is an empty target; A first target position in the first target chamber (8) is provided with a first bremsstrahlung target (82), and the angle between the target surface of the first bremsstrahlung target (82) and the electron beam is 45°; a second target position in the first target chamber (8) is provided perpendicularly to the first target position; A 2# bremsstrahlung target (92) is provided on the first target position in the 2# target chamber (9), and the target surface of the 2# bremsstrahlung target (92) is perpendicular to the electron beam flow; and the angle between the second target position in the 2# target chamber (9) and the electron beam flow is 45 degrees.

5. The high repetition rate ultrafast pulse X-ray source generation system according to claim 4, characterized in that: The repetition frequency of the driving laser (1) is ≥100 Hz, the output wavelength is less than 300 nm, the ultraviolet laser pulse length is ≤10 ps, ​​and the ultraviolet laser single pulse energy is ≥10 μJ; The photocathode material of the electron gun (2) is Cs2Te or mirror-surface oxygen-free Cu or Mg, and the working vacuum degree of the chamber of the electron gun (2) is higher than 10 -6 Pa, double-sidedly coated with an antireflection film corresponding to the output wavelength of the driving laser (1); The material of the first bremsstrahlung target (82) is metal W or metal Ta; The material of the second # bremsstrahlung target (92) is metal Al or metal Mo.

6. A high repetition rate ultrafast pulse X-ray source generation system according to claim 5, characterized in that: The laser incident window of the electron gun (2) is made of quartz glass; The material of the gate valve (3) is metal, and its vacuum degree is ≤10 -6 Pa; The effective length of the first correction magnet (4) is ≥100 mm, the integral field is ≥0.25 Gs·m, and the kicking amount is ≥15 mrad; The effective length of the electromagnetic solenoid (5) is ≥100 mm, and the average integral field is ≥3698 Gs 2 •m; The effective length of the second correction magnet (6) is ≥100 mm, the integral field is ≥0.63 Gs·m, and the kicking amount is ≥35 mrad; The material of the 1# target chamber (8) is 316L stainless steel, and the flange and window of the 1# target chamber must be sealed with a knife edge; the thickness of the 1# bremsstrahlung target (82) is ≥1mm, and the diameter is ≥2cm; The second target chamber (9) is made of metal Al, metal Au, or metal Ag, with a thickness of ≤500 μm, and the target surface of the second target chamber (9) is processed by a metal polishing process; the thickness of the second bremsstrahlung target (92) is ≤500 μm; The first lead-out window (81) and the second lead-out window (91) are both Be windows, and both have a thickness of 100 μm to 250 μm.

7. A method for generating a high repetition rate ultrafast pulse X-ray source, based on a high repetition rate ultrafast pulse X-ray source generating system according to any one of claims 1 to 6, characterized in that: The following steps are involved: 1) A laser beam is incident on the photocathode of the electron gun (2) to generate ultrafast pulsed photoelectrons. At the same time, a DC high-voltage source (21) is used to apply a negative high voltage to the photocathode of the electron gun (2), thereby forming an electrostatic electric field between the cathode and anode of the electron gun (2). The electrostatic electric field accelerates the photoelectrons, so that after the photoelectrons reach a set energy, an electron beam current is output from the anode of the electron gun (2); 2) using the first correction magnet (4) to adjust the electron beam position so that the electron beam passes through the center of the electromagnetic solenoid (5); 3) After the electron beam is focused laterally by the electromagnetic solenoid (5), the second correction magnet (6) is used to control the direction of the electron beam and adjust the target position of the electron beam; 4) Make the electron beam hit the target to produce X-rays.

8. The method for generating a high repetition rate ultrafast pulse X-ray source according to claim 7, characterized in that: Step 4) is as follows: 4.

1. Using the ICT probe (7) to measure the charge of the electron beam, and adjusting the internal current of the electromagnetic solenoid (5), the electron beam is focused on the No. 1 bremsstrahlung target (82) and reflection-type target shooting is performed to generate high-energy spectrum X-rays; 4.

2. Measure the electron beam focal spot, then focus the electron beam on the 2# bremsstrahlung target (92) and perform in-situ transmission target shooting to generate low-energy spectrum X-rays.

9. The method for generating a high repetition rate ultrafast pulse X-ray source according to claim 8, characterized in that: In step 1), the incident angle of the pulsed laser beam incident on the photocathode of the electron gun (2) is ≤30°; In step 5.1, the angle between the target surface of the reflective type 1# bremsstrahlung target (82) and the electron beam is 45°; In step 5.2, the transmission type is that the 2# bremsstrahlung target (92) and the electron beam are vertically transmitted.

Images