Wedge-shaped supersonic gas target for laser accelerator

Abstract

The invention belongs to the technical field of laser accelerators, and particularly relates to a wedge-shaped supersonic gas target for a laser accelerator, which comprises a target body and a base, the upper end face and the lower end face of the target body are respectively provided with a gas nozzle and a gas inlet, a wedge-shaped cavity is arranged in the target body, and the wedge-shaped cavity is respectively communicated with the gas inlet and the gas nozzle. The density distribution of the supersonic gas jet jetted from the gas jet hole in the laser transmission direction is in positive gradient distribution; the target body is arranged on a base, and the base is provided with a slit matched with the air inlet. According to the wedge-shaped supersonic gas target conforming to the electron transport characteristic of the laser accelerator, the density distribution of supersonic gas jet in the laser transmission direction is in positive gradient distribution, and the gas density can be continuously increased; the phase loss effect of phase mismatching generated by the electron beam is avoided, the energy and monoenergy of the electron beam are effectively improved, and the high-quality electron beam is provided for the laser accelerator in the aspects of ultrafast radiation source driving, ultrafast process diagnosis, high-resolution imaging and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of laser accelerators, and in particular relates to a wedge-shaped supersonic gas target for a laser accelerator. Background Art

[0002] With the continuous development of ultrashort and ultra-intense laser technology, new particle accelerators driven by lasers can generate accelerating electric fields up to 100GV / m because there is no breakdown threshold limitation. The acceleration gradient is increased by 3-4 orders of magnitude compared with traditional radio frequency accelerators. It is expected to provide a new technical approach for the miniaturization of accelerators, which has important scientific research and application significance.

[0003] The interaction between ultrafast and ultra-intense lasers and targets can accelerate charged particles. In 1979, American scientists John Dawson and Toshi Tajima first proposed the concept of laser wakefield-driven electron acceleration (LWFA). Through the wakefield driven by the propagation of lasers in plasma, an extremely high accelerating electric field can be obtained, thereby obtaining a high-energy electron beam within a very short scale range. Among them, supersonic gas targets, as important experimental targets in laser accelerators, can produce gas media with local high density and high repetition rate under vacuum conditions. The generated gas jet has a long range, steep edges and a nearly flat-topped density distribution, which can provide a gas jet with a specific density for acceleration experiments. Due to the high controllability of this type of target, the density of the ionized plasma can be regulated in real time, thereby affecting the quality of the electron beam accelerated by the laser-driven wakefield. The energy, charge, and monoenergeticity of the electron beam can be effectively improved, and it can be used in high repetition rate experiments. It is an important solution to achieve effective control and quality improvement of the electron beam. The high-quality quasi-monoenergetic electron beam produced by the supersonic gas target can be used to further carry out work in laser-driven high-energy Betatron radiation, Compton scattering γ source, ultrafast electron diffraction, beam imaging, etc., which has important significance and application value.

[0004] At present, the international research focus on supersonic gas targets for laser accelerators is mainly on improving the density of gas jets. The supersonic gas targets used mainly produce conical and long strip gas jets, and the gas density distribution provided is a flat-top distribution. The team of Jena University in Germany developed two conical nozzles with Mach numbers of 3 and 6, and obtained 10 18 / cm 3The research team of ELI-Beamline conducted an experimental study on a long supersonic gas target with a diameter of 6.8mm×1.26mm, and obtained simulation and experimental data of density distribution at different distances from the nozzle. In addition, Source LAB in France and Smartshell in Japan have developed a variety of conical and long gas targets.

[0005] This type of gas target is axially symmetrically distributed, with mature technology and certain advantages in terms of gas density uniformity. However, in practical applications, due to the characteristics of electron transport during the acceleration process of the laser wakefield, the flat-top gas density distribution will cause the electrons injected into the wakefield to gradually move from the acceleration phase to the deceleration phase, resulting in a dephasing effect, which inhibits the increase in the energy of the electron beam. According to the principle of laser accelerator, there are currently two main solutions to solve the electron beam dephasing effect. One is to use the laser to attenuate in the plasma to reduce the scale of the wakefield, which requires the control of the light beam generated by a large ultrashort and ultra-intense laser device. The operation is complex and difficult. The other is to adjust the wakefield to generate the longitudinal density of the plasma, which can be achieved by designing and developing the structure of the experimental target without significantly changing the overall experimental system. It is highly operational and easy to implement.

[0006] Since the existing supersonic gas target structure is not conducive to reducing the influence of the dephasing effect in the laser accelerator to produce the high-energy and high-quality electron beam required for practical applications, the key problem to be solved by the present invention is to design and develop a new supersonic gas target for the purpose of reducing the influence of the dephasing effect in combination with the electron transport characteristics in the laser accelerator. Summary of the invention

[0007] In view of the problems existing in the prior art, the purpose of the present invention is to provide a wedge-shaped supersonic gas target for a laser accelerator. This scheme targets the dephasing effect produced in the process of laser-driven electron acceleration. Combining the characteristics of electron transport for the first time, it can effectively reduce the limitation on the energy gain of the electron beam due to the dephasing effect, so as to obtain an electron beam with higher energy and better monochromaticity, and provide high-quality beam for the further application of laser accelerators in multiple fields.

[0008] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

[0009] A wedge-shaped supersonic gas target for a laser accelerator comprises a target body and a base, wherein the upper and lower end surfaces of the target body are respectively provided with a jet port and an air inlet, and a wedge-shaped cavity is arranged inside the target body, wherein the wedge-shaped cavity is respectively connected with the air inlet and the jet port, so that the density distribution of the supersonic gas jet ejected from the jet port along the laser transmission direction presents a positive gradient distribution; the target body is arranged on the base, and the base is provided with a slit matched with the air inlet.

[0010] Furthermore, the wedge-shaped cavity is wider at one end and narrower at the other end in the horizontal direction, and is wider at the upper end and narrower at the lower end in the vertical direction.

[0011] Furthermore, the jet port is trapezoidal, and the air inlet is a rectangular slit, so as to ensure that the target body has a narrow throat structure.

[0012] Furthermore, the upper base of the jet trapezoid is L1, the lower base is L2, and the height is H. The length of the rectangular slit of the air inlet is d, the width is w, and the total height of the target body can all be adjusted and designed according to the required gas density and density gradient distribution, satisfying: L2>L1≥w, H≥d.

[0013] Furthermore, the L1, L2 and w are all in the order of hundreds of micrometers to millimeters; and the H and d are all in the order of millimeters to centimeters.

[0014] Furthermore, the lower surface of the base is tightly connected to the gas supply solenoid valve to provide high-pressure gas input for the target body.

[0015] Furthermore, the lower surface of the base is fixed to the air supply solenoid valve via a plurality of positioning screws and a rubber sealing ring.

[0016] Furthermore, the base is a disc structure.

[0017] Furthermore, the roughness of the inner wall of the wedge-shaped cavity needs to be controlled to achieve a smooth transition between the target body contour and the inner wall.

[0018] Furthermore, the gas target is arranged in a vacuum target field, and the laser beam is emitted toward the gas target along the length direction of the wedge-shaped cavity, interacting with the gas jet to generate a quasi-monoenergetic electron beam.

[0019] Furthermore, the gas is hydrogen, helium, nitrogen or argon, or the above types of gases are mixed in different proportions according to different laser acceleration mechanisms.

[0020] The beneficial effects of the present invention are as follows:

[0021] 1. A new wedge-shaped supersonic gas target that meets the electron transport characteristics of the laser accelerator makes the supersonic gas jet density distribution along the laser transmission direction present a positive gradient distribution, and the gas density can be continuously increased;

[0022] 2. Different from the flat-top density distribution produced by the existing supersonic gas target, the new gas target can obtain a plasma with a positive gradient density distribution, avoiding the dephasing effect caused by phase mismatch of the electron beam, thereby effectively improving the energy and monoenergeticity of the electron beam, and providing high-quality electron beams for laser accelerators in driving ultrafast radiation sources, ultrafast process diagnosis, high-resolution imaging, life sciences, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The figure is a schematic diagram of a wedge-shaped supersonic gas target for a laser accelerator according to the present invention.

[0024] Figure 2 The figure is a cross-sectional view of a wedge-shaped supersonic gas target for a laser accelerator according to the present invention.

[0025] Figure 3 Schematic diagram of the interaction between laser and gas medium generated by wedge-shaped supersonic gas target.

[0026] Figure 4 This is a schematic diagram of the connection between the wedge-shaped supersonic gas target and the gas supply solenoid valve.

[0027] Figure 5 Schematic diagram of the layout of a laser wake electron accelerator device using a wedge-shaped supersonic gas target. DETAILED DESCRIPTION

[0028] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0029] The present invention designs a supersonic gas target with a wedge-shaped structure, which can generate a supersonic gas jet with a positive gradient density distribution, compensate for the phase shift produced during the electron acceleration process, thereby solving the electron dephasing effect and obtaining an electron beam with higher energy gain. The specific technical solution is as follows:

[0030] In the process of laser wakefield driven electron acceleration, since the wakefield is located at the tail of the laser, its propagation speed is slightly lower than the speed of light, and the speed of the electron beam accelerated by the wakefield can reach close to the speed of light, so during the transmission process, the relative position of the generated electron beam and the laser wakefield will change, and the electron beam will continue to move toward the front end of the cavity, and gradually enter the deceleration phase from the acceleration phase, resulting in a phase loss effect, which is not conducive to the effective acceleration of the electron beam. Therefore, in order to keep the accelerated electron beam in the acceleration phase, the present invention regulates the plasma density generated.

[0031] The relationship between plasma wavelength and plasma density is:

[0032]

[0033] Where n is the plasma density, n c is the critical density, λ laser is the laser wavelength. It is known that the plasma wavelength decreases with the increase of density, and the cavitation size is approximately the plasma wavelength. Therefore, when the generated plasma density gradually increases, the cavitation size will continue to decrease, so that the position of the electron in the cavitation of the wake field is maintained at a fixed acceleration phase, effectively accelerating and realizing the improvement of electron energy.

[0034] Therefore, the present invention proposes a new type of supersonic gas target, which satisfies the requirement that the density distribution of the supersonic gas presents a positive gradient distribution in the propagation direction of the laser pulse. After laser ionization, a plasma density distribution that is approximately conical and gradually increases can be obtained, ensuring that the accelerated electrons can be locked in a fixed acceleration phase as much as possible.

[0035] like Figure 1 and Figure 2 As shown, a wedge-shaped supersonic gas target for a laser accelerator comprises a target body and a base, wherein the upper and lower end faces of the target body are respectively provided with a jet port and an air inlet, and a wedge-shaped cavity is arranged inside the target body, wherein the wedge-shaped cavity is respectively connected with the air inlet and the jet port, so that the density distribution of the supersonic gas jet ejected from the jet port along the laser transmission direction presents a positive gradient distribution; the target body is arranged on the base, and the base is provided with a slit matched with the air inlet.

[0036] The wedge-shaped cavity of the wedge-shaped supersonic gas target is a hollowed-out long wedge-shaped structure embedded in the target body, which is wide in front and narrow in the back, wide at the top and narrow at the bottom. In order to ensure the generation of supersonic gas jets, the wedge-shaped supersonic gas target adopts a De-Lava l structure, the gas outlet cross-section (jet port) is trapezoidal, and the bottom cross-section (air inlet) is a rectangular slit to ensure that the target body has a narrow throat structure. The gas target base can be designed as a disc structure or other shapes according to the type of gas supply solenoid valve. The slit end on the lower surface of the base is tightly connected to the gas supply solenoid valve mainly through fixing screws and rubber rings to provide high-pressure gas input for the gas target. The upper base (L1), lower base (L2), height (H) of the jet port trapezoid, the length (d), width (w) of the rectangular slit of the air inlet, and the total height of the target body can all be adjusted and designed according to the required gas density and density gradient distribution. In view of the different particle acceleration energies and application scenario requirements of laser accelerators, the gas density distribution that needs to be generated is generally between 10 17 -10 21 / cm 3 Therefore, the geometric dimensions of the jet trapezoid and the rectangular slit of the air inlet of the wedge-shaped supersonic gas target can be adjusted within a certain range to meet the following requirements: L2>L1≥w (both in the order of hundreds of micrometers to millimeters), H≥d (both in the order of millimeters to centimeters). The size of the designed wedge-shaped supersonic gas target can be simulated by FLUENT fluid dynamics to obtain the gas density distribution, and optimized according to the results to obtain the gas density distribution required for experiments or practical applications.

[0037] In laser accelerator experiments, the gases that can be used in wedge-shaped supersonic gas targets mainly include hydrogen, helium, nitrogen, argon, etc. The above types of gases can also be mixed in different proportions according to different laser acceleration mechanisms to obtain better beam quality.

[0038] Different from the flat-top density distribution produced by the existing supersonic gas target, the wedge-shaped supersonic gas target of the present invention can obtain a density distribution close to Figure 3 The positive gradient density distribution plasma shown in the figure avoids the dephasing effect caused by phase mismatch in the electron beam, thereby effectively improving the energy and monoenergeticity of the electron beam, and providing high-quality electron beams for laser accelerators in driving ultrafast radiation sources, ultrafast process diagnosis, high-resolution imaging, life sciences, etc.

[0039] At the same time, in order to avoid the generation of shock waves in supersonic gas, the roughness of the inner wall of the wedge-shaped cavity needs to be controlled, and the roughness should be kept as small as possible during the actual processing process to achieve a smooth transition between the target contour and the inner wall of the wedge-shaped cavity. Figure 4 As shown, the wedge-shaped supersonic gas target is connected to the gas supply solenoid valve, and the two are fixed by multiple positioning screws and rubber sealing rings to ensure that no gas overflows from the interface between the two during the jetting process.

[0040] Example

[0041] Taking the laser wake electron accelerator using a wedge-shaped supersonic gas target as an example, the specific device layout is as follows: Figure 5 As shown:

[0042] TW or PW-level ultrashort and ultra-intense laser enters the vacuum target range, and after the optical path is adjusted and focused by the corresponding reflector group (M1, M2) and parabolic mirror, it is transmitted to the wedge-shaped supersonic gas target located in the target range, and interacts with the supersonic gas jet with a positive gradient density distribution generated by the gas target, thereby generating a quasi-monoenergetic electron beam with high energy.

[0043] The wedge-shaped supersonic gas target includes a target body and a base. The upper and lower end faces of the target body are respectively provided with a jet and an air inlet. A wedge-shaped cavity is arranged inside the target body. The wedge-shaped cavity is respectively connected with the air inlet and the jet, so that the density distribution of the supersonic gas jet ejected from the jet along the laser transmission direction presents a positive gradient distribution; the target body is arranged on the base, and the base is provided with a slit adapted to the air inlet. The wedge-shaped cavity is a hollow-out long wedge-shaped structure embedded in the target body, which is wide in front and narrow in the back, wide in the top and narrow in the bottom. In order to ensure the generation of supersonic gas jets, the wedge-shaped supersonic gas target adopts a De-Laval structure, the gas outlet cross section (jet) is a trapezoid, and the bottom cross section (air inlet) is a rectangular slit, so as to ensure that the target body has a narrow throat structure. The base of the gas target is a disc structure, and the slit end of the lower surface of the base is tightly connected to the gas supply solenoid valve mainly through fixing screws and rubber rings, so as to provide high-pressure gas input for the gas target. The gas input into the wedge-shaped supersonic gas target is a mixture of helium and nitrogen, with a volume ratio of helium to nitrogen of 99:1. The upper base (L1), lower base (L2), height (H) of the jet trapezoid, the length (d), width (w) of the rectangular slit at the gas inlet, and the total height of the target are L1=2mm, L2=4mm, H=10mm, d=7mm, w=0.5mm, respectively. The total height of the target is 14mm, of which the height above the slit is 10mm and the height below the slit is 4mm.

[0044] The generated electron beam is deflected after passing through the magnet of the electron magnetic spectrometer, and the energy spectrum of the deflected electron beam can be measured using the Fuji Film SR series imaging plate (IP). Since electrons of different energies will be deflected to different distances after passing through the uniform magnetic field generated by the magnetic spectrometer, the relationship between the deposited electron beam excitation intensity (PSL) and position can be obtained from the imaging plate, and the energy spectrum of the electron beam can be obtained through processing. The generated electron beam can be further used for high-resolution imaging, generating high-energy Betatron radiation, generating Compton scattering γ source, ultrafast electron diffraction and other applications.

[0045] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A wedge-shaped supersonic gas target for a laser accelerator, comprising a target body and a base, Features: The upper and lower end faces of the target body are respectively provided with a jet port and an air inlet, and a wedge-shaped cavity is arranged inside the target body, and the wedge-shaped cavity is respectively connected with the air inlet and the jet port, so that the density distribution of the supersonic gas jet ejected from the jet port along the laser transmission direction presents a positive gradient distribution; the target body is arranged on a base, and the base is provided with a slit adapted to the air inlet.

2. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, Features: The wedge-shaped cavity is wide at one end and narrow at the other end in the horizontal direction, and is wide at the upper end and narrow at the lower end in the vertical direction.

3. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, Features: The jet port is trapezoidal, and the air inlet is a rectangular slit, so as to ensure that the target body has a narrow throat structure.

4. The wedge-shaped supersonic gas target for a laser accelerator according to claim 3, Features: The upper base of the jet trapezoid is L 1 , the lower base is L 2 , height is H, the length of the rectangular slit of the gas inlet is d, the width is w, and the total height of the target can be adjusted and designed according to the required gas density and density gradient distribution to meet: L 2 >L 1 ≥w, H≥d.

5. The wedge-shaped supersonic gas target for a laser accelerator according to claim 4, Features: The L 1 , L 2 and w are both in the order of hundreds of micrometers to millimeters; and H and d are both in the order of millimeters to centimeters.

6. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, Features: The lower surface of the base is tightly connected to the gas supply electromagnetic valve to provide high-pressure gas input for the target body.

7. The wedge-shaped supersonic gas target for a laser accelerator according to claim 6, Features: The lower surface of the base is fixed to the air supply solenoid valve via a plurality of positioning screws and a rubber sealing ring.

8. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, Features: The base is a disc structure.

9. The wedge-shaped supersonic gas target for a laser accelerator according to any one of claims 1 to 8, Features: The inner wall of the wedge-shaped cavity needs to have controlled roughness to achieve a smooth transition between the target body contour and the inner wall.

10. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, Features: The gas target is arranged in a vacuum target field, and the laser beam is shot toward the gas target along the length direction of the wedge-shaped cavity, interacts with the gas jet, and generates a quasi-monoenergetic electron beam.

11. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, Features: The gas is hydrogen, helium, nitrogen or argon, or a mixture of the above types of gases in different proportions according to different laser acceleration mechanisms.

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