A wedge-shaped supersonic gas target for laser accelerator

Abstract

The present invention belongs to the technical field of laser accelerators, and specifically relates to a wedge-shaped supersonic gas target for a laser accelerator, comprising a target body and a base. The upper and lower end surfaces of the target body are respectively provided with an air jet and an air inlet. A wedge-shaped cavity is provided within the target body, the wedge-shaped cavity being connected to the air inlet and the air jet, respectively, so that the density distribution of the supersonic gas jet emitted from the air jet presents a positive gradient distribution along the direction of laser transmission. The target body is disposed on the base, which is provided with a slit that matches the air inlet. The wedge-shaped supersonic gas target, which conforms to the electron transport characteristics of a laser accelerator, presents a positive gradient distribution of the supersonic gas jet density along the direction of laser transmission, and the gas density can continuously increase. This avoids the dephasing effect of phase mismatch in the electron beam, effectively improving the energy and monoenergeticity of the electron beam, and providing a high-quality electron beam for laser accelerators in driving ultrafast radiation sources, ultrafast process diagnosis, and high-resolution imaging.

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, ultraintense lasers and targets can accelerate charged particles. The concept of laser wakefield-driven electron acceleration (LWFA) was first proposed by American scientists John Dawson and Toshi Tajima in 1979. The wakefield driven by laser propagation in a plasma can generate an extremely high accelerating electric field, thereby producing a high-energy electron beam over a very short timescale. Supersonic gas targets, important experimental targets in laser accelerators, can produce a gas medium with locally high density and high repetition rate under vacuum conditions. The resulting gas jet has a long range, steep edges, and a nearly flat-topped density distribution, providing a gas jet with a specific density for acceleration experiments. Due to the high controllability of this target, the density of the ionized plasma can be regulated in real time, thereby affecting the quality of the electron beam accelerated by laser-driven wakefield acceleration. This can effectively improve the energy, charge, and monoenergeticity of the electron beam, and can be used in high-repetition-rate experiments, providing an important solution for achieving effective control and quality improvement of the electron beam. The high-quality quasi-monoenergetic electron beam generated 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, and beam imaging, which is of great 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, providing a flat-top gas density distribution. The team of Jena University in Germany developed two conical nozzles with Mach numbers of 3 and 6, and achieved 10 18 / cm 3The ELI-Beamline research team conducted experimental research on a long, 6.8mm x 1.26mm supersonic gas target, obtaining both simulation and experimental data on density distribution at various distances from the nozzle. Furthermore, companies such as Source LAB in France and Smartshell in Japan have developed various conical and long gas targets.

[0005] This type of gas target has an axisymmetric distribution, mature technology, and certain advantages in terms of gas density uniformity. However, in actual applications, due to the characteristics of electron transport during laser wakefield acceleration, 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 electron beam energy. According to the principles of laser accelerators, there are currently two main solutions to the electron beam dephasing effect. One is to use the laser to attenuate in the plasma to reduce the scale of the wakefield. This requires the control of the light beam generated by a large ultrashort and ultra-intense laser device, which is complex and difficult to operate. The other is to adjust the wakefield to generate the longitudinal density of the plasma. This 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] Existing supersonic gas target structures are not conducive to reducing the dephasing effect in laser accelerators to produce the high-energy, high-quality electron beams required for practical applications. Therefore, the key issue addressed by this invention is the design and development of a new supersonic gas target, designed to minimize the dephasing effect and taking into account the electron transport characteristics of laser accelerators. Summary of the Invention

[0007] In response to 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 solution targets the dephasing effect generated during laser-driven electron acceleration and, for the first time, combines the characteristics of electron transport to effectively reduce the limitation on electron beam energy gain due to the dephasing effect, thereby obtaining an electron beam with higher energy and better monochromaticity, providing high-quality beam flow 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. The upper and lower end surfaces of the target body are respectively provided with an air jet and an air inlet. A wedge-shaped cavity is provided inside the target body. The wedge-shaped cavity is respectively connected to the air inlet and the air jet, so that the density distribution of the supersonic gas jet ejected from the air jet presents a positive gradient distribution along the laser transmission direction. The target body is arranged on the base, and the base is provided with a slit adapted to 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, ensuring 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 and width of the rectangular slit of the air inlet are d and 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 to 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 shot 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 a mixture of the above types of gases 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 laser accelerators 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. Unlike the flat-top density distribution produced by existing supersonic gas targets, this new gas target can obtain a plasma with a positive gradient density distribution, avoiding 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, and other aspects. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0024] Figure 2 This 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 will be 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, capable of generating a supersonic gas jet with a positive gradient density distribution, compensating for the phase shift produced during electron acceleration, thereby resolving the electron dephasing effect and obtaining an electron beam with higher energy gain. The specific technical solution is as follows:

[0030] During the laser wakefield-driven electron acceleration process, the wakefield, located at the tail of the laser, propagates at a speed slightly lower than the speed of light. However, the electron beam accelerated by the wakefield can reach a speed close to the speed of light. Therefore, during transmission, the relative position of the generated electron beam and the laser wakefield changes, causing the electron beam to continuously move toward the front end of the cavity and gradually enter the deceleration phase from the acceleration phase, resulting in a dephasing effect that is detrimental to the effective acceleration of the electron beam. Therefore, to ensure that the accelerated electron beam remains in the acceleration phase, the present invention regulates the generated plasma density.

[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 increasing density, and the cavitation bubble size is approximately the plasma wavelength. Therefore, when the generated plasma density gradually increases, the cavitation bubble size will continue to decrease, so that the position of the electron in the cavitation bubble in the wake field remains at a fixed acceleration phase, effectively accelerating and achieving an increase in 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 direction of laser pulse propagation. 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 includes a target body and a base. The upper and lower end surfaces of the target body are respectively provided with an air jet and an air inlet. A wedge-shaped cavity is provided inside the target body. The wedge-shaped cavity is connected to the air inlet and the air jet, respectively, so that the density distribution of the supersonic gas jet ejected from the air jet presents a positive gradient distribution along the laser transmission direction. The target body is set on the base, and the base is provided with a slit adapted to 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, ensuring 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 response to 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 trapezoidal jet nozzle and rectangular inlet slit of the wedge-shaped supersonic gas target can be adjusted within a certain range to meet the following requirements: L2>L1≥w (both on the order of hundreds of microns to millimeters) and H≥d (both on the order of millimeters to centimeters). The dimensions of the designed wedge-shaped supersonic gas target can be used to obtain the gas density distribution through FLUENT fluid dynamics simulation, and then optimized based on 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 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, and other aspects.

[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 optical path adjustment and focusing through the corresponding reflector group (M1, M2) and parabolic mirror, it is transmitted to the wedge-shaped supersonic gas target located in the target range, interacting 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 comprises a target body and a base. The upper and lower end surfaces of the target body are respectively provided with an air jet and an air inlet. A wedge-shaped cavity is disposed within the target body, communicating with the air inlet and air jet, respectively, so that the density distribution of the supersonic gas jet emitted from the air jet exhibits a positive gradient along the direction of laser transmission. The target body is mounted on a base, which is provided with a slit that aligns with the air inlet. The wedge-shaped cavity is a hollowed-out, elongated wedge-shaped structure embedded within the target body, wide at the front and narrow at the back, and wide at the top and narrow at the bottom. To ensure the generation of a supersonic gas jet, the wedge-shaped supersonic gas target utilizes a De-Laval structure, with a trapezoidal gas outlet cross-section (air jet) and a rectangular slit at the bottom (air inlet), ensuring a narrow throat structure within the target body. The gas target base is a disc-shaped structure, and the slit end on the lower surface of the base is tightly connected to the gas supply solenoid valve, primarily through fixing screws and rubber rings, providing high-pressure gas input to the gas target. The gas input to 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), and height (H) of the trapezoidal jet nozzle, the length (d) and width (w) of the rectangular inlet slit, and the total height of the target are L1 = 2 mm, L2 = 4 mm, H = 10 mm, d = 7 mm, and w = 0.5 mm, respectively. The total height of the target is 14 mm, with a height above the slit of 10 mm and a height below the slit of 4 mm.

[0044] The generated electron beam is deflected after passing through the magnet of the electron magnetic spectrometer. 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 are 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. Through processing, the energy spectrum of the electron beam can be obtained. The generated electron beam can be further used for applications such as high-resolution imaging, generation of high-energy Betatron radiation, generation of Compton scattering gamma sources, and ultrafast electron diffraction.

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

Claims

1. A wedge-shaped supersonic gas target for a laser accelerator, comprising a target body and a base, characterized in that: The upper and lower end faces of the target body are respectively provided with an air jet and an air inlet, and a wedge-shaped cavity is provided inside the target body. The wedge-shaped cavity is wide at one end and narrow at the other end in the horizontal direction, and wide at the upper end and narrow at the lower end in the vertical direction. The wedge-shaped cavity is connected with the air inlet and the air jet respectively. The air jet is trapezoidal, and the air inlet is a rectangular slit. The upper base of the air jet trapezoid is L1, the lower base is L2, and the height is H. The length d, width w of the rectangular slit of the air inlet and the total height of the target body can be adjusted and designed according to the required gas density and density gradient distribution, satisfying: L2>L1≥w, H≥d, ensuring that the target body has a narrow throat structure, so that the density distribution of the supersonic gas jet ejected from the air jet port along the laser transmission direction presents a positive gradient distribution; the target body is set 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, wherein: The L1, L2 and w are all in the order of hundreds of micrometers to millimeters; the H and d are all in the order of millimeters to centimeters.

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

4. The wedge-shaped supersonic gas target for a laser accelerator according to claim 3, wherein: 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.

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

6. The wedge-shaped supersonic gas target for a laser accelerator according to any one of claims 1 to 5, characterized in that: 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.

7. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, wherein: 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, interacting with the gas jet to generate a quasi-monoenergetic electron beam.

8. The wedge-shaped supersonic gas target for a laser accelerator according to claim 1, wherein: 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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