Method for generating quasi-monochromatic proton beams based on picosecond laser-driven CH target

Abstract

The application discloses a quasi-monochromatic proton beam generation method based on a picosecond laser driven CH target, and comprises the following steps: a linearly polarized Gaussian laser with a long pulse is shot into a plasma CH target with a density gradient converter, the linearly polarized Gaussian laser and the plasma CH target continuously interact with each other, and then a long-time annular magnetic field is formed by the current formed by accelerated electrons and the electron backflow; an electric field generated by the annular magnetic field accelerates various particles to different degrees, separates the particles, and makes the protons in the negative gradient of the acceleration electric field, so that the protons are accumulated in the phase space, and a quasi-monochromatic proton beam is obtained. The application is applied to the field of particle acceleration, can generate a quasi-monochromatic proton beam with high energy and low energy dispersion, has low requirements on laser intensity, has a certain robustness, and has high energy, good monochromaticity and low energy dispersion (as low as 3.1%) of the obtained proton beam.

Description

Technical Field

[0001] The present invention relates to the field of particle acceleration technology, and in particular to a quasi-monoenergetic proton beam generation method based on a picosecond laser driven CH target. Background Art

[0002] High-quality ion beams are widely used in a variety of fields, including cancer treatment, proton imaging, and fast ignition. Producing high-energy, monoenergetic, high-quality ion beams is crucial. Compared to traditional particle accelerators, laser particle accelerators offer advantages such as low cost and high acceleration gradients. With the continuous development of laser technology, laser-driven charged particle acceleration has attracted increasing attention.

[0003] To date, scientists have proposed numerous ion acceleration mechanisms, of which target normal sheath acceleration (TNSA) and radiation pressure acceleration (RPA) are the two most widely studied. However, these two acceleration mechanisms suffer from inherent drawbacks that severely limit their practical application. For example, the ion beam energy spectrum obtained by TNSA is generally exponentially distributed, resulting in poor monoenergetic properties. RPA is an important mechanism for efficiently obtaining monoenergetic ion beams. However, its high matching requirements for laser and plasma, coupled with rapidly growing Rayleigh-Taylor and Weibel instabilities, can disrupt the stable acceleration structure, limiting the ion energy achieved in experiments.

[0004] In recent years, a magnetic vortex acceleration (MVA) mechanism has been theoretically proposed, involving the interaction of relativistic intensity laser pulses with near-critical plasma targets. Vortex magnetic field structures can be induced at plasma density gradients. Simultaneously, magnetic pressure expels electrons from regions filled with magnetic fields, establishing longitudinal acceleration fields and transverse collimation fields. This has certain advantages for ion beam alignment. Therefore, within the MVA mechanism, more research is focused on utilizing magnetic vortex mechanisms to achieve ion beam alignment. In 2005, Bulanov et al. studied the effect of plasma density distribution on magnetic vortex ion acceleration in a gas target. In 2022, Hu et al. used a Laguerre-Gaussian laser to drive a near-critical plasma, generating a collimated proton beam of 100 MeV through magnetic vortex acceleration, an improvement of approximately three times compared to Gaussian lasers.

[0005] In summary, people have proposed a variety of laser-driven ion acceleration mechanisms and schemes for generating high-quality ion beams, but they all place high demands on experimental conditions or have problems such as wide energy spectrum distribution. There is relatively little research on quasi-monoenergetic ion acceleration based on magnetic vortex acceleration mechanisms. Summary of the Invention

[0006] In view of the above-mentioned deficiencies in the prior art, the present invention provides a method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target, which can effectively obtain a quasi-monoenergetic proton beam with high energy and low energy dispersion.

[0007] To achieve the above objectives, the present invention provides a method for generating a quasi-monoenergetic proton beam based on a picosecond laser-driven CH target, comprising the following steps:

[0008] A long-pulse linearly polarized Gaussian laser is injected into a plasma CH target with a density gradient transformation, so that the linearly polarized Gaussian laser and the plasma CH target continuously interact with each other, thereby causing the current formed by the accelerated electrons and the electron return flow to jointly form a long-lasting circular magnetic field;

[0009] The electric field generated by the annular magnetic field accelerates various particles to varying degrees, separating the particles while placing the protons in the negative gradient of the accelerating electric field, thereby causing the protons to accumulate in the phase space and obtaining a quasi-monoenergetic proton beam.

[0010] In one embodiment, the wavelength of the linearly polarized Gaussian laser is , pulse width .

[0011] In one embodiment, the dimensionless parameter of the linearly polarized Gaussian laser is , the waist radius is .

[0012] In one embodiment, the plasma CH target is composed of electrons, protons and carbon ions, and its density distribution is ,in, is the maximum density, is the target thickness of the plasma CH target along the laser incident direction, is the position coordinate of the plasma CH target along the laser incident direction.

[0013] In one embodiment, the target thickness of the plasma CH target along the laser incident direction is ,in, is the wavelength of the linearly polarized Gaussian laser.

[0014] In one embodiment, the carbon-hydrogen ratio of the plasma CH target is .

[0015] In one of the embodiments, the maximum density wherein, is the plasma critical density.

[0016] Compared with the prior art, the present application has the following beneficial technical effects:

[0017] 1. The present application uses long pulse laser to interact with near-critical plasma, high-energy, low-energy dispersion quasi-monochromatic proton beam, not only the laser intensity requirement is not high, and has certain robustness, at the same time, the energy of the obtained proton beam is higher, the monochromaticity is good, and the energy dispersion can be as low as 3.1%;

[0018] 2. The present application can obtain similar quality proton beams within a certain laser intensity range, which reduces the application difficulty to a certain extent. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained according to the structures shown in these drawings without creative labor.

[0020] Figure 1 It is a flow chart of the quasi-monochromatic proton beam generation method based on picosecond laser driven CH target in the embodiments of the present application;

[0021] Figure 2 It is a principle schematic diagram of the quasi-monochromatic proton beam generation method based on picosecond laser driven CH target in the embodiments of the present application;

[0022] Figure 3 It is a particle simulation result schematic diagram of quasi-monochromatic proton beam obtained by interaction of long pulse linearly polarized Gaussian laser and plasma CH target in the embodiments of the present application, wherein: (a) is an electron density distribution schematic diagram at moment, (b) is a current density distribution schematic diagram at moment, (c) is a proton energy density distribution schematic diagram at moment, (d) is a proton and carbon ion energy spectrum distribution schematic diagram at moment;

[0023] Figure 4Schematic diagram of proton acceleration under different laser pulse width conditions in an embodiment of the present invention, wherein: (a) is a magnetic field distribution diagram at 250T when the laser pulse width is 0.5ps, (b) is a magnetic field distribution diagram at 400T when the laser pulse width is 0.5ps, (c) is a magnetic field distribution diagram at 600T when the laser pulse width is 0.5ps, (d) is a magnetic field distribution diagram at 200T when the laser pulse width is 0.1ps, (e) is a magnetic field distribution diagram at 300T when the laser pulse width is 0.1ps, and (f) is a schematic diagram of the change of maximum energy with time under laser pulse widths of 0.5ps and 0.1ps;

[0024] Figure 5 : The electric field and the distribution of electrons, protons, and carbon ions on the axis in an embodiment of the present invention, wherein: (a) is a schematic diagram of a CH target with a density gradient change at 400 T, (b) is a schematic diagram of a CH target with a density gradient change at 800 T, (c) is a schematic diagram of a CH target with a uniform density distribution at 400 T, (d) is a schematic diagram of a CH target with a uniform density distribution at 800 T, (e) is a schematic diagram of a single-component pure hydrogen target at 400 T, and (f) is a schematic diagram of a single-component pure hydrogen target at 800 T.

[0025] Figure 6 Schematic diagram of the effect of laser intensity on proton beam quality in an embodiment of the present invention, where: (a) is a schematic diagram of the change of the maximum energy of the proton beam with time when a=5, 8, 10, 15, and 20, and (b) is Schematic diagram of energy spectrum distribution at each moment.

[0026] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0027] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0028] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0029] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in this field can implement it. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0030] This embodiment discloses a quasi-monoenergetic proton beam generation method based on a picosecond laser driven CH target, which uses a long pulse linearly polarized Gaussian laser to interact with a near-critical plasma to obtain a quasi-monoenergetic proton beam. Figure 1 , specifically including the following steps:

[0031] A long-pulse linearly polarized Gaussian laser is injected into a plasma CH target with density gradient transformation, so that the linearly polarized Gaussian laser and the plasma CH target continuously interact with each other, thereby causing the current formed by the accelerated electrons and the electron return flow to jointly form a long-lasting circular magnetic field.

[0032] The electric field generated by the circular magnetic field accelerates various particles to varying degrees, separating the particles while placing the protons in the negative gradient of the accelerating electric field, causing the protons to accumulate in the phase space and obtaining a quasi-monoenergetic proton beam.

[0033] The quasi-monoenergetic proton beam generation method in this embodiment accelerates electrons by injecting Gaussian laser light into a plasma with a varying density gradient. The current generated by the accelerated electrons, together with the return current from the channel walls, forms a typical annular quasi-static magnetic field. In the region where the magnetic field exists, the cold electron flow is expelled from the magnetized region by magnetic pressure, forming a positively charged region and an electrostatic sheath electric field. This charge separation field accelerates ions near the channel axis. Simultaneously, the bimodal electric field formed by the multi-component target achieves excellent acceleration and compression of the proton beam, ultimately producing a high-energy, low-dispersion quasi-monoenergetic proton beam.

[0034] Specifically in this embodiment, the wavelength of the linearly polarized Gaussian laser is , pulse width , dimensionless parameters , the waist radius is The plasma CH target is composed of electrons, protons and carbon ions, and its density distribution is ,in, is the maximum density, is the plasma critical density, is the target thickness of the plasma CH target along the laser incident direction (i.e. Figure 2 The thickness of the plasma CH target in the y-axis and z-axis directions is 20-30 μm. is the position coordinate of the plasma CH target along the laser incident direction, and the target thickness of the plasma CH target along the laser incident direction is , the carbon-hydrogen ratio of the plasma CH target is . refer to Figure 2 , after the laser interacts with the CH target, the current formed by the accelerated electrons Together with the electron backflow, a circular magnetic field is formed The electric field it generates accelerates the particles, and because different particles are accelerated at different speeds, the particles separate, such as Figure 2 Carbon ions in , proton With electronics As shown, the circular magnetic field The resulting charge separation field causes the protons Good acceleration and confinement are achieved, and a quasi-monoenergetic proton beam is eventually formed.

[0035] refer to Figure 3 The particle simulation results of the quasi-monoenergetic proton beam obtained by the interaction of a long pulse linearly polarized Gaussian laser with a plasma CH target are shown in Figure 2. Figure 3 (a)- Figure 3 (c) It can be seen that after the long pulse linearly polarized Gaussian laser is incident on the plasma CH target, a plasma channel is formed under the action of the ponderomotive force. This is because the ponderomotive force is proportional to the spatial gradient of the laser intensity, which pushes electrons from areas with high laser intensity to areas with low laser intensity. The electrons in the channel are accelerated forward to generate a negative current, while the electrons on the channel wall flow back to generate a positive current, as shown in Figure 2. Figure 3 (b) Such a structure can generate a circular magnetic field to accelerate ions. Figure 3 (d) The accelerated ion energy spectrum shows that a carbon ion beam of about 50 MeV / u and a proton beam with a maximum energy of 167 MeV / u and a peak energy of about 76 MeV / u can be produced, with an energy spread of about 3.1%.

[0036] For the quasi-monoenergetic proton beam generation method in this embodiment, the pulse width of the laser pulse plays a very important role. Figure 4 This is a schematic diagram of proton acceleration under different laser pulse width conditions, showing the magnetic field evolution with a pulse width of 0.5ps and a pulse width of 0.1ps, as well as the corresponding energy change over time. Under normal circumstances, the magnetic field strength will gradually decay over time, but under long-pulse laser conditions, since the laser can continuously interact with the plasma, the circular current it generates can be maintained for a longer time, delaying the decay of the magnetic field and allowing the proton beam to be accelerated for a longer time. The continuous evolution of the vortex magnetic field can cause the ions to be continuously accelerated. It can be seen that the duration of the magnetic field corresponds to the main acceleration period of the proton beam, such as Figure 4 (f) shown.

[0037] Furthermore, in order to explain the formation of proton quasi-beam monoenergeticity, Figure 5The electric field and particle density distributions under three different target conditions are shown. Compared with the uniform CH target and the single-layer hydrogen target, the multi-component target with density gradient can form a more obvious double-peak electric field structure, such as Figure 5 (b) This causes some protons to be in the negative gradient of the accelerating electric field. As a result, the lagging protons experience a larger accelerating field to catch up with the leading protons, causing them to accumulate in phase space, resulting in a quasi-monoenergetic proton beam. In comparison, a pure hydrogen target does not form a double-peak structure, and the proton beam is not in the negative gradient of the acceleration phase. The resulting proton beam energy spectrum exhibits an exponential distribution and is not monoenergetic.

[0038] refer to Figure 6 The results of proton beam generation when the laser intensity distribution is a = 5, 8, 10, 15 and a = 20 are shown. It can be found that the energy of the proton beam increases with the increase of laser intensity, such as Figure 6 (a). And at the same time, it maintains the characteristics of quasi-monopotency, such as Figure 6 As shown in (b), it can be seen that the quasi-monoenergetic proton beam generation method in this embodiment is applicable in the range of a=5 to a=20 and has good robustness.

[0039] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made by using the contents of the present invention description and drawings under the inventive concept of the present invention, or direct / indirect application in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A quasi-monoenergetic proton beam generation method based on picosecond laser driven CH target, characterized in that: The steps include: A long-pulse linearly polarized Gaussian laser is injected into a plasma CH target with a density gradient transformation, so that the linearly polarized Gaussian laser and the plasma CH target continuously interact with each other, thereby causing the current formed by the accelerated electrons and the electron return flow to jointly form a long-lasting circular magnetic field; The electric field generated by the annular magnetic field accelerates various particles to varying degrees, separating the particles while placing the protons in the negative gradient of the accelerating electric field, thereby causing the protons to accumulate in the phase space and obtaining a quasi-monoenergetic proton beam.

2. The method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target according to claim 1, characterized in that: The linearly polarized Gaussian laser has a wavelength λ0 = 0.8 μm and a pulse width τ = 0.5 ps.

3. The method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target according to claim 2, characterized in that: The dimensionless parameter a0 of the linearly polarized Gaussian laser is 8, and the beam waist radius is σ0=10λ0.

4. The method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target according to claim 1, 2 or 3, characterized in that: The plasma CH target is composed of electrons, protons and carbon ions, and its density distribution is n e =n0·(x 2 / L 2 ), where n0 is the maximum density, L is the target thickness of the plasma CH target along the laser incident direction, and x is the position coordinate of the plasma CH target along the laser incident direction.

5. The method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target according to claim 4, characterized in that: The target thickness of the plasma CH target along the laser incident direction is L=100λ0, where λ0 is the wavelength of the linearly polarized Gaussian laser.

6. The method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target according to claim 4, characterized in that: The carbon-hydrogen ratio of the plasma CH target is 7. The method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target according to claim 4, characterized in that: The maximum density n0=0.5n c , where n c is the critical density of plasma.

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