Quasi-monoenergy proton beam generation method based on picosecond laser driven CH target

Abstract

The invention discloses a quasi-monoenergy proton beam generation method based on a picosecond laser driven CH target, which comprises the following steps of: driving long-pulse linear polarization Gaussian laser into a plasma CH target with density gradient transformation, so that the linear polarization Gaussian laser continuously interacts with the plasma CH target, the current formed by the accelerated electrons and the electron backflow jointly form a long-time annular magnetic field; the electric field generated by the annular magnetic field accelerates various particles to different degrees, and the protons are located in the negative gradient of the accelerating electric field while the particles are separated, so that the protons are accumulated in the phase space, and a quasi-monoenergetic proton beam is obtained. The device is applied to the field of particle acceleration, the quasi-monoenergy proton beam with high energy and low energy dispersion can be generated, the requirement for laser intensity is not high, certain robustness is achieved, meanwhile, the energy of the obtained proton beam is high, the monoenergy performance is good, and the energy dispersion can be as low as 3.1%.

Description

Technical Field

[0001] The invention relates to the technical field of particle acceleration, 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 cancer treatment, proton imaging, fast ignition and other fields. How to obtain high-energy, monoenergetic, high-quality ion beams is very important. Compared with traditional particle accelerators, laser particle accelerators have the advantages of low cost and high acceleration gradient. With the continuous development of laser technology, laser-driven charged particle acceleration has attracted more and more attention.

[0003] So far, scientists have proposed many ion acceleration mechanisms, among which target normal sheath acceleration (TNSA) and radiation pressure acceleration (RPA) are the two most widely studied acceleration mechanisms. However, due to some inherent defects of these two acceleration mechanisms, their practical application is severely limited. For example, the ion beam energy spectrum obtained by TNSA is mostly exponentially distributed and has poor monoenergeticity. RPA is an important mechanism for efficiently obtaining monoenergetic ion beams. However, its matching conditions for laser and plasma are relatively high, and the rapidly growing Rayleigh-Taylor and Weibel instabilities will destroy the stable acceleration structure, limiting the ion energy obtained in the experiment.

[0004] In recent years, people have theoretically proposed the magnetic vortex acceleration (MVA) mechanism of the interaction between relativistic intensity laser pulses and near-critical plasma targets. The vortex magnetic field structure can be induced at the plasma density gradient. At the same time, the magnetic pressure establishes a longitudinal acceleration field and a transverse collimation field by expelling electrons from the area filled with the magnetic field, which has certain advantages for the collimation of the ion beam. Therefore, in the MVA mechanism, more schemes are to study the use of magnetic vortex mechanisms to achieve ion beam collimation. In 2005, Bulanov et al. studied the effect of plasma density distribution on magnetic vortex ion acceleration in gas targets. In 2022, Hu et al. used Laguerre-Gaussian lasers to drive near-critical plasmas and produced collimated proton beams of 100 MeV through magnetic vortex acceleration, which is about 3 times higher than that of Gaussian lasers.

[0005] In summary, people have proposed a variety of laser-driven ion acceleration mechanisms and schemes for producing high-quality ion beams, but they all place high demands on experimental conditions or have problems such as wide energy spectrum distribution. There are relatively few studies 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 quasi-monoenergetic proton beam generation method 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 object, 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: 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 making the current formed by the accelerated electrons and the electron reflux jointly form a long-term circular magnetic field; The electric field generated by the annular magnetic field accelerates various particles to different degrees, separating the particles while placing protons in the negative gradient of the accelerating electric field, thereby causing protons to accumulate in the phase space to obtain a quasi-monoenergetic proton beam.

[0008] In one embodiment, the wavelength of the linearly polarized Gaussian laser is , Pulse Width .

[0009] In one embodiment, the dimensionless parameter of the linearly polarized Gaussian laser is The waist radius is .

[0010] 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.

[0011] 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.

[0012] In one embodiment, the carbon-hydrogen ratio of the plasma CH target is .

[0013] In one embodiment, the maximum density ,in, is the critical density of plasma.

[0014] Compared with the prior art, the present invention has the following beneficial technical effects: 1. The present invention uses a long pulse laser to interact with a near-critical plasma to produce a quasi-monoenergetic proton beam with high energy and low energy dispersion. This not only does not require high laser intensity, but also has a certain robustness. At the same time, the proton beam obtained has high energy and good monoenergeticity, and the energy dispersion can be as low as 3.1%; 2. The present invention can obtain proton beams of similar quality within a certain laser intensity range, thus reducing its application difficulty to a certain extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0016] Figure 1 This is a flow chart of a method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target in an embodiment of the present invention; Figure 2 Schematic diagram of the principle of a method for generating a quasi-monoenergetic proton beam based on a picosecond laser driven CH target in an embodiment of the present invention; Figure 3 Schematic diagram of particle simulation results of a quasi-monoenergetic proton beam obtained by the interaction between a long pulse linearly polarized Gaussian laser and a plasma CH target in an embodiment of the present invention, wherein: (a) Schematic diagram of electron density distribution at time, (b) Schematic diagram of current density distribution at the moment, (c) Schematic diagram of proton energy density distribution at time, (d) Schematic diagram of proton and carbon ion energy spectrum distribution at each moment; Figure 4 Schematic 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, (f) is a schematic diagram of the maximum energy variation with time when the laser pulse width is 0.5ps and 0.1ps; Figure 5 : is a diagram of the electric field on the axis and the distribution of electrons, protons and carbon ions in an embodiment of the present invention, wherein: (a) is a schematic diagram of a CH target with a density gradient change at 400T, (b) is a schematic diagram of a CH target with a density gradient change at 800T, (c) is a schematic diagram of a CH target with uniform density distribution at 400T, (d) is a schematic diagram of a CH target with uniform density distribution at 800T, (e) is a schematic diagram of a single-component pure hydrogen target at 400T, and (f) is a schematic diagram of a single-component pure hydrogen target at 800T; Figure 6 Schematic diagram of the effect of the same laser intensity on the quality of the proton beam in the embodiment of the present invention, wherein: (a) is a schematic diagram of the change of the maximum energy of the proton beam with time when a=5, 8, 10, 15, 20, (b) is Schematic diagram of energy spectrum distribution at each moment.

[0017] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] 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 components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0020] 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 the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0021] 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: A long pulse of 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, and then the current formed by the accelerated electrons and the electron reflux jointly form a long-term circular magnetic field; 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 to obtain a quasi-monoenergetic proton beam.

[0022] The quasi-monoenergetic proton beam generation method in this embodiment accelerates electrons by injecting Gaussian laser into a plasma with density gradient change. The current formed by the accelerated electrons and the channel wall backflow together form a typical annular quasi-static magnetic field. In the area where the magnetic field exists, the cold electron flow is expelled from the magnetized area by magnetic pressure, thereby forming a positively charged area and an electrostatic sheath electric field. This charge separation field accelerates the ions near the channel axis. At the same time, the double-peak electric field formed by the multi-component target can achieve good acceleration and compression of the proton beam, and finally obtain a quasi-monoenergetic proton beam with high energy and low energy dispersion.

[0023] 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 critical density of plasma, 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 generated by it accelerates the particles. At the same time, different particles are accelerated at different speeds, causing the particles to 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.

[0024] 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 as follows: 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 the electrons from the area with high laser intensity to the area with low laser intensity. The electrons in the channel are accelerated forward to generate negative current, while the electrons on the channel wall flow back to generate positive current, such as 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 Me / u can be produced, and the energy spread is about 3.1%.

[0025] 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 for pulse widths of 0.5ps and 0.1ps, as well as the corresponding energy changes over time. Normally, 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) as shown.

[0026] Further, to illustrate the formation of proton quasi-beam monoenergeticity, Figure 5 The 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 variation can form a more obvious double-peak electric field structure, such as Figure 5 (b) shows. Some protons are placed in the negative gradient of the accelerating electric field. As the lagging protons have a larger accelerating electric field to catch up with the protons in front, they are accumulated in the phase space to obtain a quasi-monoenergetic proton beam. In comparison, the pure hydrogen target does not form a double peak structure, and the proton beam is not in the negative gradient of the accelerating phase. The energy spectrum of the proton beam produced by it is exponentially distributed and does not have monoenergeticity.

[0027] refer to Figure 6The 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, the quasi-monopotency characteristics are maintained, 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.

[0028] 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 changes made by using the contents of the present invention specification and drawings under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for generating a quasi-monoenergetic proton beam based on a 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 density gradient transformation, so that the linearly polarized Gaussian laser and the plasma CH target continuously interact with each other, thereby making the current formed by the accelerated electrons and the electron reflux jointly form a long-term circular magnetic field; The electric field generated by the annular magnetic field accelerates various particles to different degrees, separating the particles while placing protons in the negative gradient of the accelerating electric field, thereby causing protons to accumulate in the phase space to obtain 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, wherein 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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