A method for obtaining electron microbunches by laser ionization injection

Abstract

A method for obtaining electron microbunches by laser ionization injection, which is based on a laser wakefield acceleration system. The method includes the following steps: (1) providing a laser pulse; (2) allowing the laser pulse to enter an injection stage, where the injection stage uses a mixed gas plasma of a high atomic number gas and a low atomic number gas; the laser pulse interacts with the mixed gas to form electron microbunches; (3) ionizing the inner-layer electrons of high atomic number atoms near the peak intensity of the laser pulse; (4) the electrons with an initial spatial distribution released by ionization slide towards the tail of the plasma bubble; (5) allowing the laser pulse and the electron microbunches to enter an acceleration stage, where the acceleration stage uses a low atomic number gas, and the electron microbunches are continuously bunched and accelerated. The present invention obtains electron microbunches by laser ionization injection, greatly reducing the experimental conditions and saving costs.

Description

Technical Field

[0001] The present invention relates to the technical field of generating high-quality electron beams by laser wakefield acceleration, and particularly to a method for obtaining electron microbunches by laser ionization injection. Background Art

[0002] The laser wakefield accelerator driven by an ultrashort and ultra-intense laser device has an acceleration gradient three orders of magnitude higher than that of a traditional radio frequency electron linear accelerator (on the order of gigavolts per meter), providing the possibility for new desktop particle acceleration technologies and solutions. It can accelerate particles to very high energies in a very short distance, and the accelerated particle beam has unique characteristics such as high peak current, femtosecond to attosecond bunch duration, low transverse emittance, and ultra-high brightness.

[0003] High-brightness, short-wavelength, and ultra-short pulse duration high-energy X- and γ-ray sources can be widely applied to basic scientific research, industrial applications, and medicine. With the development of a new generation of compact particle accelerators, generating microbunches using a laser wakefield accelerator to provide a compact and low-cost radiation source has attracted in-depth research. For example, using a temporally asymmetric driving laser pulse to enhance the transverse oscillatory motion of the accelerated electrons to obtain microbunches for generating femtosecond coherent radiation with photon energies in the kilo-electron-volt range; based on a combination of a laser-plasma accelerator and a plasma mirror to generate a high-brightness broadband X-ray spectrum up to several hundred kilo-electron-volts. However, due to factors such as laser self-modulation instability, suppression of filamentary instability and hose instability, the quality of the injected electron beam and the stable transmission of the beam-plasma are affected.

[0004] The ionization injection mechanism is a new injection scheme in which electron injection is easier and more stable. The emittance of the electron beam can be limited to the residual momentum and initial radius of the electrons from the ionization process itself, and the ionization injection process can be controlled by controlling the laser intensity distribution, thereby controlling the quality of the electron beam. In addition, due to the effect of discrete ionization phases in the ionization injection mechanism, the ionized electrons naturally have a microbunching effect in the time distribution, and it is a simple and low-cost method for generating electron microbunches under appropriate parameter conditions.

[0005] Plasmas under the ionization injection mechanism usually use gases with a large ionization potential difference between the outer shell layer and the inner shell electrons (such as nitrogen), and high atomic number Z or a mixture of high Z and low Z gases are required as the medium. The outer electrons of high Z atoms and the electrons of low Z atoms can be completely ionized at the front of the relativistic intense laser pulse, exciting plasma waves; while the inner electrons of high Z atoms will be ionized near the peak intensity of the laser pulse, and these electrons are directly generated in the wakefield bubble. The peak intensity of the laser pulse is near the center of the bubble on the propagation axis, and there are multiple different peaks in the laser electric field greater than the ionization threshold of the inner electrons of high Z atoms. Electrons are ionized at different initial phases of the laser electric field, and the electrons ionized at different initial phases are in different energy phase spaces and density spaces. This discrete initial phase distribution makes the electrons show a periodic distribution in the phase space and density space after injection. However, some early experimental results show that ionization injection usually leads to continuous injection, thus destroying the microbunching structure of electrons; and the distance between the initial injection positions of electrons will result in a large final energy spread.

[0006] Therefore, the method of generating electron microbunches by applying the ionization injection mechanism needs to be further studied and improved to make the quality of the generated electron microbunches controllable and practically applicable to basic scientific research, industrial applications, and medicine, etc. Summary of the Invention

[0007] To overcome the above problems, the present invention provides a method for obtaining electron microbunches by laser ionization injection under the conditions of an ultrashort and ultra-intense pulsed laser (tens of terawatts, 30 - 50 femtoseconds).

[0008] The technical solution adopted by the present invention is: a method for obtaining electron microbunches by laser ionization injection, which is realized based on a laser wakefield acceleration system. The laser wakefield acceleration system includes a laser and a gas target. The laser pulse interacts with the gas target after being focused, driving the excitation of plasma waves and ionizing to generate an injected electron beam; the gas target includes a first part and a second part. The first part is a mixed gas composed of a low atomic number gas and a high atomic number gas, and the ratio of the mixed gas is adjustable; the second part is a pure low atomic number gas.

[0009] The method includes the following steps:

[0010] (1) Provide a laser pulse with a peak power in the terawatt range, a pulse width in the femtosecond range, and a beam waist radius in the micrometer range;

[0011] (2) Cause a laser pulse to enter an injection stage, where the injection stage uses a mixed gas of a gas with a high atomic number Z and a gas with a low atomic number Z; the laser pulse interacts with the mixed gas, ionizing the outer shell electrons of the low atomic number gas and the high atomic number gas to form a background plasma, and forming a plasma wakefield bubble under the action of the driving laser;

[0012] (3) Ionize the inner layer electrons of high atomic number Z atoms near the peak intensity of the laser pulse. Since the ionization rate is related to the phase of the laser electric field, the initially generated electrons due to ionization have a natural modulation in space, and its longitudinal spatial modulation frequency is twice the laser wave vector;

[0013] (4) The electrons with an initial spatial distribution after being ionized and released slide towards the tail of the plasma bubble. Electrons at different initial positions are projected onto different acceleration phases and are accelerated by the longitudinal electric field of the wakefield to form electron microbunches;

[0014] (5) Cause the laser pulse and the electron microbunches to enter an acceleration stage, where the acceleration stage uses a gas with a low atomic number Z, and the electron microbunches are continuously bunched and accelerated.

[0015] Further, in the injection stage, the density distribution of the gas with a high atomic number Z shows a trend of first increasing, then remaining flat, and finally decreasing along the laser transmission direction; among them, the length of the region where the density distribution of the gas with a high atomic number Z shows an increasing trend is L 1 , the length of the region where the density distribution of the gas with a high atomic number Z shows a flat trend is L 2 , the length of the region where the density distribution of the gas with a high atomic number Z shows a decreasing trend is L 3 .

[0016] Further, in step (2), after the ultra-intense and ultra-short laser pulse enters the mixed gas, the pulse front completely ionizes all the electrons of the atoms in the gas with a low atomic number Z and the outer layer electrons of the atoms in the gas with a high atomic number Z to form a background plasma. To ensure the stable transmission of the laser, the plasma density in the acceleration stage is kept consistent with the background plasma density in the injection stage of the mixed gas stage.

[0017] Further, in order to suppress continuous injection and reduce the absolute energy spread, the length of the mixed gas is as short as possible. The length of the mixed gas is on the order of hundreds of micrometers to control the energy spread below 2%; at the same time, in order to control the transverse emittance of the electron beam, we control the vector potential of the laser near the ionization threshold of the inner layer electrons of high-Z atoms, thereby controlling the transverse emittance of the electron beam below 2 millimeters milliradians.

[0018] Further, in the step 5, the energy of the electron microbunch is 200 to 1000 MeV; the charge is 1 to 100 pC; the overall transverse emittance is below 2 mm mrad; the overall energy spread is below 2%, and the microbunch energy spread is below 1%; the spatial density modulation period is 2k 0 ~3k 0 or so, where k 0 is the wave vector of the laser.

[0019] Further, in the step 1, the laser pulse is a Gaussian beam; in the step 2, the mixed gas is a helium-nitrogen mixed gas, the length of the injection stage is <200 μm; the length of the gas distribution in the acceleration stage is of the order of millimeters.

[0020] Further, in the steps 3-4, analyze the evolution of the six-dimensional phase space of the microbunch electron beam, and select appropriate laser plasma parameters to enable stable laser transmission and maintain a stable microbunch structure of the electron beam during the acceleration process.

[0021] The principle of the present invention is as follows: in a laser wakefield electron acceleration system, under the laser ionization injection mechanism, the laser tunneling ionization rate is related to the electric field phase. Due to the discrete phase-related ionization effect, the electron beam released by ionization naturally has a density modulation effect in the longitudinal space (time distribution). The electron beam released by ionization slides to the tail of the bubble and still maintains the corresponding density modulation during this process. Under the optimized laser and plasma parameter conditions, the microbunch structure initially possessed by the electron beam is well maintained during the acceleration process, and the quality of the electron microbunch is improved as much as possible, finally obtaining an electron microbunch with sub-femtosecond low energy spread. The ionization injection mechanism used in the present invention is a cascaded acceleration method with separate injection and acceleration stages.

[0022] A method for obtaining an electron microbunch by ionization injection under a two-stage acceleration model (including an injection stage and an acceleration stage). Under the ionization injection mechanism, due to the effect of discrete ionization phases, the injected electron beam naturally has a microbunching effect in the time distribution. By shortening the length of the injection stage, suppressing continuous ionization injection, and restricting the phase space volume of the injected electron beam in the wakefield, the microbunch structure of the electron beam can be maintained. Under the optimized laser and plasma parameter conditions, an electron microbunch with sub-femtosecond low energy spread is obtained.

[0023] The beneficial effects of the present invention are as follows: the present invention uses laser ionization injection to obtain an electron microbunch, greatly reducing the experimental conditions and saving costs. When a strongly relativistic laser pulse interacts with a plasma, under the action of discrete ionization phases, the inner electrons of high-Z atoms exhibit a microbunch distribution in density and energy at birth. Under the optimized laser plasma parameter conditions, an ultrashort multi-color low energy spread electron microbunch can be obtained, and effective acceleration of the electron beam can be achieved while maintaining the microbunch structure. Brief Description of the Drawings

[0024] Figure 1 is a schematic diagram of the injection stage and acceleration stage structures of the present invention.

[0025] Figure 2 is a spatial density distribution diagram of electron microbunches obtained under a typical parameter condition of the present invention. Detailed Embodiments

[0026] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

[0027] In the description of the present invention, it should be noted that, as terms such as "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are used to indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, it is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation of the present invention. In addition, as terms such as "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0028] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, as terms such as "installation", "connection", "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood in specific situations.

[0029] Through a large number of experimental studies by the present inventors, the results show that the density (n p ) of the background plasma is crucial for energy gain. The higher the plasma density, the greater the charge of the injected electron beam. However, as the plasma density increases, the dephasing length of the electron beam becomes shorter, which will reduce the energy gain of the electron beam. And the high-Z gas doping concentration (C) and the mixed gas length (L mix ) of the acceleration stage have a great influence on the electron beam monochromaticity. Based on the principle of wakefield acceleration of ultrashort and ultra-intense lasers propagating in low-density gases and the above experimental basis, the present invention optimizes the laser parameters and gas parameters.

[0030] Figure 1 Schematic diagram of the two - stage acceleration model structure (injection stage and acceleration stage) for obtaining electron micro - bunches by laser ionization injection of the present invention. As Figure 1 shown, the two - stage acceleration model of the present invention includes an injection stage 1 and an acceleration stage 2. In the injection stage, a mixed - gas plasma of high atomic number Z and low atomic number Z is used to generate injected electron micro - bunches; in the acceleration stage, a low - Z gas is used to ensure the stable transmission of the laser, so that the injected electron beam can be continuously accelerated. The density distribution of the high - Z gas in the injection stage is a rising edge L 1 , a plateau region L 2 and a falling edge L 3 . After the ultra - intense and ultra - short laser enters the gas, the leading edge of the pulse can completely ionize all the electrons of the low - Z atoms and the outer - layer electrons of the high - Z atoms to form a background plasma. Except for a transition with a length of L 1 that linearly rises from vacuum, the density of the background plasma remains unchanged. The inner - layer electrons of the high - Z atoms can only be ionized at the peak of the pulse intensity due to their large ionization potential. To suppress continuous injection and reduce the absolute energy spread, we make the length of the mixed gas as short as possible, about on the order of hundreds of micrometers, and the energy spread can be controlled below 2%. At the same time, to control the transverse emittance of the electron beam, we control the vector potential of the laser near the ionization threshold of the inner - layer electrons of the high - Z atoms, so as to control the transverse emittance of the electron beam below 2 mm mrad.

[0031] Specifically, after the laser pulse generated by the laser enters the injection stage 1, it interacts with the mixed gas to form a background plasma with a density of n p , except for a transition with a length of L 1 that linearly rises from vacuum, n p remains unchanged. The density distribution of the high - Z gas in the injection stage is a rising edge L 1 , a plateau region L 2 (where n ~ c%n p ) and a falling edge L 3 . The laser pulse and the electron micro - bunches formed by ionization enter the acceleration stage 2, and a low - Z gas is used in the acceleration stage. In the acceleration stage, the wakefield continuously accelerates the electron micro - bunches, and high - quality electron micro - bunches with low energy spread and stable structure are obtained at an appropriate acceleration distance.

[0032] In the present invention, the low - atomic - number gas used is preferably the inert gas helium with high safety, but is not limited to using hydrogen or other gases.

[0033] In the present invention, the high - atomic - number gas used is preferably the gas nitrogen with high safety, but is not limited to using oxygen, argon or other gases.

[0034] The laser used in the present invention is a Gaussian beam.

[0035] The laser pulse in the watt level described in the present invention refers to a peak power of dozens of terawatts.

[0036] The femtosecond-level laser pulse described in the present invention refers to a pulse width of 30 to 50 femtoseconds.

[0037] The micron-level laser pulse described in the present invention refers to a beam waist radius of several microns to dozens of microns.

[0038] The following will describe the preferred embodiments of the present invention in detail with reference to the accompanying drawings. It should be understood that the embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0039] Embodiment 1

[0040] This embodiment adopts the Figure 1 two-stage acceleration model of obtaining electron microbunching by laser ionization injection described above. The laser beam used is a Gaussian beam, with a wavelength of 800 nanometers, a beam waist radius of 20 microns, a pulse width of 30 femtoseconds, and a peak power of 30 terawatts. The high atomic number gas used is nitrogen, and the low atomic number gas used is helium. In the injection stage, helium and nitrogen are mixed at a molecular number density ratio of 5:1, where the nitrogen molecular number density is 2.5×10 17 per cubic centimeter (where n N2 ~c%n p ). In the acceleration stage, pure helium is used, with a molecular number density of n He = 1 / 2n p . The length of the mixed gas region is 100 microns (that is, the rising edge L 1 and the falling edge L 3 are each 50 microns, and the platform region L 2 is 0 microns). After the ultra-intense and ultra-short laser enters the gas, the leading edge of the pulse can completely ionize all the electrons of He atoms and the outer 5 electrons of N atoms, forming a background plasma with a density of n p = 2n He + 10n N2 . The two electrons N 5+ , N 6+ in the K shell of N atoms can only be ionized at the peak of the pulse intensity due to their relatively large ionization potentials (553 electron volts and 667 electron volts respectively).

[0041] The numerical simulation results show that under the mechanism of discrete-phase ionization, the ionized electrons naturally have a microbunching effect in the time distribution. The trapped electron beam enters the acceleration stage and is continuously accelerated under the action of a longitudinal electric field with an acceleration gradient of the order of gigavolts per meter. Under the above parameter conditions, after the electrons are injected, they are modulated periodically in density space and phase space, and the modulation period is about 3k0 , where k 0 is the wave vector of the laser, and this periodic microbunching distribution can be well maintained throughout the acceleration process. The overall energy spread of the electron beam can reach a minimum of 1.92%, and the energy spread of each microbunch can be maintained at a lower level during the acceleration process and its minimum energy spread can be obtained at an appropriate acceleration distance, reaching the order of one thousandth.

[0042] Figure 2 shows the spatial density distribution diagram of the electron microbunch, indicating that the generated electron beam has a good microbunching structure in the density space.

[0043] The present invention optimizes the laser parameters and gas parameters, utilizes the effect of discrete ionization phases, enables the ionized electrons to naturally have a microbunching structure in the time distribution, and maintains this structure well during the acceleration process. Thus, electron microbunches with several hundred MeV energy, an energy spread below 2%, and an energy spread of each microbunch reaching the order of one thousandth can be obtained using laser pulses of dozens of terawatts.

[0044] The parts not elaborated in detail in the present invention belong to the well-known technologies of those skilled in the art. The purpose of disclosing the embodiments is to help further understand the present invention. However, the above embodiments are only the preferred embodiments of the present invention, and do not elaborate all details and the matching of plasma parameters under different laser parameter conditions. Without departing from the design spirit of the present invention, those skilled in the art can make modifications and substitutions to the technical solutions of the present invention, and all should fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for obtaining electron microbunches by laser ionization injection, characterized in that, this method is implemented based on a laser wakefield acceleration system, which includes a laser and a gas target. The laser pulse interacts with the gas target after being focused, driving the excitation of plasma waves and ionizing to generate an injected electron beam; the gas target includes a first part and a second part. The first part is a mixed gas composed of a low atomic number gas and a high atomic number gas, and the ratio of the mixed gas is adjustable; the second part is a pure low atomic number gas; the method includes the following steps: (1) Provide a laser pulse with a peak power in the terawatt range, a pulse width in the femtosecond range, and a beam waist radius in the micrometer range; (2) Let the laser pulse enter the injection stage, and the injection stage uses a mixed gas of a high atomic number gas and a low atomic number gas; the laser pulse interacts with the mixed gas, ionizing the outer shell electrons of the low atomic number gas and the high atomic number gas to form a background plasma, and forming a plasma wakefield bubble under the action of the driving laser; (3) Ionize the inner layer electrons of high atomic number atoms near the peak intensity of the laser pulse. Since the ionization rate is related to the phase of the laser electric field, the initially generated electrons have a natural modulation in space, and its longitudinal spatial modulation frequency is twice the laser wave vector; (4) The electrons released by ionization and having an initial spatial distribution slide to the tail of the plasma bubble, and electrons at different initial positions are projected to different acceleration phases and are accelerated by the longitudinal electric field of the wakefield to form electron microbunches; (5) Let the laser pulse and the electron microbunches enter the acceleration stage, and the acceleration stage uses a low atomic number gas, and the electron microbunches are continuously bunched and accelerated.

2. A method for obtaining electron microbunches by laser ionization injection as described in claim 1, characterized in that: The density distribution of the high atomic number gas in the injection stage first increases, then levels off, and finally decreases along the length of the region where the mixed gas is distributed; among them, the length of the region where the density distribution of the high atomic number gas shows an increasing trend is L 1 , the length of the region where the density distribution of the high atomic number gas shows a leveling-off trend is L 2 , the length of the region where the density distribution of the high atomic number gas shows a decreasing trend is L 3 .

3. A method for obtaining electron microbunches by laser ionization injection as described in claim 2, characterized in that: In step (2), after the ultra-intense ultra-short laser pulse enters the mixed gas, the leading edge of the pulse completely ionizes all the electrons of the atoms in the low atomic number gas and the outer layer electrons of the atoms in the high atomic number gas to form a background plasma; in order to ensure the stable transmission of the laser, the plasma density in the acceleration stage is kept consistent with the background plasma density in the injection stage of the mixed gas stage.

4. A method for obtaining electron microbunches by laser ionization injection as described in claim 3, characterized in that: In order to suppress continuous injection and reduce the absolute energy spread, the length of the mixed gas is as short as possible, and the length of the mixed gas is in the order of hundreds of micrometers to control the energy spread below 2%; at the same time, in order to control the transverse emittance of the electron beam, we control the vector potential of the laser near the ionization threshold of the inner layer electrons of the high atomic number gas atoms, so as to control the transverse emittance of the electron beam below 2 millimeters milliradians.

5. A method for obtaining electron microbunches by laser ionization injection as described in claim 4, characterized in that: In the said step (3), the energy of the electron microbunch is from 200 to 1000 MeV; the charge is from 1 to 100 pC; the overall transverse emittance is below 2 mm mrad; the overall energy spread is below 2%, and the microbunch energy spread is below 1%; the spatial density modulation period is about 3k 0 or so, where k 0 is the wave vector of the laser.

6. A method for obtaining electron microbunches by laser ionization injection as described in claim 4, characterized in that: The laser pulse in the step (1) is a Gaussian beam; the mixed gas in the step (2) is a helium-nitrogen mixed gas, the length of the injection stage < 200 microns; the length of the gas distribution in the acceleration stage is of the order of millimeters.

7. A method for obtaining electron microbunches by laser ionization injection as described in claim 4, characterized in that: In the step (3), analyze the evolution of the six-dimensional phase space of the microbeam electron bunches, and select appropriate laser plasma parameters so that the laser can be stably transmitted and the electron bunches can maintain a stable microbunch structure during the acceleration process.

Images