CN114845458A - Electron beam acceleration method and plasma medium density setting method - Google Patents
Electron beam acceleration method and plasma medium density setting method Download PDFInfo
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Abstract
The present disclosure relates to an electron beam acceleration method and a plasma medium density setting method, the method including: injecting an electron beam having a predetermined amount of charge; and enabling the electron beam to dynamically keep moving at a preset position of a wake field excited by the laser pulse in the plasma medium, and obtaining an accelerated target electron beam, wherein the density of the plasma medium is increased along the moving direction of the electron beam. According to the electron beam acceleration method of the embodiment of the disclosure, the wavelength of the wake field excited by the plasma medium with the density increased along the moving direction of the electron beam and the laser pulse is changed in the moving process of the laser pulse, so that the electron beam is dynamically kept at the preset position in the wake field, the electron beam can be accelerated with higher acceleration gradient, the acceleration efficiency and the energy conversion efficiency are improved, the electric charge amount adaptive to the density of the medium can be loaded, the energy chirp of the electron beam is offset, the energy dispersion of the electron beam is reduced, and the electron beam with higher quality is obtained.
Description
Technical Field
The disclosure relates to the field of particle accelerators, and in particular to an electron beam acceleration method and a plasma medium density setting method.
Background
The basic principle of laser plasma electron accelerators was proposed by Tajima and Dawson in 1979. Because the plasma is not limited by a breakdown threshold, the laser plasma electron accelerator has ultrahigh acceleration gradient which is three orders of magnitude higher than that of the traditional radio frequency accelerator, so that the acceleration distance of electrons can be greatly shortened. With the continuous progress of ultrafast laser technology in recent years, the laser plasma electron accelerator technology has been developed greatly, the overall scale of the device is greatly reduced, and high-quality electron beams of the order of MeV to GeV can be generated on the desktop scale at present. Benefiting from a series of advantages of the laser plasma electron accelerator, the laser plasma electron accelerator is expected to drive new-generation large scientific devices such as colliders, free electron lasers and the like, and is applied to scientific front-edge exploration. And secondary particles and ray sources (positrons, seeds, gamma rays and the like) driven by the device are expected to play an important role in the fields of ultra-high precision industrial nondestructive detection and the like.
In a laser plasma electron accelerator, laser pulses excite an accelerating wake field in a plasma medium, and electron beams at proper positions in the wake field can be continuously accelerated. However, the propagation speed of the laser pulse in the plasma medium is slightly slower than that of the high-energy electron beam, so that the electron beam moves away from the optimal acceleration position relative to the wake field of the laser pulse during the acceleration process, which is called the dephase of the electron beam in the wake of the laser plasma, and the energy conversion efficiency from laser to electron beam is greatly reduced. In addition, when a common laser plasma electron accelerator is loaded with a large amount of electric charge, the beam loading effect in the wake wave can greatly distinguish the acceleration energy obtained at different longitudinal positions of the electron beam, so that the electron beam has large energy dispersion, which is not favorable for the application of the electron beam as a particle source or a radiation source.
Disclosure of Invention
The present disclosure provides an electron beam acceleration method and a plasma medium density setting method.
According to an aspect of the present disclosure, there is provided an electron beam acceleration method including: injecting an electron beam having a predetermined amount of charge; and enabling the electron beam to dynamically keep moving at a preset position of a tail wave field excited by laser pulses in a plasma medium, and obtaining an accelerated target electron beam, wherein the density of the plasma medium is increased along the moving direction of the electron beam and the laser pulses, the moving speed of the laser pulses is less than that of the electron beam, and the density of the plasma medium is inversely related to the wavelength of the tail wave field.
In one possible implementation, the preset position comprises a tail of the wake field.
In a possible implementation manner, the preset charge amount is greater than a first charge loading capacity corresponding to the density of the initial section of the plasma medium and is less than a second charge loading capacity corresponding to the density of the final section of the plasma medium.
In one possible implementation, the manner in which the density of the plasma medium increases along the direction of motion comprises any one of: increases linearly along the direction of motion; increases in a non-linear manner along the direction of motion; the number of steps increases stepwise along the direction of motion, the number of steps being greater than or equal to 3; and the density of the plasma medium is set in a segmented manner along the movement direction, and in the two adjacent sections of plasma medium, the density of the rear section of plasma medium is greater than or equal to that of the front section of plasma medium.
In one possible implementation, the normalized vector potential of the laser pulse is greater than or equal to 2.
In one possible implementation, the density distribution of the plasmonic medium in a direction perpendicular to the direction of motion of the laser pulse corresponds to the shape of the laser pulse.
According to an aspect of the present disclosure, there is provided a plasma medium density setting method, including: setting the density of a plasma medium carrying the laser pulse and the electron beam to move according to the movement speed of the laser pulse and the movement speed of the electron beam, and obtaining a target plasma medium, wherein the density of the target plasma medium enables the electron beam to dynamically keep moving at a preset position of a tail wave field excited by the laser pulse, the density of the target plasma medium is increased along the movement direction of the electron beam and the laser pulse, the movement speed of the laser pulse is smaller than the movement speed of the electron beam, and the density of the plasma medium is inversely related to the wavelength of the tail wave field.
In one possible implementation, the first charge carrying capacity corresponding to the initial density of the target plasma medium is smaller than the amount of charge of the electron beam, and the second charge carrying capacity corresponding to the final density of the target plasma medium is larger than the amount of charge of the electron beam.
In one possible implementation, the preset position comprises a tail of the wake field.
According to an aspect of the present disclosure, a plasma medium for carrying a laser pulse and a movement of an electron beam is provided, wherein a density of the medium increases in a direction of movement of the electron beam and the laser pulse such that the electron beam dynamically remains moving at a preset position of a wake field excited in the medium by the laser pulse, a speed of movement of the laser pulse is smaller than a speed of movement of the electron beam, the density of the plasma medium being inversely related to a wavelength of the wake field.
According to the electron beam acceleration method disclosed by the embodiment of the disclosure, the wavelength of the wake wave field excited by the plasma medium with the density increased along the moving direction of the electron beam and the laser pulse can be changed in the moving process of the laser pulse, so that the electron beam is dynamically kept at the preset position in the wake wave field, the electron beam can be accelerated with higher acceleration gradient, and the acceleration efficiency and the energy conversion efficiency are improved.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. Other features and aspects of the present disclosure will become apparent from the following detailed description of exemplary embodiments, which proceeds with reference to the accompanying drawings.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the disclosure.
FIG. 1 shows a flow diagram of an electron beam acceleration method according to an embodiment of the present disclosure;
FIG. 2 shows a schematic diagram of electron beam acceleration in a wake field according to an embodiment of the present disclosure
FIG. 3 shows a schematic diagram of the density of a plasmonic medium according to an embodiment of the present disclosure;
fig. 4 shows an application schematic diagram of an electron beam acceleration method according to an embodiment of the present disclosure.
Detailed Description
Various exemplary embodiments, features and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers can indicate functionally identical or similar elements. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
The word "exemplary" is used exclusively herein to mean "serving as an example, embodiment, or illustration. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.
The term "and/or" herein is merely an association describing an associated object, meaning that three relationships may exist, e.g., a and/or B, may mean: a exists alone, A and B exist simultaneously, and B exists alone. In addition, the term "at least one" herein means any one of a plurality or any combination of at least two of a plurality, for example, including at least one of A, B, C, and may mean including any one or more elements selected from the group consisting of A, B and C.
Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a better understanding of the present disclosure. It will be understood by those skilled in the art that the present disclosure may be practiced without some of these specific details. In some instances, methods, means, elements and circuits that are well known to those skilled in the art have not been described in detail so as not to obscure the present disclosure.
In view of the above problems, the present disclosure provides an electron beam acceleration method, which can continuously change the length of a wake field excited in a plasma medium by a laser pulse through a plasma medium with a density rising multiple times or continuously rising to continuously replace the dephased electron beam back to an optimal acceleration phase to increase the energy absorbed by the electron beam from the wake field, so that the electron beam is dynamically maintained at an optimal position in the wake field, thereby accelerating the electron beam with higher efficiency.
Fig. 1 shows a flow diagram of an electron beam acceleration method according to an embodiment of the present disclosure, as shown in fig. 1, the method comprising:
in step S11, injecting an electron beam having a preset amount of charge;
in step S12, the electron beam is dynamically kept moving at a preset position of a wake field excited by the laser pulse in the plasma medium, and an accelerated target electron beam is obtained, wherein the density of the plasma medium increases along the moving direction of the electron beam and the laser pulse, the moving speed of the laser pulse is less than that of the electron beam, and the density of the plasma medium is inversely related to the wavelength of the wake field.
According to the electron beam acceleration method disclosed by the embodiment of the disclosure, the wavelength of the wake wave field excited by the plasma medium with the density increased along the moving direction of the electron beam and the laser pulse can be changed in the moving process of the laser pulse, so that the electron beam is dynamically kept at the preset position in the wake wave field, the electron beam can be accelerated with higher acceleration gradient, and the acceleration efficiency and the energy conversion efficiency are improved.
In one possible implementation, the electron beam in the laser plasma electron accelerator may be an electron beam externally injected into the plasma medium (e.g., an electron beam generated by another electron source and injected into the plasma medium), which is not limited by the present disclosure.
In one possible implementation, a laser plasma electron accelerator can accelerate an electron beam by a wake field excited by laser pulses in a plasma medium. In an example, the normalized sagittal potential of the laser pulse is greater than or equal to 2. The laser pulses of different normalized sagittal potentials differ in their acceleration effects on the electron beam, for example, the wake field excited in the plasma medium may differ due to the difference in normalized sagittal potentials. In an example, the laser pulse having the normalized vector potential greater than or equal to 2 belongs to a laser pulse having a relatively strong energy, and the specific value of the normalized vector potential is not limited by the present disclosure.
Fig. 2 shows a schematic diagram of electron beam acceleration in a wake field according to an embodiment of the present disclosure. The laser pulse excites a wake wave in a plasma medium, the structure of a wake wave field formed by the wake wave is approximately an elliptical space of which the plasma is evacuated by the laser pulse, a high-density electron shell is arranged on the boundary of the space, and an electron beam is generally positioned in the wake wave field and close to the tail part.
During acceleration, the rate of change of the acceleration field of the electron beam along the direction of propagation of the electron beam can be determined by the following equation (1):
wherein E is z Is the acceleration field of the electron beam, xi is the length unit in the propagation direction, I b The beam current of the electron beam, n p Density of the plasma medium at the location of the wake field, r b The longitudinal distance between the center of the electron beam and the shell of the plasma is shown, and A and B are positive constants.
In the example, the electron beam having an energy of 100MeV moves at 0.999987c, c being the speed of light in vacuum; the 200MeV electron beam has a velocity of 0.999996c, although the energy difference is doubled, the velocity difference is small and very close to c. Therefore, the energy of the electron beam is often used to represent the velocity to increase the contrast. The laser pulse has a speed of motion in the plasma medium which is less than the speed of light in vacuum, c, and is between about 0.99c and 0.999c, although also close to c, but which is already a considerable difference from the speed of a 100MeV or 200MeV electron beam, i.e. the speed of the electron beam is greater than the speed of the laser pulse in the plasma medium.
In the acceleration process, when the electron beam is at the preset position in the wake field, the acceleration gradient is the largest, and the acceleration efficiency is also the highest, but as the acceleration process proceeds, the velocity of the electron beam in the plasma medium is higher than that of the laser pulse in the plasma medium, so that the electron beam cannot be maintained at the preset position in the wake field excited by the laser pulse in the plasma medium, but is relatively displaced, i.e., shifted forward, with respect to the laser pulse and its wake field, and after leaving the preset position, the acceleration gradient of the electron beam is reduced, and thus the acceleration efficiency is also reduced.
In a possible implementation, as shown in fig. 2, the preset position comprises the tail of the wake field, and at the tail of the wake field, the electric field strength is stronger, so that the acceleration gradient obtained by the electron beam is larger, for example, a certain range of the end of the length direction of the wake field, for example, 90% -100% of the length direction of the wake field (the direction is set in a way that 0% of the length direction of the wake field is the beginning of the wake field and 100% is the tail end of the wake field) can be used as the preset position. That is, in the case where the position of the electron beam in the wake field is the tail position of the wake field, the acceleration gradient of the electron beam is large, and the acceleration efficiency is high. However, as described above, the velocity of the electron beam moving in the plasma medium is higher than the velocity of the laser pulse in the plasma medium, and therefore, the electron beam gradually catches up with the laser pulse during the acceleration in the wake field excited by the laser pulse, i.e., gradually moves forward relatively in the wake field, and if the wake field remains unchanged, the electron beam cannot always remain at the tail of the wake field excited by the laser pulse.
In one possible implementation, the wavelength of the wake field can be changed, i.e. during the progressive advance of the electron beam from the tail of the wake field, the electron beam can be made to remain at the tail of the wake field if the wavelength of the wake field is also correspondingly shorter. In an example, the first case where the wavelength of the wake field is not changed, the electron beam is shifted forward with respect to the wake field, i.e. stepwise to the middle or front of the wake field. The other case is that the wavelength of the wake field is correspondingly shorter, if the electron beam is also shifted forward, for example, to a position in the middle or in the front of the wake field in the first case, but the wavelength of the wake field is also changed correspondingly, for example, if the wavelength is also decreased stepwise such that the wavelength of the wake field in the second case is shorter than the wavelength of the wake field in the first case, for example, such that the position at which the wake field in the second case ends is close to or the same as the position in the middle or in the front of the wake field in the first case, the electron beam is at the tail position of the shortened wake field in the second case despite the shift forward, and therefore, in the second case, the electron beam can be held at the tail position of the wake field for a longer time, thereby obtaining a higher acceleration gradient and acceleration efficiency.
In one possible implementation, the wavelength of the wake field excited in the plasmonic medium by the laser pulse is related to the density of the plasmonic medium. In an example, the density of the plasmonic medium is inversely related to the wavelength of the wake field, i.e. the higher the density of the plasmonic medium, the shorter the wavelength of the wake field excited by the laser pulse in the plasmonic medium, and the relationship between the wavelength of the wake field excited by the laser pulse in the plasmonic medium and the density of the plasmonic medium is as shown in equation (2) below:
wherein λ is p Wavelength of the wake field excited in the plasma medium by the laser pulse, epsilon 0 Is a vacuum dielectric constant, m e For electron mass, e is the charge of the electron beam, c is the speed of light, n p Is the density of the plasma medium. From the formula (2), the electron mass m is determined in the case of an electron beam e And the charge e of the electron beam are both known constants, the vacuum dielectric constant epsilon 0 C and pi are also constants, so that the wavelength lambda of the wake field excited by the laser pulse in the plasma medium p Density n of plasma medium only p Are related and the relationship between the two is reversedCorrelation, i.e. density n of the plasma medium p The larger the wavelength λ of the wake field excited in the plasma medium by the laser pulse p The smaller. Thus, the density of the plasmonic medium increases in the direction of motion of the electron beam and the laser pulse, and the wavelength of the wake field excited by the laser pulse in the increasing plasmonic medium may be gradually reduced.
In a possible implementation, as can be seen from the above description, in a plasma medium with a density that increases gradually in the direction of motion of the electron beam and the laser pulse, the wavelength of the wake field excited by the laser pulse decreases gradually, making it possible for the electron beam to dynamically remain at a position at the end of the gradually shortened wake field. Further, in order to reduce the energy dispersion of the electron beam, that is, to make the energy distribution of each electron in the electron beam more concentrated, the amount of charge of the electron beam can be adjusted to the density of the plasma medium.
In one possible implementation, a certain density of the plasma medium may correspond to a certain charge loading capacity, which represents a threshold value of the charge amount loaded by the plasma medium, and if the charge amount of the electron beam moving in the plasma medium with a certain density is larger than the loading capacity of the plasma medium, the electron beam may generate negative energy chirp, that is, the front end energy of the electron beam is relatively increased and the rear end energy of the electron beam is relatively decreased (the energy of the electrons in the electron beam is increased during acceleration, but the front end energy is increased faster and the rear end energy is increased slower, so that the front end energy is increased relative to the rear end energy and the rear end energy is decreased relative to the front end energy), thereby increasing the energy dispersion. Conversely, if the electron beam moving in a plasma medium of a specific density has a smaller charge amount than the loading capacity of the plasma medium, the electron beam is subjected to positive energy chirp, that is, the back end energy of the electron beam is relatively increased and the front end energy is relatively decreased (the energy of the electrons in the electron beam during acceleration is increased, but the back end energy is increased faster and the front end energy is increased slower, so that the front end energy is decreased relative to the back end energy and the back end energy is increased relative to the front end energy), thereby decreasing the energy dispersion.
In one possible implementation, the energy chirp of the electron beam can be controlled in two ways, in the example, the energy chirp of the electron beam can be obtained by integrating the right side of equation (1) over time, and the energy chirp is related to energy dispersion. Thus, by controlling the energy chirp of the electron beam, the energy spread of the electron beam can be controlled.
In the example shown by equation (1), the controllable variable on the right side of equation (1) comprises the density n of the plasma medium p And the longitudinal distance r between the center of the electron beam and the plasma sheath b . Therefore, the ways of controlling the energy chirp of the electron beam may include two ways: one is to control the energy chirp of the electron beam by controlling the relationship between the charge quantity carried by the electron beam and the density of the plasma medium (the higher the density of the plasma medium is, the stronger the charge loading capacity is), and the other is to control the position of the electron beam in the wake field, thereby changing r b And thereby controlling the energy chirp (r) of the electron beam b The larger, the stronger its charge loading capability, in an example, the right side of equation (1) may be kept at 0, and r b The larger, the I representing the charge loading capability b The larger, i.e., the more charge loading capability is made).
In one possible implementation, based on the above description, in step S11, the energy chirp of the electron beam may be controlled by controlling the relationship between the amount of charge carried by the electron beam and the density of the plasma medium. The electron beam may be caused to carry a predetermined amount of charge that is an amount of charge that is appropriate to the density of the plasma medium, which is varied in density (i.e., increased in steps). In an example, if it is desired to produce an electron beam with a small energy chirp and a small energy spread, the adaptive relationship may include: the preset charge amount is larger than a first charge loading capacity corresponding to the density of the initial section of the plasma medium and smaller than a second charge loading capacity corresponding to the density of the final section of the plasma medium.
In a possible implementation manner, if the predetermined charge amount is greater than the first charge loading capability, when the electron beam moves in the initial section of the plasma medium, a negative energy chirp is generated, that is, the front end energy of the electron beam is relatively increased, and the rear end energy of the electron beam is relatively decreased, and as the electron beam moves, in the moving direction of the electron beam, the density of the plasma medium is gradually increased, and the corresponding charge loading capability is also gradually increased, and after moving to a certain position, the charge loading capability of the plasma medium exceeds the predetermined charge amount of the electron beam, so that the electron beam generates a positive energy chirp, and thus the front end energy of the electron beam is relatively decreased, and the rear end energy of the electron beam is relatively increased, and when moving to the final section of the plasma medium, the density of the final section of the plasma medium corresponds to a second charge loading capability greater than the predetermined charge amount. Therefore, the positive energy chirp and the negative energy chirp in the whole path of the movement of the electron beam in the plasma medium can be mutually offset, namely, the energy of the front end of the electron beam is increased and then reduced relative to the energy of the rear end of the electron beam, and the energy of the rear end of the electron beam is decreased and then increased relative to the energy of the front end of the electron beam, so that the energy of each electron of the electron beam is not gradually dispersed, the concentration degree of energy distribution is improved, the energy dispersion is reduced, a higher-quality electron beam is obtained, and support is provided for obtaining a higher-quality secondary ray or a more precise electron beam application and the like.
In a possible implementation manner, the above effect of adapting the density of the plasma medium to the preset charge amount of the electron beam can be achieved by adjusting the density of the first segment and the last segment of the plasma medium, or by adjusting the charge amount of the electron beam and the density of the first segment and the last segment of the plasma medium comprehensively. The present disclosure is not so limited.
Therefore, in the case of being loaded with an appropriate amount of electric charge, the plasma medium whose density is gradually increased may reduce the energy dispersion of the electron beam, that is, as the density of the plasma medium is gradually increased, a positive energy chirp may be generated such that the front end energy is decreased with respect to the rear end energy and the rear end energy is increased with respect to the front end energy in the electron beam to reduce the energy dispersion of the electron beam.
Conversely, if a more energy dispersive electron beam is desired, a less dense (e.g., tapered) plasma medium may be used, e.g., the charge loading capability corresponding to the density is always less than the charge of the electron beam, thereby continuously increasing the negative energy chirp, resulting in greater energy dispersion.
In a possible implementation manner, in step S12, after the preset charge amount and the density of the plasma medium are set, the preset position of the electron beam in the wake field excited by the laser pulse in the plasma medium may be moved, and as the electron beam moves, the electron beam may get closer to the laser pulse, but the density of the laser pulse and the density of the plasma medium where the electron beam is located also get larger, and the wavelength of the wake field excited by the laser pulse in the plasma medium also gets shorter, so that the electron beam can be kept at the tail of the wake field to obtain a larger acceleration gradient and an acceleration efficiency in a longer time and a longer path, thereby increasing the energy of the finally obtained target electron beam. Alternatively, the path length in the plasma medium can be shortened in the case where the required energy of the target electron beam is constant.
In one possible implementation, the manner in which the density of the plasma medium increases along the direction of motion comprises any one of: increases linearly along the direction of motion; increases in a non-linear manner along the direction of motion; the number of steps increases stepwise along the direction of motion, the number of steps being greater than or equal to 3; and the density of the plasma medium is set in a segmented manner along the movement direction, and in the two adjacent sections of plasma medium, the density of the rear section of plasma medium is greater than or equal to that of the front section of plasma medium.
Fig. 3 shows a schematic diagram of the density of a plasmonic medium, which may increase linearly in the direction of motion, as shown in graph (a) in fig. 3, according to an embodiment of the present disclosure. Alternatively, as shown in (b) of fig. 3, the density of the plasmonic medium may increase in a non-linear manner along the direction of motion, for example, in a parabolic, exponential, logarithmic, etc. curve manner, and the non-linear manner is not limited by the present disclosure. Alternatively, as shown in (c) of fig. 3, the density of the plasmonic medium may be increased stepwise along the direction of movement, i.e., once per a certain distance of movement, and the density of the plasmonic medium may be increased by a plurality of steps, e.g., the number of steps is greater than or equal to 3, so that the density of the plasmonic medium increases stepwise, and the electron beam is dynamically maintained at the tail of the wake field. Further alternatively, as shown in fig. 3 (d), the density of the plasma may be set in stages in the moving direction, and the setting manner of each stage may be arbitrary as long as the density of the plasma medium of the rear stage is greater than or equal to that of the plasma medium of the front stage, that is, the density of the plasma medium is increased stepwise in the moving direction. The present disclosure is not limited to the particular manner in which the plasma density is set.
In a possible implementation manner, the density of the plasma medium is set as above, so that when the electron beam relatively moves forward in the wake field excited by the laser pulse, the wavelength of the wake field is gradually shortened, and the electron beam can be kept at the tail position of the wake field, so that the electron beam obtains larger acceleration gradient and higher acceleration efficiency.
In one possible implementation, the density distribution of the plasmonic medium in a direction perpendicular to the direction of motion of the laser pulse corresponds to the shape of the laser pulse. In an example, the density may also be set on a plane tangential to the plasma medium (in a direction perpendicular to the moving direction of the laser pulse), for example, the density may be distributed in a parabolic shape on the plane so as to correspond to the shape of the laser pulse, so that the laser pulse may be kept in a region with a smaller density in the paraboloid to extend the moving distance of the laser pulse, thereby making the acceleration distance of the electron beam longer and obtaining higher acceleration efficiency.
According to the electron beam acceleration method of the embodiment of the disclosure, the electron beam has the charge amount which is larger than the first charge loading capacity corresponding to the density of the initial section of the plasma medium and smaller than the second charge loading capacity corresponding to the density of the final section of the plasma medium, so that the positive and negative chirps of the electron beam can be offset, the target electron beam with low energy dispersion can be obtained, and the wavelength of the wake wave field excited by the plasma medium with the density increased along the moving direction of the electron beam and the laser pulse can be changed in the moving process of the laser pulse, so that the electron beam can be dynamically kept at the preset position in the wake wave field, the electron beam can be accelerated with higher acceleration gradient, and the acceleration efficiency and the energy conversion efficiency are improved.
In one possible implementation, more types of density variations may be provided in addition to the density setting of the plasmonic medium represented in fig. 3 above.
The present disclosure provides a plasma medium density setting method, the method comprising: setting the density of a plasma medium carrying the laser pulse and the electron beam to move according to the movement speed of the laser pulse and the movement speed of the electron beam, and obtaining a target plasma medium, wherein the density of the target plasma medium enables the electron beam to move at a preset position of a tail wave field excited by the laser pulse, the density of the target plasma medium is increased along the movement directions of the electron beam and the laser pulse, the movement speed of the laser pulse is smaller than the movement speed of the electron beam, and the density of the plasma medium is inversely related to the wavelength of the tail wave field.
In an example, the length of the appropriate wake field, and thus the density of the corresponding plasma medium, may be determined based on the velocity difference between the laser pulse and the velocity of motion of the electron beam, and thus set according to the determined density.
In an example, the preset location comprises a tail of the wake field. The acceleration gradient of the electron beam can be determined by assuming that the electron beam is always at the position of the tail of the wake field excited by the laser pulse, so that the velocity of the electron beam at each time (i.e., the electron beam accelerates at a faster and faster speed) and the position of each time can be determined, and the distance difference between the electron beam and the laser pulse at each time (i.e., the distance difference between the time when the laser pulse moves to each position) can be determined based on the velocity and the position of each time of the laser pulse, and the length of the wake field excited by the laser pulse at each position can be determined based on the distance difference, so that the density of the plasma medium at each position can be found based on the length of the wake wave (i.e., the wavelength of the wake field) at each position, and equation (2). After setting based on the density, a target plasma medium can be obtained.
In another example, simulation may also be performed by a computer to determine the density of the plasmonic medium at each location based on the above parameters, such as the speed of movement of the laser pulses and the speed of movement of the electron beam. Alternatively, the density of the plasma medium may be set in any of several ways as shown in FIG. 3. The present disclosure does not limit the manner in which the density of the plasma medium is set.
In a possible implementation manner, further, in order to make the energy dispersion of the electron beam low, a first charge loading capacity corresponding to the initial density of the target plasma medium may be smaller than the charge amount of the electron beam, and a second charge loading capacity corresponding to the final density of the target plasma medium may be larger than the charge amount of the electron beam. The relationship between the charge amount and the charge loading capability can be realized by setting the charge amount of the electron beam, setting the density of the target plasma medium, and setting both the density of the target plasma medium and the charge amount of the electron beam, which is not limited by the present disclosure.
The present disclosure also provides a plasma medium for carrying laser pulses and a movement of an electron beam, wherein a density of the medium increases in a direction of movement of the electron beam and the laser pulses such that the electron beam moves at a preset position of a wake field excited in the medium by the laser pulses, a movement speed of the laser pulses is smaller than a movement speed of the electron beam, and a density of the plasma medium is inversely related to a wavelength of the wake field.
In one possible implementation, during the acceleration of the electron beam by the plasma medium, the electron beam may obtain an acceleration gradient in the wake field generated by the laser pulse to accelerate the electron beam, and as the velocity of the electron beam increases (greater than the velocity of the laser pulse), the electron beam may be advanced relative to the wake field, and the density of the plasma medium increases along the moving direction of the electron beam and the laser pulse, so that the wake field generated by the laser pulse is shorter and shorter, and thus, the electron beam may be dynamically maintained at a preset position of the wake field even if the velocity is greater than the laser pulse, for example, the preset position includes a tail of the wake field, so as to obtain a greater acceleration gradient and a higher acceleration efficiency.
Further, the first charge carrying capacity corresponding to the initial section density of the medium is smaller than the charge quantity of the electron beam, and the second charge carrying capacity corresponding to the final section density of the medium is larger than the charge quantity of the electron beam. So that the positive and negative chirps of the electron beam are offset to obtain an electron beam with lower energy dispersion, i.e. more concentrated energy distribution and higher quality.
Fig. 4 is a schematic diagram illustrating an application of an electron beam acceleration method according to an embodiment of the present disclosure, where, in a process of accelerating an electron beam by a wake field excited in a plasma medium by laser pulses, the electron beam is located at a tail of the wake field, so that the electron beam can obtain a maximum acceleration gradient and acceleration efficiency, but the moving speed of the electron beam in the plasma medium can be greater than that of the laser pulses, and thus, as shown in a state (2) in fig. 4, the electron beam can gradually approach the laser pulses and advance with respect to the wake field.
In one possible implementation, the density of the plasmonic medium can be increased along the direction of motion of the electron beam and the laser pulse, wherein the wavelength of the wake field is inversely related to the density of the plasmonic medium, and thus, the increase in the density of the plasmonic medium decreases the wavelength of the wake field, as shown in state (3) in fig. 4, and as the density of the plasmonic medium increases, the wavelength of the wake field decreases, which can cause the electron beam to return to the tail of the shortened wake field.
In one possible implementation, as shown in state (4) of fig. 4, the electron beam may continue to accelerate with greater acceleration efficiency, and move forward again with respect to the wake field, and as shown in state (5) of fig. 4, the density of the plasma medium may be increased again, and the wavelength of the wake field may be decreased again, so that the electron beam is located at the tail … … of the decreased wake field again, and the electron beam may be accelerated with greater acceleration efficiency by the process of increasing the density of the plasma medium and decreasing the wavelength of the wake field, so that the electron beam is dynamically maintained at the tail of the wake field.
In contrast, the electron beam is accelerated in the plasma medium with unchanged density, the energy conversion efficiency is usually only about 1%, and the density of the plasma medium is set, so that the electron beam moves in the plasma medium with gradually increased density, the position of the electron beam is dynamically maintained at the tail of the wake wave field, the energy conversion efficiency can be improved to more than 20%, and the improvement effect is obvious.
In a possible implementation manner, the charge amount of the electron beam is larger than a first charge loading capacity corresponding to the density of the first section of the plasma medium and smaller than a second charge loading capacity corresponding to the density of the last section of the plasma medium, so that the positive and negative chirping of the electron beam is cancelled, and the electron beam with smaller energy dispersion is obtained. The chirp of the electron beam is controlled by the density of the plasma medium, so that the energy dispersion can be reduced to below 1%.
It is understood that the above-mentioned method embodiments of the present disclosure can be combined with each other to form a combined embodiment without departing from the logic of the principle, which is limited by the space, and the detailed description of the present disclosure is omitted. Those skilled in the art will appreciate that in the above methods of the specific embodiments, the specific order of execution of the steps should be determined by their function and possibly their inherent logic.
Having described embodiments of the present disclosure, the foregoing description is intended to be exemplary, not exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen in order to best explain the principles of the embodiments, the practical application, or improvements made to the technology in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.
Claims (10)
1. A method of accelerating an electron beam, comprising:
injecting an electron beam having a predetermined amount of charge;
and enabling the electron beam to dynamically keep moving at a preset position of a tail wave field excited by laser pulses in a plasma medium, and obtaining an accelerated target electron beam, wherein the density of the plasma medium is increased along the moving direction of the electron beam and the laser pulses, the moving speed of the laser pulses is less than that of the electron beam, and the density of the plasma medium is inversely related to the wavelength of the tail wave field.
2. The method of claim 1, wherein the preset position comprises a tail of the wake field.
3. The method of claim 1, wherein the predetermined amount of charge is greater than a first charge carrying capacity corresponding to a density of an initial segment of the plasma medium and less than a second charge carrying capacity corresponding to a density of a final segment of the plasma medium.
4. The method of claim 1, wherein the manner in which the density of the plasma medium increases along the direction of motion comprises any one of:
increases linearly along the direction of motion;
increases in a non-linear manner along the direction of motion;
the number of steps increases stepwise along the direction of motion, the number of steps being greater than or equal to 3;
and the density of the plasma medium is set in a segmented manner along the movement direction, and in the two adjacent sections of plasma medium, the density of the rear section of plasma medium is greater than or equal to that of the front section of plasma medium.
5. The method of claim 1, wherein the normalized sagittal potential of the laser pulse is greater than or equal to 2.
6. The method of claim 1, wherein a density distribution of the plasmonic medium in a direction perpendicular to a direction of motion of the laser pulse corresponds to a shape of the laser pulse.
7. A method of plasma medium density setting, the method comprising:
setting the density of a plasma medium carrying the laser pulse and the electron beam to move according to the movement speed of the laser pulse and the movement speed of the electron beam, and obtaining a target plasma medium, wherein the density of the target plasma medium enables the electron beam to dynamically keep moving at a preset position of a tail wave field excited by the laser pulse, the density of the target plasma medium is increased along the movement direction of the electron beam and the laser pulse, the movement speed of the laser pulse is smaller than the movement speed of the electron beam, and the density of the plasma medium is inversely related to the wavelength of the tail wave field.
8. The method of claim 7, wherein a first charge carrying capacity corresponding to an initial density of the target plasma medium is less than an amount of charge possessed by the electron beam, and a second charge carrying capacity corresponding to a final density of the target plasma medium is greater than the amount of charge possessed by the electron beam.
9. The method of claim 7, wherein the preset position comprises a tail of the wake field.
10. A plasmonic medium, characterized in that the medium is configured to carry a laser pulse and a movement of an electron beam, wherein a density of the medium increases in a direction of the movement of the electron beam and the laser pulse such that the electron beam dynamically remains moving at a predetermined position in a wake field excited by the laser pulse in the medium, wherein a speed of the movement of the laser pulse is smaller than a speed of the movement of the electron beam, and wherein the density of the plasmonic medium is inversely related to a wavelength of the wake field.
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Inventor after: Lu Wei Inventor after: Liu Shuang Inventor after: Hua Jianfei Inventor before: Lu Wei Inventor before: Hua Jianfei Inventor before: Liu Shuang |