RU2826450C1 - Method of spatially directed laser acceleration of beam of charged particles in plasma with inhomogeneous concentration of electrons - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention relates to a method for spatially directed acceleration of particles by ultrashort laser pulses of high intensity in plasma with inhomogeneous distribution of electron concentration. Method involves directing a focused accelerating laser pulse of femtosecond duration with power of more than 0.1 terawatt through plasma, forming a plasma channel along the optical axis of the laser pulse and gaining energy by particles over the length of the plasma channel. Plasma channel curvature is provided, caused by deviation of the axis of propagation of the accelerating pulse relative to the initial direction, due to refraction of light in the presence of a transverse gradient of concentration of electrons in the plasma, as well as changing the direction of the beam of accelerated electrons in accordance with the curvature of the plasma channel and the axis of propagation of the accelerating pulse while maintaining the average energy of particles in the beam and divergence of the beam.

EFFECT: possibility of obtaining a collimated electron beam with divergence less than 0.1 rad and average particle energy of more than 1 MeV by means of pulsed laser acceleration of the beam in plasma with a transverse gradient of electron concentration relative to the axis of propagation of the accelerating laser pulse.

5 cl, 3 dwg

Description

Field of technology

The invention relates to the field of producing beams of charged particles, in particular, to methods of simultaneously accelerating and controlling the directionality of a collimated beam of high-energy electrons and, more precisely, to methods of spatially directed acceleration of particles by ultra-short high-intensity laser pulses in a plasma with a non-uniform distribution of electron concentration.

State of the art

There are various methods for obtaining a collimated electron beam with high particle energy, in particular, when femtosecond laser radiation with high peak intensity passes through plasma, accompanied by the formation of a plasma channel as a result of self-focusing of radiation and/or ponderomotive action of the laser pulse. The compact size and high efficiency of laser sources of charged particle beams determine the prospects for their use in medical physics, nuclear physics, generation of terahertz radiation and other applications of both fundamental and applied nature.

The prior art includes a method for acceleration in a plasma channel using electromagnetic fields of laser radiation (the so-called "direct laser acceleration"), disclosed, for example, in the publication by A. E. Hussein et al. "Towards the optimisation of direct laser acceleration" // New Journal of Physics (2021) 23 023031. Also known is a method for acceleration in a plasma channel using longitudinal fields of a plasma wave ("wakefield acceleration") at the trailing edge of a laser pulse, disclosed, for example, in the publication by T. Tajima et al. "Wakefield acceleration" // Reviews of Modern Plasma Physics (2020) 4:7.

The above sources to varying degrees disclose the physical principle of plasma channel formation and the mechanism of electron beam acceleration in it, but do not formulate approaches to controlling the electron beam directionality. The intensity of laser radiation with a peak power above 0.1 terawatt is achieved by focusing in vacuum conditions (less than 0.1 Torr) values above 10 ¹⁷ W/cm ² . The passage of such laser radiation in the waist region through optically transparent plasma (with an electron concentration below the critical value, which for a wavelength of 800 nm is equal to ~1.7x10 ²¹ cm ^-3 or more, taking into account relativistic effects) is accompanied by phase self-modulation, self-focusing due to the displacement of electrons from the laser pulse axis by the ponderomotive action of light, the formation of a plasma channel and the capture of background electrons with subsequent acceleration to an average energy of 1 MeV or more on a length scale of about 0.05-10 mm. The collimation of the electron beam is 0.1 rad or less, and the charge reaches units of nC.

In a number of tasks, in addition to high energy and beam charge, it is necessary to be able to control the direction of particle motion after acceleration. Magnetic optics and deflectors are usually used for this purpose.

A system for correcting the trajectories of a flow of charged particles in a magnetic field is known according to Russian Patent No. 2643507, including a high-voltage pulse generator, lines that provide the creation of a magnetic field on the path of the particle flow, electrically interconnected, a means for transmitting a high-voltage pulse from the high-voltage pulse generator to a line that provides the formation of a magnetic field on the path of the particle flow, characterized in that the lines that provide the creation of a magnetic field on the path of the particle flow form a current loop with a wave impedance ρ, the means for transmitting a high-voltage pulse from the high-voltage pulse generator is made in the form of transmission lines, each of which has a wave impedance ρ equal to the wave impedance of the current loop, and the high-voltage pulse generator is built on forming lines with a total wave impedance ρ/2 and is formed by n forming lines connected to an external static power source, wherein the wave impedance of each forming line is equal to nρ/2, wherein the forming lines are electrically connected to a controlled arrester, connected to transmission lines and then to the current loop.

The specified system has high inertia and does not allow for rapid changes in the electron beam deflection angle. Spectral selectivity of magnetic systems reduces the useful beam charge and can reduce the degree of collimation.

A utility model is known under Russian patent No. 156716, which discloses a pyroelectric deflector of a charged particle beam, comprising a module with a bracket-heat conductor, two pyroelectric crystals, a Peltier element, characterized in that the device consists of two modules located opposite each other at a distance determined by the transverse size of the controlled beam, each of the modules contains one pyroelectric crystal connected to the Peltier element, an additional built-in heat conductor made in the form of a plate. Using this utility model, the possibility of deflecting an electron beam with an energy of 7 MeV by an angle of 26 mrad was experimentally demonstrated (V.I. Alekseev et al. “Pyroelectric deflector of relativistic electron beam” // Chinese Journal of Physics 2022, 77, p.2298).

The disadvantage of this solution is the small deflection angle, the slow response of the deflection field to changes in system parameters, and the need for a deep vacuum to prevent breakdown between the pyroelectric crystals.

The closest in technical essence to the claimed invention is a method for deflecting the propagation axis of an electron beam from the initial direction using optical methods during acceleration of the electron beam by longitudinal plasma fields in a curved plasma channel. For this purpose, plasma is created by an electric discharge in a gas-filled capillary made inside a solid by milling, through which an accelerating powerful femtosecond laser pulse passes (X. Zhu et al. "Experimental Demonstration of Laser Guiding and Wakefield Acceleration in a Curved Plasma Channel" // PHYSICAL REVIEW LETTERS 130. 215001, 2023).

The disadvantage of this technical solution is the constancy of the angle of deviation of the electron beam from the initial axis, which is determined by the milling of the capillary. In addition, the capillary is subject to gradual degradation due to optical breakdown of its walls by laser radiation. Also, working with gas targets at a high pulse repetition rate creates a high load on the vacuum gas pumping system.

Thus, the technical problem solved by the claimed invention consists in the need to overcome the shortcomings inherent in the above-mentioned analogs and prototype, by creating a method for simultaneous acceleration and rapid controlled control over significant angular limits of the directionality of an electron beam obtained in a laser accelerator directly at the stage of gaining energy by particles in the plasma.

Brief description of the invention

The technical result of the invention consists in obtaining a collimated electron beam with high energy and charge with controlled directionality and achieved in a scheme of pulsed laser beam acceleration in plasma with a transverse gradient of electron concentration relative to the axis of propagation of the accelerating laser pulse.

The stated technical result is achieved as a result of implementing a method of spatially directed laser acceleration of a beam of charged particles with a divergence of less than 0.1 rad and an average particle energy of more than 1 MeV in a plasma with a non-uniform electron concentration, which includes the following actions:

- direction of a focused accelerating laser pulse of femtosecond duration with a power of more than 0.1 terawatt through the plasma, while the electron concentration in the plasma for the central wavelength of the accelerating pulse is below the critical value and has a transverse gradient that determines the gradient of the refractive index of the plasma relative to the axis of the direction of the femtosecond pulse,

- formation along the optical axis of the laser pulse of a plasma channel with a length exceeding the waist length of the focused femtosecond pulse, as a result of self-focusing of the femtosecond pulse and/or the ponderomotive action of light as the laser radiation propagates through the plasma,

- energy accumulation by particles in the pulsed laser-plasma acceleration scheme along the length of the plasma channel in the field of transverse electromagnetic waves of the accelerating laser pulse or in the longitudinal electric fields of the plasma wave formed by the accelerating laser pulse,

- curvature of the plasma channel caused by the deviation of the propagation axis of the femtosecond laser pulse relative to the initial direction due to the refraction of light in the presence of a transverse gradient of electron concentration in the plasma,

- changing the direction of the beam of accelerated electrons in accordance with the curvature of the plasma channel and the axis of propagation of the accelerating femtosecond pulse while maintaining the average energy of the particles in the beam and the divergence of the beam, while the angle of deviation of the particle beam from the original axis is determined by the gradient of the electron concentration in the plasma.

The plasma has a cylindrically symmetric electron density distribution, with the gradient directed across the propagation axis of the accelerating femtosecond pulse. Parallel displacement of the initial propagation axis of the accelerating femtosecond pulse relative to the plasma symmetry axis ensures the pulse passage through the plasma with a transverse density gradient and the plasma channel curvature and change in the direction of the accelerated particle beam. In a plasma with a cylindrically symmetric electron density distribution, parallel displacement can be performed in any radial direction, which leads to the particle beam deflection also in any direction while maintaining the average particle energy in the beam and the beam divergence. Plasma with a cylindrically symmetric density distribution is preliminarily formed by ionization, ablation and breakdown of a thin solid foil with a heating laser pulse of 0.1-10 nanoseconds duration, focused onto the foil surface.

The plasma channel curvature depends on the parallel displacement of the initial axis of the accelerating femtosecond laser pulse relative to the plasma symmetry axis, while the pulse propagation through a medium with a radial dependence of the refractive index is accompanied by its deflection by an angle determined by the particle concentration gradient in the plasma in the transverse direction and the plasma length along the pulse propagation direction, while the plasma channel is also curved, which leads to a deflection of the accelerated electron beam by an angle comparable to the laser beam deflection angle and reaching tens of degrees, while maintaining the electron beam parameters (average energy and divergence) compared to the case of accelerating pulse propagation along the plasma symmetry axis (in the absence of deflection). The radial symmetry of the electron concentration distribution in the plasma allows deflecting the electron beam in any radial direction determined by the direction of the transverse parallel displacement of the initial axis of propagation of the accelerating femtosecond pulse relative to the plasma symmetry axis.

Brief description of the drawings

The invention is illustrated by the following images, where

Fig. 1 a) shows a diagram of the effect of a heating pulse 2 on a target made of foil 1, leading to ionization, ablation and breakdown of the film;

Fig. 1 b) shows a formed cylindrical symmetrical breakdown 3 after a certain period of time following the impact of the heating pulse 2, during which the density of particles in the breakdown area decreased due to the dispersion of the substance of the initially dense foil 1;

Fig. 2 a) shows the passage of a focusing femtosecond pulse 4, accelerating an electron beam 5 in the plasma as the pulse propagates along the breakdown axis;

Fig. 2 b) shows the passage and deflection of a focusing femtosecond pulse 4, accelerating an electron beam 5 in the plasma during pulse propagation with a parallel displacement relative to the breakdown axis;

Fig. 3 a) shows the diagram of the effect of pulses on the target and the location of the scintillation detector for the electron beam;

Fig. 3 b) shows an image of the plasma channel in optical light, obtained using camera 6, during the propagation of the accelerating pulse along the breakdown axis. The solid lines indicate the position of the target. The white arrow indicates the direction of the incident laser radiation on the target;

Fig. 3c) shows the registered electron beam on the scintillation detector, where the circle indicates the direction that coincides with the direction of propagation of the initial laser pulse entering the plasma;

Fig. 3 d) shows an image of the plasma channel in optical light, obtained using camera 6, with a deviation from the initial axis during the propagation of the accelerating pulse with a parallel shift relative to the breakdown axis by 10 μm. The solid lines indicate the position of the target. The white arrow indicates the direction of the incident laser radiation on the target;

Fig. 3 d) shows the registered shifted electron beam on the scintillation detector, where the indicated deviation on the detector corresponds to the beam emission angle of 10 degrees relative to the initial axis of propagation of the accelerating pulse.

The following positions are indicated on the drawings:

1 - foil target,

2 - heating laser pulse

3 - breakthrough in the target

4 - accelerating laser pulse

5 - electron beam

6 - a camera for recording the glow of the plasma channel in the optical range

7 - position-sensitive scintillation electron detector

Implementation of the invention

The invention is implemented as follows.

Target 1 in the form of a foil several tens of micrometers thick is placed near the waist of the heating laser pulse 2. A plastic (or lavsan) film or another thin target of solid density, placed in vacuum conditions with a residual gas pressure below 1 Torr, can be used as the foil. The effect of the heating laser pulse on the target is ensured. Radiation in the IR or visible range with a duration of 0.1-10 ns with a peak intensity in the waist sufficient for plasma formation on the film surface (over 10 ¹⁰ W/cm ² ) can be used as the heating laser pulse. The energy input to the plasma must be sufficient to form a through radially symmetric breakdown 3 in the film. The plasma dynamics and breakdown development on times of the order of nanoseconds after the peak of the heating pulse lead to the formation of a transverse concentration gradient - a decrease in the particle concentration in the plasma on the breakdown axis compared to the peripheral regions. The dynamics of plasma expansion can be established experimentally, for example, by interferometric diagnostics methods, or numerically, for example, by hydrodynamic modeling of laser plasma expansion.

Next, a powerful accelerating laser pulse of femtosecond duration 4 with a power higher than 0.1 terawatt is passed through the plasma, focusing to a peak intensity higher than 10 ¹⁷ W/cm ² . In this case, the initial position of the foil plane is within the waist length of the femtosecond pulse, and the pulse propagates coaxially with the plasma symmetry axis in the foil breakdown. The propagation of the pulse through the plasma is accompanied by self-focusing and/or ponderomotive action of light, leading to the formation of a plasma channel with a length comparable to the longitudinal size of the plasma. In the channel, electrons gain energy using the method of direct laser acceleration or wake acceleration to an average energy of about 1 MeV or more in the form of a collimated beam 5 with a divergence of about 0.1 rad.

A parallel shift of the initial propagation axis of the accelerating pulse relative to the plasma symmetry axis in the region of the foil breakdown by a value within the breakdown diameter leads to a deviation of the laser radiation due to the presence of a radial dependence of the plasma refractive index in the breakdown. The plasma refractive index, in turn, is determined by the distribution of the electron concentration in the plasma. The deviation angle θ as the laser radiation propagates along the breakdown axis z is related to the dependence along the radial axis r of the refractive index of the medium n as . In this case, the maximum refraction is achieved in the region where the gradient of the refractive index in the radial direction is the highest. Refraction and deflection of the laser pulse 4 also leads to curvature of the plasma channel. The beam of accelerated electrons 5 also deflects from the initial axis by an angle comparable to the deflection angle of the accelerating pulse, which reaches tens of degrees, while maintaining the electron beam parameters (average energy and divergence) identical to the case of propagation of the accelerating pulse along the plasma symmetry axis. In this case, the parallel displacement of the accelerating femtosecond pulse relative to the plasma symmetry axis can be carried out in any direction, which, due to the radial symmetry of the electron concentration in the plasma, makes it possible to control the emission angle of the electron beam θ in any radial direction.

The displacement of the accelerating pulse axis relative to the plasma symmetry axis in the breakdown is achieved by a small rotation of the optomechanical mirror holder of the optical path of the heating or accelerating pulse. In this case, the use of automation and control using piezo actuators or other types of motors (step or direct current) provides a displacement of up to tens of micrometers in less than 10 ms. The displacement accuracy can be better than 0.1 μm, which determines the accuracy of the beam deflection angle better than 0.1 degree. Thus, the angle of the electron beam emission direction can be controlled with high accuracy in each accelerating laser pulse with frequency generation of the electron beam (up to 1 kHz).

Example of specific implementation

In order to confirm the efficiency of the claimed invention, an experimental implementation of the claimed method was carried out in the circuit shown in Fig. 3a. Heating laser radiation with a pulse duration of 8 ns at half-maximum intensity at a wavelength of 1064 nm and a pulse energy of 100 mJ is focused under vacuum conditions (residual pressure of 0.1 Torr) into a spot with a diameter of 13 μm at half-maximum intensity to a peak intensity of about 10 ¹³ W/cm ² on the surface of a 16 μm thick lavsan film at an angle close to the normal. The lavsan film is a 10 mm wide tape wound on a reel and rewound onto a receiving reel. The constancy of the position of the film plane in the waist plane of the heating pulse is maintained when the tape passes near two posts, the position of which is fixed. When exposed to a heating pulse, due to the high intensity, the film substance is heated and ionized, and a high-temperature plasma is formed. Plasma expansion and ablation from the impact area lead to breakdown in the film. After 2 ns from the impact of the heating pulse peak on the breakdown axis, the diameter of which is about 50 μm, the residual density of the substance is at the level of thousandths of the initial density, according to the calculation model. Due to the axial symmetry of the impact, the concentration in the breakdown area has a cylindrically symmetric distribution; with distance in the radial direction from the breakdown axis, the concentration increases. At this moment, a laser pulse with a peak power of 1 terawatt, a duration of 50 fs and an energy of 50 mJ at a central wavelength of 800 nm enters the rarefied plasma in the direction parallel to the film breakdown axis, focusing into the breakdown area to a peak intensity of over 10 ¹⁸ W/cm ² . The passage of the laser pulse through the plasma is accompanied by nonlinear optical self-action and the formation of a plasma channel due to the ponderomotive action of light. The image of the channel glow in the optical range is recorded by the camera. Fig. 3b shows the channel during propagation of a femtosecond pulse along the film breakdown axis. The length of the luminous region of the channel is about 150 μm. The combined effect of the channel and pulse fields on the particles leads to the formation and acceleration of an electron beam, the total charge in which for particles with an energy of more than 1 MeV is about 100 pC, and the divergence at the exit from the accelerating stage does not exceed 0.1 rad. The electron beam is recorded on a scintillation screen located in the path of the particle beam, Fig. 3c. A parallel shift of the laser pulse axis relative to the film breakdown axis by 5 μm leads to the passage of radiation through a region with a refractive index gradient (due to the particle concentration gradient) in the radial direction, entailing a deviation of the laser pulse from the original direction of propagation. The plasma channel also experiences a deviation, Fig. 3d, the angle is approximately 6 degrees from the original axis. On the scintillation screen, the trace of the electron beam also shifts by a distance corresponding to a deflection angle of 6 degrees, Fig. 3d. In this case, the brightness of the scintillation screen glow and the transverse size of the beam do not change significantly relative to Fig. 3b, where the electron beam is obtained during the coaxial passage of the laser pulse through the film breakdown. Thus, in each laser pulse, the accelerated electron beam can be output at a given angle. In this case, due to the axial symmetry of the film breakdown, the particle beam can be deflected both vertically and horizontally. The rate of shift of the pulse axis relative to the breakdown axis is determined by the optomechanical components used. In this example, a shift of 5 μm occurs in less than 5 ms with an accuracy of better than 0.5 μm, which corresponds to an accuracy of control of the emission angle of better than 0.5 degrees. Consequently, at a repetition rate of laser pulses, as well as electron beams, above 100 Hz, the direction of emission of the beam of charged particles can be controlled for each electron bunch.

Claims (10)

1. A method of spatially directed laser acceleration of a beam of charged particles with a divergence of less than 0.1 rad and an average particle energy of more than 1 MeV in a plasma with a non-uniform electron concentration, including - direction of a focused accelerating laser pulse of femtosecond duration with a power of more than 0.1 terawatt through the plasma, wherein the electron concentration in the plasma for the central wavelength of the accelerating pulse is below the critical value and has a transverse gradient that determines the gradient of the refractive index of the plasma, relative to the axis of the direction of the femtosecond pulse, - formation along the optical axis of the laser pulse of a plasma channel with a length exceeding the waist length of the focused femtosecond pulse, as a result of self-focusing of the femtosecond pulse and/or the ponderomotive action of light as the laser radiation propagates through the plasma, - energy accumulation by particles in the pulsed laser-plasma acceleration scheme along the length of the plasma channel in the field of transverse electromagnetic waves of the accelerating laser pulse or in the longitudinal electric fields of the plasma wave formed by the accelerating laser pulse, - curvature of the plasma channel caused by the deviation of the propagation axis of the femtosecond laser pulse relative to the initial direction due to the refraction of light in the presence of a transverse gradient of electron concentration in the plasma, - changing the direction of the beam of accelerated electrons in accordance with the curvature of the plasma channel and the axis of propagation of the accelerating femtosecond pulse while maintaining the average energy of the particles in the beam and the divergence of the beam, while the angle of deviation of the particle beam from the original axis is determined by the gradient of the electron concentration in the plasma. 2. The method according to claim 1, characterized in that the plasma has a cylindrically symmetric distribution of electron concentration, wherein the gradient is directed across the axis of propagation of the accelerating femtosecond pulse. 3. The method according to item 2, characterized in that the parallel displacement of the initial axis of propagation of the accelerating femtosecond pulse relative to the axis of symmetry of the plasma ensures the passage of the pulse through the plasma with a transverse concentration gradient, and the curvature of the plasma channel, and a change in the direction of the beam of accelerated particles. 4. The method according to paragraph 3, characterized in that in a plasma with a cylindrically symmetric distribution of electron concentration, parallel displacement is carried out in any radial direction, which leads to a deflection of the particle beam also in any direction while maintaining the average energy of the particles in the beam and the divergence of the beam. 5. The method according to item 2, characterized in that the plasma with a cylindrically symmetrical concentration distribution is preliminarily formed during ionization, ablation and breakdown of a thin solid foil by a heating laser pulse with a duration of 0.1-10 nanoseconds, focused on the surface of the foil.