METHOD AND SYSTEM FOR ELECTRON ACCELERATION BY MEANS OF LASER-PLASMA INTERACTION

Description

Domaine technique :

[0001] The present invention relates to a method and system for accelerating electrons by laser-plasma interaction. It relates in particular to the field of accelerating electrons using femtosecond lasers ranging from the terawatt (TW) class to the multi-PW (petawatt) class by a laser wake mechanism ( Laser WakeField Acceleration (or LWFA in English). Technique antérieure :

[0002] Accelerating electrons to relativistic energies over very short distances using lasers has long been a goal in physics and is of great fundamental interest. This acceleration mechanism allows us to reproduce and understand physical processes that can only be found in extreme astrophysical scenarios. Furthermore, it enables the construction of accelerators that are much more compact than conventional accelerators for high-energy physics and medicine.

[0003] By focusing pulses delivered by femtosecond lasers ranging from the TW class to the multi-PW class onto focal spots of a few microns to a few tens of micrometers, it is possible to obtain intensities greater than 1018. W / cm 2< producing electric fields of several TV / m. When the normalized peak amplitude of the pulse is greater than or equal to 1, the electron oscillates in the laser field with relativistic dynamics. That is to say, the speed of this electron is close to the speed of light.

[0004] As a reminder, the normalized peak amplitude, a1, is defined by: a 1 = eE L m e ω 1 c with e And m e the charge and mass of the electron, respectively, E L the peak amplitude of the pulse (expressed in V / m), ω 1. The angular frequency of the laser and c the speed of light. For a wavelength λ = 1 µmwe have a 1 > 1 for an intensity greater than 10 18< W / cm 2< .

[0005] If a sample of matter is placed at the focus of the laser, the extreme electric fields ionize the matter almost instantaneously and form an ultra-relativistic plasma, with electrons accelerated to speeds close to that of light on timescales smaller than one fs. This allows the study of ultra-relativistic, non-equilibrium, and highly non-linear physics, known as "ultra-high intensity physics".

[0006] There are several ways to enable the acceleration of electrons by such laser fields.

[0007] Among the various existing methods, laser acceleration in a vacuum ( Vacuum Laser Acceleration The VLA (or VLA in English) method has generated considerable interest and extensive theoretical study due to its apparent simplicity. In this method, electrons interact with an intense laser field in a vacuum and can be continuously accelerated, provided they remain in a specific phase of the field until they escape the laser beam. In the VLA method, the laser field itself delivers electric fields of several TV / m, which can, in principle, accelerate electrons to relativistic speeds along the Rayleigh length of the laser. Until recently, the VLA method was primarily studied from a theoretical perspective because the conditions required to correctly inject electrons into the laser field were extremely challenging. Specifically, the injected electron packet must be ultrashort (much shorter than a laser period <3fs) and injected at a very precise acceleration phase with sub-fs accuracy.

[0008] The feasibility of this injection method using a relativistic plasma mirror was demonstrated in the document Thévenet, M., et al. "Vacuum laser acceleration of relativistic electrons using plasma mirror injectors." Nature Physics 12.4 (2016): 355-360, hereinafter "Thévenet, et al. ". There figure 1A This schematically illustrates the principle of this technique. An incident laser pulse in ultrashort p-polarization (~25 fs ) is focused with an intensity greater than 10<18< W / cm 2< on a plasma mirror that is reflective to the field of the incident laser pulse. The plasma mirror is generated under vacuum by ionizing an initially solid target with a laser pre-pulse focused at an intensity greater than 1016 W / cm 2. Under the influence of this laser field, the "plasma mirror" thus formed oscillates at relativistic speeds with the laser period. During reflection on the surface of the plasma mirror, the electric field E of the incident laser pulse ejects electrons from the surface of the plasma mirror and injects them into the reflected laser field. The injected charge is then accelerated by the ultra-intense electric field of the laser pulse.

[0009] Unfortunately, this method has the disadvantage of producing poor-quality electron beams. More specifically, electron beams produced by VLA exhibit high divergence (over 500 mrad ) and a strong energy dispersion, which makes this method unusable for some applications. In addition, the maximum energy of the VLA mechanism evolves as the square root of the laser power P, which makes it not a good candidate for achieving high energies (see Zaïm, M. Thévenet, A. Lifschitz, and J. Faure, "Relativistic Acceleration of Electrons Injected by a Plasma Mirror into a Radially Polarized Laser Beam," Phys. Rev. Lett., vol. 119, no. 9, p. 094801, Aug. 2017, doi: 10.1103 / PhysRevLett.119.09480).

[0010] Another technique known to those skilled in the art (see in particular the document E. Esarey, CB Schroeder, and WP Leemans “Physics of laser-driven plasma-based electron accelerators”, Reviews Of Modern Physics, Volume 81, July - September 2009) for accelerating electrons by laser-plasma interaction is the laser wake mechanism ( Laser WakeField Acceleration (or LWFA in English). The figure 1B is a schematic representation of the operation of this mechanism. In the LWFA method, a laser delivers ultrashort pulses that are focused into a gas with a high intensity, typically greater than 1018. W / cm 2. At the laser focus, the gas is ionized almost instantaneously by the ultra-intense laser field and forms a "sub-dense" plasma, with an electron density typically between 10 and 17. cm -3 and 10 20 cm -3< . A plasma is said to be "underdense" when the plasma frequency, proportional to the square root of the density, is lower than the laser frequency. For laser intensities greater than 10 18< W / cm 2< , the laser pulse (via the podromotive force it generates) violently expels electrons from its trajectory during its propagation and forms an electron-deficient "bubble" in its wake, which can withstand large accelerating fields on the order of 100 GV / m. Some electrons from the plasma can then be trapped in this bubble and accelerated to relativistic speeds over lengths of a few millimeters to a few centimeters.

[0011] This LWFA mechanism enables the acceleration of electrons up to 8 GeV on the centimeter scale (see AJ Gonsalves et al., "Petawatt Laser Guiding and Electron Beam Acceleration to 8 GeV in a Laser-Heated Capillary Discharge Waveguide," Phys. Rev. Lett., vol. 122, no. 8, p. 084801, Feb. 2019, doi: 10.1103 / PhysRevLett.122.084801). Therefore, it is considered one of the most promising candidates for building the next generation of compact particle accelerators dedicated to high-energy physics. Currently, LWFA-type devices can already provide high-quality electron beams: ultrashort (a few fs), small (micrometer scale), with low divergence, and low energy dispersion (a few percent). However, LWFA-type devices currently suffer from a charge per electron packet ( electron bunch (in English) relatively low to high energy (about ten pC to a few GeV).

[0012] Also, there is the problem of increasing the charge per packet of electrons obtained by LWFA while maintaining its good quality in terms of shortness, spatial dimension, divergence, and energy dispersion.

[0013] The production of such high-charge beams is crucial for many applications such as very high dose rate radiotherapy, compact X-ray free-electron lasers, or the next generation of particle colliders based on multi-PW laser systems.

[0014] To this end, an object of the invention is a method and system for accelerating electrons by laser-plasma interaction based on the laser wake mechanism. Unlike prior art LWFA devices, electron injection is performed by reflecting an ultrashort laser pulse directed at oblique incidence and with s-polarization onto a previously generated dense plasma, thereby generating a laser wake within the gas. The laser pulse heats the electrons in the dense plasma to an energy such that a bunch of electrons is injected into the wake to be accelerated. Thus, it is possible to obtain an electron beam with ultrashort electron bunches exhibiting low divergence, high charge for high energy, and low energy dispersion.

[0015] Compared to prior art LWFA devices, the invention significantly increases the electron packet charge while maintaining optimal electron beam quality. Compared to VLA devices, the invention substantially improves electron beam quality (divergence and energy dispersion). Résumé de l'invention :

[0016] To this end, an object of the invention is a method for accelerating electrons by laser-plasma interaction in which at least one laser pulse is directed onto a surface of a target in the condensed state, said surface being covered with a layer of gas, the intensity of said at least one pulse being sufficient to: in step A, generate, from the target in the condensed state, a dense plasma; in step B, after reflection by the dense plasma, generate a laser wake in the gas layer; in step C, heat the electrons of said dense plasma to an energy such that a packet of said electrons is injected into said wake to be accelerated there, said at least one impulse intended to be reflected by the dense plasma and to heat the electrons of the latter, being at oblique incidence in s polarization on said target.

[0017] In a first variant of the process of the invention, the generation of the dense plasma in step A is induced by at least one laser pulse called pre-pulse and steps B and C are induced by at least one laser pulse called main pulse, in step B, said main pulse is spatially superimposed on the pre-pulse and has a time delay with respect to the pre-pulse.

[0018] Preferably, in this first variant: said time delay is sufficiently small such that a scale length of the dense plasma gradient is less than a wavelength of the main pulse, and / or said time delay is between 50 fs 200 ps

[0019] Preferably, in this first variant, the process includes a step of adjusting said time delay so as to optimize a charge of said packet of electron injected into said wake.

[0020] Preferably, in step A of this first variant, the intensity of the pre-pulse is greater than 1015 W / cm 2< on said surface and, in step B, the intensity of the main pulse is greater than 10 18< W / cm 2< after reflection by said dense plasma.

[0021] Preferably, in this first variant, the process includes a step prior to steps A and B of generating the pre-pulse and the main pulse from the same so-called initial laser pulse.

[0022] Preferably, in this first variant, the process includes a step prior to steps A and B, of increasing the temporal contrast of the main pulse by reflection on one or more additional plasma mirrors.

[0023] Preferably, in the process of the invention, the average pressure in the gas layer is between 0.1 atm and 200 atm, preferably between 0.5 atm and 50 atm.

[0024] Another object of the invention is a laser-plasma interaction electron acceleration system comprising: a target in the condensed state covered by a layer of gas, a laser system adapted to generate at least one laser pulse, an optical system adapted to direct said at least one laser pulse onto a surface (SS) of the target in the condensed state, the laser system and the optical system being further configured such that the intensity of said at least one pulse is sufficient to: generate, from the target in the condensed state, a dense plasma; after reflection by the dense plasma, generate a wake in the gas layer; heat the electrons of said dense plasma to an energy such that a bunch of said electrons is injected into said wake to be accelerated therein the optical system being further adapted so that said at least one impulse intended to be reflected by the dense plasma and to heat the electrons of the latter, is at oblique incidence in s polarization on said target.

[0025] Preferably, according to a first variant, the laser system and the optical system are configured to: Generate a first pulse, called pre-pulse, and direct it towards the target to generate said dense plasma, Generate a second pulse, called main pulse, and direct it towards the target to generate said wake in the gas layer and induce said heating of the electrons of said dense plasma, said main pulse, when reflected by the dense plasma, being spatially superimposed on the pre-pulse.

[0026] Preferably, in this first variant, said laser system is adapted so that the temporal contrast of the main pulse is greater than 10< 8< , preferably greater than 10< 10< .

[0027] Preferably, the system of the invention comprises a gas nozzle connected to a gas reservoir, the gas nozzle being adapted to deliver a gas jet configured to form the gas layer. Even more preferably, the system of the invention comprises a gas cell sealed or partially sealed by the target, the gas nozzle being adapted to deliver the gas jet into the gas cell. Again preferably, the target is formed by a portion of ribbon that unwinds from a reel, said system comprising a motor assembly adapted to unwind said ribbon into the cell.

[0028] Preferably, in the system of the invention, the gas reservoir comprises helium and / or hydrogen and / or nitrogen.

[0029] Preferably, in the system of the invention, the gas nozzle is adapted so that an average pressure in the gas layer is between 0.1 atm and 200 atm, preferably between 0.5 atm and 50 atm. Brève description des figures :

[0030] Other features, details and advantages of the invention will become apparent from the description provided with reference to the accompanying drawings given by way of example, which represent, respectively: [ Fig.1A] et [Fig.1B ], a schematic illustration of the methods known in the art for accelerating electrons by laser-plasma interaction in a vacuum and by laser wake mechanism respectively, [ Fig.2 ], a schematic diagram of the system according to a preferred variant of the invention, [ Fig.3 ], three instantaneous representations a, b and c of a PIC code simulation, obtained at different times in the process according to the preferred variant of the invention, [ Fig.4A ] a graphical representation of a spectrum of the electron beam generated by the system of the invention obtained by simulation, [ Fig.4B ] a graphical representation of an experimental result: a spectrum of the electron beam generated by the system of the invention [ Fig.5 ], a schematic representation of a system according to a first embodiment of the invention, [ Fig.6 ], a schematic representation of the system according to a second embodiment of the invention, [ Fig.7 ], a schematic representation of the system according to a third embodiment of the invention,

[0031] In the figures, unless otherwise indicated, the elements are not to scale. Description détaillée :

[0032] There figure 2 Figure 1 schematically illustrates a system according to a first preferred embodiment of the invention for accelerating electrons by laser-plasma interaction. It consists primarily of a laser system SL, for delivering an initial laser beam FI carrying a first pulse, called the pre-pulse IL1, and a second pulse, called the main pulse IL2, and an optical system SO for focusing these pulses IL1 and IL2 onto a solid-state target CS (more generally, a condensed-state target; the use of a liquid target is also possible). The FI beam is focused by the optical system SO onto a surface SS of the target CS so as to form a focused laser beam FF. In the invention, the surface SS of the target CS is coated with a layer of gas CG.

[0033] The SL laser system and the SO optical system are adapted so that the intensity of the pre-pulse IL1 is sufficient to generate, in step A, a dense plasma MP at the surface SS of the target by ionizing the target CS. Preferably, the intensity of the pre-pulse IL1 is greater than 1015 W / cm 2< on the SS surface in order to ionize the CS target and thus generate the dense MP plasma. The intensity in W / cm 2< mentioned here and in the rest of the document corresponds here to the peak intensity of the IL1 or IL2 impulse at the surface where the IL1 or IL2 impulse is focused.

[0034] By "dense plasma," we mean here that the electron density of the plasma is such that the plasma frequency ω p = n e e 2 m e ε 0 ( ε (where 0 is the permittivity of free space) is greater than the frequency of the laser pulse intended to be reflected by the dense plasma MP (i.e., the main pulse IL2). This condition allows the reflection of the main pulse IL2 onto the dense plasma MP in a step subsequent to step A. Otherwise, the laser pulse would propagate through the plasma instead of being reflected by it.

[0035] Furthermore, the SL laser system and the SO optical system are adapted so that the intensity of the main pulse IL2 is sufficient to: In step B, after reflection by the dense plasma MP, generate a laser wake WF in the gas layer. The laser beam reflected by the dense plasma MP is denoted by the reference FR in the figure 2 This trail is similar to the one detailed above in the description of the figure 1B In step C, the electrons in the dense plasma are heated to an energy such that a packet EB of electrons is injected into the wake WF to be accelerated. Thus, the reflection of the beam FF on the dense plasma MP generates an electron beam FE. The minimum energy required for injection is typically 100 keV.

[0036] In the first embodiment of the invention, it is necessary that the main pulse IL2 be focused towards the target CS at oblique incidence and with s polarization for reasons that will be explained later. Furthermore, as explained above, the plasma frequency ω p The frequency of the plasma generated by the pre-pulse L1 must be greater than the frequency of the main pulse IL2, denoted ω2, in order to allow the reflection of the main pulse IL2 by the dense plasma MP. Finally, it is necessary that the main pulse IL2, upon reflection by the dense plasma, be spatially superimposed on the pre-pulse IL1 to optimize the ejection of electrons from the dense plasma MP. This is denoted Δ t the time delay of the main pulse IL2 when focused on the dense plasma relative to the pre-pulse IL1 when it generates the dense plasma.

[0037] In prior art VLA-type devices (e.g., Thévenet, et al .), the injection is caused by the laser E field present in the normal direction nat the surface of the plasma mirror, which "pulls" electrons from the mirror and injects them into the laser E field in a region where the plasma electrostatic field does not allow acceleration. This generates a low-quality FE electron beam (high divergence and strong energy dispersion).

[0038] In the invention, the injection mechanism is different. Crucially, the main laser pulse IL2, intended to be reflected by the dense plasma (and to heat its electrons), is directed at oblique incidence and with s-polarization onto the target CS. Consequently, there is no electric field E along the normal direction. nat the surface of the dense plasma MP. In the invention, the injection is triggered in two stages: first, by heating the electrons of the dense plasma via the laser pulse as it reflects off the dense plasma. Then, some of these electrons, heated to sufficient energy, are trapped in the electrostatic field of the laser wake bubble with a phase adapted to be accelerated over lengths of several millimeters to a few centimeters.

[0039] Thanks to the high electron density of the dense MP plasma, this mechanism allows the injection of a high electron charge of a few MeV to tens of MeV with a modest laser energy (typically greater than 10 pC with a few hundred millijoules), and preferably greater than 100 pC, or even 1 nC, for a high laser energy of several joules to tens of joules. Critically, the electron bunches EB are injected with an appropriate phase into the laser wake bubble, which allows the production of a high-quality FE electron beam (low divergence and low energy dispersion) with energies of several hundred MeV to a few GeV at the end of the wake acceleration.

[0040] Thus, it is possible to obtain an FE electron beam with ultrashort bunches and low divergence, a large charge for high energy, and low energy dispersion. Compared to prior art LWFA-type devices (see figure 1B ), the invention therefore makes it possible to considerably increase the charge of the electron packet while maintaining optimal beam quality. Indeed, the electron density of the dense plasma is much greater than that of the ionized gas. Compared to VLA-type devices (see figure 1A ), the invention therefore makes it possible to significantly improve the quality of the electron beam (divergence and dispersion of energy) because the electrons are injected at the rear of the wake bubble in the invention.

[0041] There figure 3 groups three instantaneous representations a, b and c of a PIC code simulation (method Particle In Cell (in English, known in the art), each of the representations being obtained at different times during the process of the invention according to the second embodiment of the invention. The three instantaneous representations a, b and c of the figure 3 These diagrams illustrate the propagation of the main pulse IL2 at different times. The electron density DE of the plasma within the dense gas layers CG MP is shown in light gray, while the laser field of the IL2 pulse is shown in darker gray. Diagram a) illustrates the laser field of the IL2 pulse before its reflection from the dense plasma MP, which has already ionized the gas along its path. Diagram b) illustrates the laser field of the IL2 pulse during its reflection from the dense plasma MP. Finally, diagram c) illustrates the laser field of the IL2 pulse after its reflection from the dense plasma MP. In diagram c), the laser wake mechanism accelerating an electron bunch EB is also observed; the injection of this bunch into the laser wake was made possible by the heating of the electrons in the dense plasma MP by the IL2 pulse.

[0042] For steps B and C, it is preferable that the main pulse IL2 causing the heating of the electrons and generating the wake has an intensity greater than 1018 W / cm 2< during its reflection by the dense MP plasma. This allows for a laser field sufficient to enable the wake mechanism and to accelerate electrons to energies of several hundred MeV for a charge typically greater than 10 pC, preferably greater than 100 pC. In general, to achieve such intensities, it is necessary for the SO optical system to strongly focus the FF beam, typically to focal spots of a few µm diameter.

[0043] According to a second embodiment of the invention, different from that illustrated in the figure 2 The SL laser system is adapted to deliver a single laser pulse. In this second variant, the SL laser system and the SO optical system are adapted so that the IL0 pulse has an intensity such that the rising time edge of the pulse ionizes the CS target and generates the dense MP plasma, and so that the IL0 pulse generates the wake in the gas layer and induces the heating of the electrons in the dense plasma, causing injection. As mentioned above, it is preferable for the IL0 pulse to have an intensity greater than 1018. W / cm 2< During its reflection by the dense plasma, electrons are accelerated to energies of several hundred MeV. In this second variant, the IL0 pulse must be focused towards the target at oblique incidence and with s polarization to allow injection solely through the heating of plasma electrons.

[0044] This second variant is not the preferred variant of the invention because it does not allow for the separate optimization of the different injection and acceleration parameters. However, it has the advantage of being very simple to implement because it requires no alignment between multiple beams and no fine-tuning of timing between different pulses.

[0045] The first variant of the invention is more complex to implement than the second variant of the invention because it requires the alignment of two pulses IL1, IL2 and the fine control of the time delay. Δt between these pulses (see below). However, it is preferred over the second variant because it allows for better control of the injection and acceleration parameters, for example via the time delay Δ t between the pre-pulse and the main pulse, or the temporal contrast of the main pulse.

[0046] Indeed, preferably, in the first variant of the invention, the SL laser system is adapted so that the temporal contrast of the main pulse is greater than 10⁸, preferably greater than 10¹⁰, during the generation of the dense plasma MP. The choice of such a temporal contrast is preferred for a focused laser beam intensity FF greater than 10¹⁸. W / cm 2< on the target, so that the pedestal of the main pulse's time profile does not have sufficient intensity to ionize the solid target between a few picoseconds and a few nanoseconds before the main pulse's intensity peak. This prevents the dense plasma from extending into the vacuum along an exponential density profile over a scale length L greater than one wavelength λ 2 of the main pulse, before the reflection of the intensity peak of the main pulse IL2 on the dense plasma MP. If we hadL > λ 2 before the reflection of the peak intensity of the main pulse IL2 on the dense plasma MP, the spatiotemporal properties of the beam reflected FR by the dense plasma MP would be too strongly degraded to allow the acceleration of electrons by wake mechanism.

[0047] Similarly, and even more preferably, the SL laser system is adapted so that the temporal contrast of the pre-pulse is also greater than 10⁸, preferably greater than 10¹⁰, during the generation of the dense MP plasma. Here too, the choice of such a temporal contrast ensures that the foot of the pre-pulse temporal profile does not have sufficient intensity to ionize the solid target between a few picoseconds and a few nanoseconds before the main pulse. This prevents the dense plasma from extending into the vacuum along an exponential density profile over a scale length L greater than one wavelength. λ 2 of the main pulse, before the reflection of the main pulse IL2 on the dense plasma MP.

[0048] Preferably, the method of the first variant includes a step prior to steps A and B, increasing the temporal contrast of the main pulse (and possibly the pre-pulse) by reflection on one or more additional plasma mirrors to ensure that the temporal contrast of the main pulse (and the pre-pulse, if applicable) is greater than 10⁸, preferably greater than 10¹⁰. Increasing the temporal contrast of a laser pulse using a plasma mirror is a method well known to those skilled in the art (see, for example, Lévy, Anna, et al. "Double plasma mirror for ultrahigh temporal contrast ultraintense laser pulses." Optics letters 32.3 (2007): 310-312).

[0049] Preferably, according to a preferred embodiment of the first variant of the invention, denoted MD, the laser system SL and the optical system SO are adapted so that the time delay Δ t be sufficiently small so that the scale length L of the dense plasma gradient is less than one wavelength λ 2 of the main impulse IL2. As explained above, this ensures that the spatiotemporal properties of the beam reflected FR by the dense plasma MP are good enough to allow the acceleration of electrons.

[0050] More generally, the control of the time delay Δ t allows precise control of the scale length L of the dense plasma gradient MP during the reflection of the IL2 pulse on the latter. Indeed, after the generation of the dense plasma by IL1 and during the delay Δ t Before the arrival of IL2, the plasma expands towards the vacuum (according to the normaln to the dense plasma MP) at a speed ranging from a few nm / ps to a few 100 nm / ps. Now, the higher the delay, the greater the scale length L of the gradient of the dense plasma MP, and the easier it is to extract electrons from the dense plasma MP during the reflection of the IL2 pulse on it in order to accelerate them. Therefore, preferably, the method of the invention includes a step for adjusting the time delay Δ t in order to optimize the charge of the electron packet injected into the wake. This time delay Δ t can for example be controlled via a delay line on the optical path of the main pulse IL2.

[0051] Preferably, in embodiment MD, the time delay Δt is between 50 fs and 200 ps, preferably 100 fs and 50 ps. Indeed, through simulations and experiments, the inventors realized that this range is optimal for injecting and accelerating a high payload into the laser wake. This time delay range Δ t between 50 fs and 200 ps corresponds (depending on the energy of the pre-impulse IL1) to a scale length L between ∼ λ 2 / 40 and ~ λ 2 / 5 respectively. By choosing a time delay Δt between 50 fs and 200 ps, The inventors observed through simulation that it is possible to obtain an injection of a highly localized electron packet EB into the wake WF with a charge up to nC with a main pulse of 10J, of 30fs, when the ladder length L is λ 2 / 10 and that the gas density is 1018 cm-3. As an illustrative example, the figure 4A This is a graphical representation of the spectrum of the FE electron beam generated by the system of the invention. The x-axis represents the energy of the electron bunch and the y-axis represents the charge of the electrons. This spectrum is obtained by simulation after an acceleration length of 1.5 mm for a laser source SL delivering IL2 main pulses of 25 fs and with an energy of 2 J. It can be observed that system 1 of the invention makes it possible to obtain an electron bunch with a charge of 400 pC with a divergence of the order of mrad and for an energy of 185 MeV and for a low energy dispersion (approximately 5%)

[0052] The inventors conducted experiments with primary laser pulses exhibiting an energy of 450 mJ with a duration of 30 fs focused on the dense plasma with an intensity of 3 x 10¹⁸ W / cm². figure 4B is a graphical representation of 5 EB packet spectra generated by the system of the invention for 5 different IL2 pulses under the aforementioned conditions. figure 4B This clearly illustrates that EB packets are stable for an energy of 180 MeV with an energy dispersion of less than 10% and for a charge of 17 pC per packet.

[0053] Preferably, the method implemented in the first embodiment of the invention includes a step, prior to steps A and B, for generating the first and second laser pulses from a single initial laser pulse. For this purpose, system 1 comprises an optical element (typically either a 95% / 5% semi-reflective plate or a sub-mirror smaller than the laser beam to select only a sub-portion) that splits the initial pulse into two pulses: the pre-pulse IL1 and the main pulse IL2. The main pulse IL2 is then focused by the optical system SO with a time delay Δt, typically controlled by a delay line along the optical path of the main pulse IL2. This embodiment has the advantage of allowing the use of a single laser in system 1 of the invention.Alternatively, according to another embodiment, the IL1 and IL2 pulses are generated from two different laser sources. Furthermore, in the first variant of the invention, it is not necessary for the IL1 and IL2 pulses to have the same wavelength, although this may be preferable in order to use the same optical components (mirrors, parabolic reflectors, or lenses) of the SO optical system to focus these pulses.

[0054] Preferably, the optical path of the laser beam is carried out under high vacuum (pressure less than 10⁻³ < mbar ), preferably under ultra-high vacuum (pressure less than 10⁻⁶ mbar ), a minima from the point where it is focused by the SO optical system to avoid any non-linear effects during the propagation of the laser beam. As is known, system 1 comprises one or more primary pumps and one or more turbomolecular pumps to obtain this high or primary vacuum.

[0055] In the first variant of the invention, it is preferable that the angle of incidence of the pre-pulse IL1 on the target CS and of the main pulse IL2 on the dense plasma MP does not exceed 75° to avoid their respective focal spot being too spread out on the surface of the solid target CS thus reducing the peak amplitude of the electric field induced by these pulses.

[0056] There figure 5 This illustrates a first embodiment of the invention in which system 1 comprises a gas nozzle GN connected to a gas reservoir (not shown). The gas nozzle is adapted to deliver a jet of gas GJ configured to form the gas layer CG. This first embodiment has the advantage of being simple to implement.

[0057] There figure 6 illustrates a particular embodiment of the embodiment of the figure 5 in which the system further comprises a gas cell CG sealed or partially sealed by the target CS. The walls of the gas cell are impermeable to the gas delivered into the gas cell by the gas nozzle GN and include two openings: one to allow the incident laser beam to pass through, and the other to allow the reflected beam and accelerated electrons to exit. Thus, the target forms a wall that makes the gas cell partially airtight, which significantly reduces gas leakage in system 1. This, among other things, reduces the load on the primary pumps and the turbomolecular pumps used to produce the high vacuum or ultra-high vacuum. Furthermore, it limits the propagation of the focused laser beam FF in the gas before its reflection by the dense plasma MP, thereby limiting the nonlinear effects that can occur in this very high-intensity region.

[0058] There figure 7 illustrates a particular embodiment of the embodiment of the figure 6 in which the target CS is formed by a portion of ribbon that unwinds from a reel BR, so as to be renewed from one shot to the next. The cell is sealed or partially sealed by the ribbon reel BR. In this embodiment, system 1 includes a motor assembly MT adapted to unwind the ribbon in the cell, for example by translation along a direction u This embodiment allows for easy target renewal without having to modify the alignment of the setup, for example, when the FF beam has excessively degraded the SS surface of the CS target where the FF beam was focused. This therefore allows for a higher electron beam repetition rate. According to a different embodiment than that illustrated in figure 7 System 1 does not include a CG gas cell, although this results in a larger gas leak.

[0059] Preferably, in the system of embodiments of figures 5 à 7 The gas reservoir contains helium and / or hydrogen and / or nitrogen. Using a low atomic mass gas, such as those mentioned above, ensures that the electrons injected in the invention originate solely from the dense plasma and not from the gas layer. Using gases with a higher atomic mass would result in greater energy dispersion in the electron beam because electrons would also be injected from the gas.

[0060] Preferably, in the invention, the gas nozzle is adapted so that the average pressure in the CG gas layer is between 0.1 atm and 200 atm, preferably between 0.5 atm and 50 atm. This pressure is adjusted according to the desired electron beam energy. Indeed, a relatively lower pressure allows for a relatively higher energy and a higher charge because, in this case, the bubble in the wake of the laser pulse is larger, thus trapping a greater number of electrons.

[0061] Indeed, the scaling laws of the LWFA mechanism show that to achieve higher energies, it is necessary to reduce the gas density. Reducing the gas density increases the phase velocity of the wake wave (which depends on the gas density) and therefore increases the distance at which the accelerated electrons 'exceed' the laser wake bubble and are slowed down. This distance is called the "dephasing length." However, lowering the gas density requires guiding the laser pulse over greater distances. This can be achieved by 'self-guiding' using nonlinear effects in the gas (but this effect requires higher laser power as the gas density decreases). It can also be achieved by modifying the gas profile (so that it acts like a lens) using capillaries or other purely optical techniques.Finally, it turns out that lowering the gas density also increases the size of the bubble and the charge that can theoretically be injected into it without affecting it.

Claims

1. Method for accelerating electrons using laser-plasma interaction, wherein at least one laser pulse (ILO) is directed onto a surface (SS) of a target in the condensed state (CS), said surface being covered with a layer of gas (CG), the intensity of said at least one pulse being enough to: - in a step A, generate, from the target in the condensed state, a dense plasma (MP); - in a step B, after being reflected by the dense plasma, generate a laser wake (WF) in the gas layer; - in a step C, heat the electrons of said dense plasma to an energy such that a packet (EB) of said electrons is injected into the wake so as to be accelerated therein, said at least one pulse that is intended to be reflected by the dense plasma and heat the electrons thereof, being at an s-polarized oblique incidence relative to said target.

2. Method according to the preceding claim, wherein: - the generating of the dense plasma in step A is induced by one of said at least one laser pulse referred to as pre-pulse (IL1), - the steps B and C are induced by one of said at least one laser pulse referred to as main pulse (IL2), - in the step B, said main pulse is spatially superimposed on the pre-pulse and has a time delay (Δt) with respect to the pre-pulse.

3. Method according to the preceding claim, wherein said time delay (Δt) is low enough so that one scale length of the gradient (L) of the dense plasma is less than one wavelength (λ2) of the main pulse.

4. Method according to the preceding claim, wherein said time delay (Δt) is comprised between 50 fs and 200 ps.

5. Method according to any of claims 2 to 4, comprising a step of adjusting said time delay (Δt) in such a way as to optimize a charge of said electron packet injected into said wake.

6. Method according to any of claims 2 to 5, wherein, in the step A, the intensity of the pre-pulse is greater than 1015 W / cm2 on said surface and wherein, in the step B, the intensity of the main pulse is greater than 1018 W / cm2 after being reflected by the dense plasma.

7. Method according to any of claims 2 to 6, comprising a step prior to steps A and B of generating the pre-pulse and the main pulse from the same so-called initial laser pulse.

8. Method according to any of claims 2 to 7, comprising a step prior to steps A and B, of increasing the time contrast of the main pulse by reflecting on one or more additional plasma mirrors.

9. Method according to any of the preceding claims, wherein an average pressure in the layer of gas is comprised between 0.1 atm and 200 atm, preferably comprised between 0.5 atm and 50 atm10. System for accelerating electrons using laser-plasma interaction (1) comprising: - a target in the condensed state (CS) covered with a layer of gas (CG), - a laser system (SL) adapted to generate at least one laser pulse (IL0) - an optical system (SO) adapted to direct said at least one laser pulse on a surface (SS) of the target in the condensed state - the laser system and the optical system being further configured in such a way that the intensity of said at least one pulse is enough to: - generate, from the target in the condensed state, a dense plasma (MP); - after being reflected by the dense plasma, generate a wake (WF) in the layer of gas; - heat the electrons of said dense plasma to an energy such that a packet (EB) of said electrons is injected into said wake so as to be accelerated therein, - the optical system being further adapted so that said at least one pulse intended to be reflected by the dense plasma and heat the electrons thereof, being at an s-polarized oblique incidence relative to the target.

11. System according to the preceding claim, wherein the laser system and the optical system are configured to: - Generate a so-called first pulse, referred to as pre-pulse (IL1), and to direct it to the target to generate said dense plasma, - Generate a second so-called pulse (IL2), referred to as main pulse, and direct it to the target to generate said wake in the layer of gas and induce said heating of the electrons of said dense plasma, said main pulse, when it is reflected by the dense plasma, being spatially superimposed on the pre-pulse.

12. System according to the preceding claim, wherein said laser system is adapted so that the time contrast of the main pulse is greater than 108, preferably greater than 1010.

13. System according to any of claims 10 to 11, comprising a gas nozzle (GN) connected to a gas reservoir, the gas nozzle being adapted to deliver a stream of gas (GJ) configured to form the layer of gas,14. System according to the preceding claim, comprising a gas cell sealed or partially sealed by the target, the gas nozzle being adapted to deliver the steam of gas in the gas cell.

15. System according to any of claims 12 to 14, wherein said target is formed by a portion of band that is unwound from a coil (BR), said system comprising a motor unit (MT) adapted to unwind said band in the cell.

16. System according to any of claims 12 to 15, wherein the gas reservoir comprises helium and / or hydrogen and / or nitrogen.

17. System according to any of claims 12 to 16, wherein the gas nozzle is adapted so that an average pressure in the layer of gas is comprised between 0.1 atm and 200 atm, preferably comprised between 0.5 atm and 50 atm.