System and method for treating material by laser shock while enclosed in a liquid

Description

DOMAINE DE L'INVENTION

[0001] The present invention relates to the field of treatment of materials by laser shock, based on the generation of a plasma confined to the surface of the target to be treated, and which generates a shock wave in the material, and more particularly a system and a method for treating a target by laser shock in a confinement regime in a liquid as defined in the preamble of claims 1 and 6 (see US 2019 / 010576 A1 for example). ETAT DE LA TECHNIQUE

[0002] Laser shock is a laser process that allows energy to be rapidly delivered to a target (typically metallic or composite material) in order to create a very high-pressure plasma. This process generates a very intense shock wave (pressures in the order of GPa), allowing for various applications.

[0003] An example of a system 5 implementing the laser shock treatment known from the state of the art is illustrated. figure 1 . It comprises a pulsed laser L generating a beam B in the form of LP pulses and an optical COD concentration device of focal length f configured to concentrate the beam B on the surface of the Tar target to be treated. Conventionally the target is not placed in the focal plane of the COD device, because for the aforementioned applications we are looking for a beam diameter on the surface of the target, at the interface with the confinement medium, of the order of mm (typically between 0.3 mm and 10 mm).

[0004] The laser creates a very high pressure PLconf plasma by laser ablation. A confinement medium is placed on the laser ablated surface. The most common and industrially practical confinement is a thin CL layer of a laser-transparent medium (water, other laser-transparent liquid, quartz, polymer tape, etc.), typically 1 to a few mm thick. The confined regime makes it possible to considerably increase the plasma pressure and its application time on the target. Optionally, a HPC thermo-protective coating is deposited on the target to be treated. This system generates a very intense OC shock wave, with pressures in the order of GPa, allowing for various applications.

[0005] Laser / material interaction and laser shock treatment are for example described in the publications: Sollier et al: “Laser-matter interaction in laser shock processing”, First international symposium on High power laser Macroprocessing, SPIE n° 4831, pages 463- 467 (2003), JT Wang et al: “Effects of laser shock peening on stress corrosion behavior of 7075 aluminum alloy laser welded joints”, Material Science & Engineering A647 pages 7-14 (2015).

[0006] To generate the plasma and therefore the shock wave in good conditions for carrying out the treatment, it is advisable to use a laser with a pulse duration τ typically between 1 ns and 30 ns and energy E between 0.5 and 10 J, focused on the target with a size between 0.3 and 10 mm, these different parameters being chosen according to the intended application.

[0007] The main applications are: adhesion tests and shock disassembly (LASAT - LAser Shock Adhesion Test); two shock waves are generated by two time-shifted laser pulses and meet at the junction of the assembly to be tested or disassembled, a high tensile stress is required (reference publication: Berthe et al: “State-of-the-art laser adhesion test (LASAT)”, Nondestructive Testing and Evaluation, Vol. 26, Nos. 3-4, pages 303-317 (2011)), surface strengthening by laser peening (LSP - Laser Shock Peening); the high pressure applied by the plasma, and transmitted to the target via the shock wave, allows the target to be plasticized and its properties to be improved (resistance, lifetime, etc.) (reference publication: Montross et al: “Laser shock processing and its effects on microstructure and properties of metal alloys: a review”, International Journal of Fatigue 24, pages 1021-1036 (2002)), the characterization of materials under high pressures.

[0008] These applications mainly concern scientific research and various fields of industrial activity such as aeronautics, nuclear power and naval energy.

[0009] The important parameter for these systems is the power density or intensity I irradiating the target expressed in GW / cm 2< , since the pressure generated is proportional to the square root of the laser intensity (see for example the publication of Fabbro et al: “Physical study of laser produced plasma in confined geometry”, Journal of Applied Physics, 68(2), pages 775-784 (1990)). P ∝ I with the laser intensity I (GW / cm 2< ) defined according to the formula: I = E τ S where E is the laser energy per pulse (J), τ the laser pulse duration (ns) and S the surface area irradiated by the laser (cm 2< )

[0010] However, it is not possible to indefinitely increase the pressure generated by increasing the laser intensity irradiating the target because the laser intensity transmitted to the target saturates by a breakdown mechanism appearing at the surface of the confinement. Indeed, the laser / matter interaction regime in the laser shock involves two different types of plasmas: the confined plasma PLconf which develops at the surface of the target, i.e. the target-confinement liquid interface illustrated figure 1 , and a PLbk / s breakdown plasma appearing at the liquid surface, shown figure 2 and due to a phenomenon of ionization of the confinement medium.

[0011] This breakdown plasma is for example studied in the publication by Sollier et al “Numerical modeling of the transmission of breakdown plasma generated in water during laser shock processing”, Eur. Phys. AP, vol 16, pages 131-139 (2001).

[0012] The resulting PLbk / s breakdown plasma is opaque to laser radiation and thus absorbs the remaining energy contained in the laser pulse. As a result of this breakdown phenomenon, the maximum intensity irradiating a target in confined laser shock is limited by the breakdown threshold intensity in the confinement medium. This phenomenon is visible on the figure 5 of the aforementioned publication: beyond a threshold incident intensity at the surface of the confinement medium (here water), which we will call Ibk, of approximately 8 GW / cm 2< (with for example: λ=1064 nm and τ = 25 ns) the intensity of the pulse transmitted by the confinement medium saturates while the incident intensity increases.

[0013] Therefore, the maximum pressure that can be generated by laser shock is also limited. As the thickness of the water layer is small compared to the focal length used (1 to 3 mm thick versus a focal length of 300 to 500 mm for example), we can consider that the intensity at the surface of the target Ist is substantially equal to the intensity at the surface of the water Isl, and therefore that the maximum intensity that can be applied to the surface of the target is also 8 GW / cm 2< . With this applied intensity of 8 GW / cm 2< the maximum pressure obtained by laser shock in the pulse duration range 5-15 ns is approximately 8 GPa.

[0014] To be able to disassemble certain strong or thick assemblies, the pressures obtained today by laser shock are too low. Similarly, a pressure greater than 2.5 times the elastic limit is generally required to optimally reinforce the target by laser peening, and current pressures therefore do not allow the treatment of all materials, especially the strongest ones.

[0015] Thus, being able to increase the maximum pressure value would make it possible to meet existing needs that are currently unmet.

[0016] Today, there are several possible solutions to increase the pressure on the target: Reduction of laser pulse duration.

[0017] It is demonstrated in the literature (experimentally and theoretically) that in a given confinement medium, the breakdown threshold laser intensity Ipk is inversely proportional to the square root of the laser pulse duration ( I pk ∝ 1 / τ ). Thus, to increase the maximum laser intensity irradiating the target (and therefore the maximum pressure created) one solution is to reduce the pulse duration of the laser system used since the breakdown threshold intensity will increase. However, although this solution makes it possible to increase the maximum pressure generated by the plasma, it poses problems of viability and usefulness: in fact, the duration of the shock wave depends on the duration of the laser pulse (approximately 2 times its value), and the damping of a shock wave increases the shorter its duration. The wave thus generated will damp more quickly in the treated target (because the laser pulse duration has been reduced) and the "useful" pressure (at the heart of the material, and not on the surface) will therefore not be increased, or will even have decreased. Use of the so-called direct regime:

[0018] A second solution consists of dispensing with the confined regime, and using the direct irradiation regime: there is therefore no longer any confinement around the target to increase its pressure. The direct regime, however, makes it possible to obtain pressures similar to the confined regime by using laser intensities 10 to 100 times higher. However, a high vacuum must be created around the target to avoid any breakdown phenomenon in the air, which is very likely at these intensity levels. This second solution is difficult to apply industrially because it requires, on the one hand, creating a vacuum around the part to be treated, and on the other hand, requires very energetic laser systems, which are therefore expensive and bulky.

[0019] Use of a magnetic or electric field (Patent No. CN201210571521 and No. US10745776) at the time of plasma creation, to provide (by transfer) additional energy to the plasma. But these solutions have not produced convincing results and do not significantly increase the pressure levels generated or are too complicated to implement and calibrate.

[0020] Document WO2015 / 062457 describes laser shock treatment in a liquid medium.

[0021] An aim of the present invention is to overcome the aforementioned drawbacks by proposing a system making it possible to increase the maximum intensity at the surface of the target, and therefore to increase the pressure transmitted to the target by laser shock. Furthermore, the system according to the invention is economical because it does not require modifying the lasers used in existing laser shock systems. DESCRIPTION DE L'INVENTION

[0022] The present invention relates to a system for treating a target by laser shock in confinement mode in a liquid defined in claim 1, the system comprising: a pulsed laser generating a beam having a pulse duration τ of between 1 ns and 30 ns and a wavelength λ, an optical concentration device having a focal length f and configured to concentrate the beam on the surface of the target, the laser beam incident on the concentration device having a diameter D, a tank filled with said liquid having a refractive index n, a desired value of the beam diameter on a surface of the target being predetermined and called Dst, a thickness e of liquid crossed by the beam before reaching the surface of the target being chosen so that a laser intensity at the surface of the liquid is less than or equal to a laser intensity at the surface of the target divided by 2.

[0023] According to the invention, the thickness e is chosen to be greater than or equal to a minimum thickness e min defined by: e min = D st 2 − 1 2 tan arcsin arctan D 2 f n

[0024] According to one embodiment, the system according to the invention further comprises an element configured to homogenize the beam and arranged on the optical path of said beam.

[0025] According to one embodiment, an energy E of the laser and the optical concentration device are configured so that the laser intensity at the surface of the target is between 0.1 GW / cm 2< and 25 GW / cm 2< and said predetermined value Dst is between 0.3 to 10 mm.

[0026] According to one embodiment, the liquid has an absorption coefficient at said wavelength less than or equal to 0.1 / m 2 < .

[0027] According to one embodiment, the liquid is water and the wavelength λ of the laser is in the range [350 nm; 600 nm].

[0028] According to another aspect the invention relates to a method of treating a target by laser shock in confinement regime in a liquid as defined in claim 6, the method comprising: having a tank filled with said liquid and containing the target, generating a beam having a pulse duration τ of between 1 ns and 30 ns with a pulsed laser, concentrating the beam on the surface of the immersed target with an optical concentration device of focal length f, the incident beam on the optical concentration device having a diameter D, positioning the target in the tank then illuminating the surface with the beam, so that the beam passes through a thickness e of liquid at least equal to a minimum thickness e min before reaching the surface of the target and so that the diameter of the beam on the surface of the target is equal to a predetermined value Dst, the minimum liquid thickness e min being defined by: e min = D st 2 − 1 2 tan arcsin arctan D 2 f n a laser intensity at the surface of the liquid then being strictly lower than a laser intensity at the surface of the target divided by 2.

[0029] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered.

[0030] The invention will be better understood and other characteristics, aims and advantages thereof will appear during the detailed description which follows and with reference to the appended drawings given as non-limiting examples and in which: There figure 1 already cited illustrates a state-of-the-art laser shock characterization system. The figure 2 already cited illustrates the two plasmas involved in the laser shock mechanism. The figure 3 illustrates the measurement protocol used to demonstrate the existence of a volume breakdown mechanism. The figure 4 illustrates the evolution of the transmission as a function of the maximum intensity Imax reached in the confinement environment, for both cases (cross points 2 mm water layer, circle points 15 cm water layer). The figure 5 illustrates a system for treating a target by laser shock according to the invention. The figure 6 illustrates the pressure applied via the confinement plasma as a function of the maximum intensity reached in the confinement medium, for the two previous cases (cross points 2 mm water layer, circle points 15 cm water layer). DESCRIPTION DETAILLEE DE L'INVENTION

[0031] The invention is based on a study by the inventors on the breakdown mechanism in the laser shock system.

[0032] First of all, the inventors' work has made it possible to demonstrate for the first time that there are in fact two breakdown modes: breakdown via the surface of the confinement medium, known from the state of the art, and breakdown in the volume of the confinement medium. This volume breakdown has not been studied in laser shock architectures because the confinement layers used are always very thin, at thicknesses for which this breakdown does not appear. This volume breakdown becomes visible when the thickness of the confinement medium crossed is increased.

[0033] The inventors experimentally demonstrated the existence of this volume breakdown mechanism using the schematic measurement protocol figure 3 Part A corresponds to the situation according to the state of the art (low water thickness, here 2 mm) and part B to a measurement carried out with a greater water thickness (15 cm).

[0034] In both cases, a transmission T=Et / Ei is measured with Ei incident energy on the outer surface of the confinement medium and Et energy transmitted after crossing the liquid thickness. The energy Ei is known and the measurement of Et is carried out with a calorimeter CAL recovering the beam transmitted via a window W located at the bottom of the tank TK.

[0035] For this measurement we have a wavelength of 532 nm, a pulse duration τ of 7.2 ns, a beam with an initial diameter of 20 mm and a concentration device with a focal length of f = 80 mm. Here we use a BH element (typically a DOE for "Diffractive Optical Element") configured to homogenize the beam and placed on the optical path of the beam.

[0036] There figure 4 illustrates the evolution of the transmission T as a function of the maximum intensity Imax reached in the confinement medium, obtained for a given intensity at the surface of the liquid Isl, which is varied (via the variation of the laser energy). The intensity Isl is easily deduced from Ei with formula (1) and knowledge of the diameter of the beam at the surface.

[0037] In case A, as the liquid thickness is thin, the intensity is substantially identical everywhere, on the surface of the liquid or in the depth of the tank. We therefore consider the intensity Imax as the incident intensity on the surface of the liquid Isl: Imax ≅ Isl. This intensity Imax will also be equal to that on the surface of the target in contact with the confinement medium, called Ist, when there is a target.

[0038] For case B, the COD system combined with the BH element is configured to concentrate the light according to a known minimum diameter of 1.5 mm inside the volume of the liquid. Knowing the incident energy and the minimum diameter, the associated intensity Imax is deduced.

[0039] Thus the abscissa Imax of the figure 4 corresponds to the maximum intensity obtained in the liquid (either on the surface or in volume).

[0040] This intensity Imax obtained in the liquid is therefore potentially the maximum intensity that can be obtained on the surface of the target for the generation of the shock wave.

[0041] On the figure 4 the cross points correspond to the values ​​obtained with the measurement along A and the circle points to the values ​​obtained with the measurement along B.

[0042] With the crosses we find the phenomenon of surface breakdown known from the state of the art, with an experimental threshold, which we will call Ibk / s, around 8GW / cm 2< . This result is consistent with the results obtained by Arnaud Sollier (see aforementioned publications) describing the appearance of a breakdown plasma on the surface of the confinement medium for breakdown intensity thresholds between 4 and 10 GW / cm 2< depending on the laser parameters used.

[0043] The innovative point is the variation of T obtained with the circles, which reveals a new threshold, which we will call Ibk / v, corresponding to a breakdown in the volume of the liquid, originating at the place where the intensity is the highest. This threshold is approximately 20-21 GW / cm 2< , i.e. higher than Ibk / s.

[0044] Thus, with this measurement, the inventors demonstrated that there are two breakdown thresholds and not just one: a surface breakdown threshold that we will call Ibk / s and a volume breakdown threshold Ibk / v. In addition, these measurements made it possible to determine a value for these two thresholds, for the same laser pulse duration and the same wavelength, and to deduce that the volume threshold is higher than the surface threshold: Ibk / v > Ibk / s.

[0045] We define the ratio R = I bk / v I bk / s

[0046] For the torque (τ=7.2 ns; λ=532 nm) we have R ≅ 2.5

[0047] The existence of this ratio R combined with the fact that it is greater than 1 is an important result. It means that when using a greater thickness of confinement material, a greater Ist intensity can be obtained on the target before breakdown than when using a smaller thickness.

[0048] The highlighting of these two breakdown thresholds, one on the surface, predominant when the confinement layer is thin, and the other in volume, appearing when the confinement layer becomes thicker, as well as the experimental determination of the parameter R>1 linking the two thresholds, is a real discovery which had never been highlighted until then.

[0049] In other words, this work demonstrates for the first time that, at constant laser parameters, the breakdown threshold in the confinement medium is higher if the breakdown occurs in the volume of the confinement medium rather than at its surface (typically from 8-10 GW / cm 2< to 20-25 GW / cm 2< for 7.2 ns pulses with water confinement).

[0050] With a sufficient thickness e of the confinement medium crossed, a breakdown at the surface of the confinement medium is avoided (the laser is not yet focused on the surface, so the laser intensity is locally low there), this breakdown being deported in volume to the heart of the confinement medium, where the threshold intensity of breakdown is higher than at the surface. Thus, the maximum intensity that can irradiate the target (for the same set of laser parameters) is increased, so the maximum pressure generated is also increased. In the previous example with a breakdown in volume, a pressure of 12 GPa can be transmitted to the target (corresponding to the threshold intensity of 20-22 GW / cm 2< ), compared to 8 GPa (8-10 GW / cm 2< ) with conventional breakdown at the surface.

[0051] Several other experimental measurements and physical deductions show that the value of this ratio R depends on the confinement material considered and remains relatively stable over a pulse duration range [1-30ns]. For water, R is between 2.5 and 3. More generally, for laser conditions and confinement materials of interest in laser shock, the inventors have determined that R is typically between 2 and 4.

[0052] We deduce from the fact that R>1 that if there is a surface breakdown, the maximum intensity that will be reached will be I bk / s and the intensity on target is at most equal to I bk / s . Similarly, for a volume breakdown, the maximum target intensity is at most equal to I bk / v .To maximize the maximum intensity on the target, it is therefore necessary to put oneself in the conditions to have a volume breakdown, and this breakdown will occur at the target level by positioning it where the laser is most concentrated (highest intensity).

[0053] In other words, the laser shock system must be designed so that when the intensity on target (Ist) is equal to I bk / v , we have less than I bk / s on the surface of the confinement medium (Isl). Indeed, if this were not the case, this would mean, for example, that when we have I bk / v on target, there is already more than I bk / s on the surface... so there is already a breakdown on the surface, so there cannot actually be I bk / v on target (absurd).

[0054] So this means that we must have I sl ≤ I st R :

[0055] Fulfilling condition (2) ensures that when there is surface breakdown ( I sl = I bk / s ) then the intensity on target is at least equal to I bk / v , and so in fact there is a breakdown in volume before the breakdown on the surface: we have maximized the possible intensity on target (as the threshold is higher in volume than on the surface, we must put ourselves in conditions so that the breakdown occurs first in volume, which is never the case with a water thickness of 1 mm).

[0056] A minimum value of R, Rmin =2, has been experimentally determined.

[0057] So the laser shock system checks: I sl ≤ I st 2

[0058] These intensities, at the surface of the liquid and in the liquid at the surface of the target, can be measured for example with a joulemeter or a photodiode.

[0059] Given what is explained above, we will always have a volume breakdown for lasers and confinement environments of interest.

[0060] Compliance with this condition (3) is a result obtained by the inventors which makes it possible to achieve a design of the laser shock system according to the invention, which favors volume breakdown. The laser shock system according to the invention exploits the experimental demonstration of the existence of a higher breakdown threshold in volume than on the surface of the confinement medium.

[0061] The invention relates to a system 10 for treating a Tar target by laser shock in confinement regime in a liquid Liq as illustrated figure 5 . The system comprises a pulsed laser L generating a beam B having a pulse duration τ between 1 ns and 30 ns and a wavelength λ and an optical concentration device COD having a focal length f and configured to concentrate the beam B at the surface St of the target. The laser beam incident on the concentration device COD has a diameter D. The system also comprises a tank TK filled with said liquid, the liquid having an index n.

[0062] In a laser shock system, the beam diameter Dst on the surface St of the target that is illuminated by the beam constitutes an input parameter, which depends on the application and the nature of the material being treated. In practice, this desired diameter Dst varies between 0.3 mm and 10 mm, preferably between 0.8 and 5 mm.

[0063] From the input parameters (D, f, n, Dst) the target is arranged in the tank so that the beam crosses a thickness e of liquid, before reaching the surface St of the target, chosen so that a laser intensity at the surface of the liquid (Isl) is less than or equal to a laser intensity at the surface of the target (Ist) divided by 2 (condition (3)).

[0064] The input parameters fixed, compliance with condition (3) makes it possible to determine a minimum thickness e min of liquid to be respected.

[0065] We have: tan θ = D 2 f 4 et tan θ r = x e voir figure 5

[0066] And with the laws of refraction: sin θ = n sin θ r n refractive index of the liquid.

[0067] Moreover : D st = D sl − 2 e tan θ r

[0068] Intensity I is defined by: I = E Sτ with E laser energy per pulse (J), τ pulse duration (ns), S irradiated surface (cm).

[0069] SO I ∝ 1 DE 2 , where DE is the diameter of the illuminated surface

[0070] From relation (2) we deduce: RD 2< st ≤ D 2< sl from where R D st ≤ D sl

[0071] From relation (6) we deduce R − 1 D sl ≤ 2 e tan θ r = > e ≥ D st R − 1 2 tan θ r

[0072] With relations (4) and (5) we can express tan(θr) as a function of the parameters D, f and n, which results in: e ≥ D st R − 1 2 tan arcsin arctan D 2 f n

[0073] By taking the minimum value of R, i.e. Rmin=2, we deduce a value of e min which is sufficient for all systems of interest: e min = D st 2 − 1 2 tan arcsin arctan D 2 f n

[0074] The laser system is, according to the invention, configured so that the thickness e of liquid crossed by the beam before reaching the surface of the target is chosen to be greater than or equal to e min . In this case, the laser intensity at the surface of the liquid Isl is less than or equal to a laser intensity at the surface of the target Ist divided by 2.

[0075] The thickness e min depends on the parameters D, f, n and Dst of system 10. As an example for D=20mm, f = 500 mm n=1.33 (water) and Dst = 4 mm on ae min = 55 mm.

[0076] In practice, the system 10 will be sized by taking, for example, 10-15 cm of water to cover all the interesting cases while ensuring compliance with condition (3).

[0077] Note that the above calculation is valid for a Gaussian laser beam from the moment we are in the far field (that is to say, far from the "waist", where the laser is a few microns in diameter, at the focus of the lens). In a laser shock system we are always in these conditions (laser spot Dst ≥300µm).

[0078] For small angles, formula (9) simplifies: e min = D st 2 − 1 2 ∗ 4 N 2 n 2 − 1 With N = f / D

[0079] The dimensioning of the laser shock system according to the invention links the numerical aperture of the system (ON=D / 2f) to the thickness of liquid used to shift the breakdown on the surface of the confinement towards the volume of the confinement.

[0080] We see in formula (9) the system / water thickness depends on the size of the spot Dst, that is to say that if 3 mm is needed to treat Titanium and 1 mm to treat Aluminum, we determine two different minimum tank thicknesses. In practice, the industrial designer of the laser shock system will use the same tank, which has a thickness greater than the largest e min , and his tank will be functional for both materials.

[0081] The laser system according to the invention can adapt to the numerical aperture (D / 2f) of the system used, according to the needs, by determining the minimum thickness of liquid to be used in order to shift the breakdown from the surface to the volume.

[0082] Note that the system 10 according to the invention is compatible with an architecture in which ae=f, which corresponds to a COD device immersed in the tank, as soon as e verifies the condition e ≥ e min.

[0083] At the other extreme, system 10 is compatible with a relatively thin water layer provided that a very open COD optic is considered, which guarantees that I sl ≤ I st 2 . This configuration, however, has the disadvantage of bringing the optics closer to the target, which is not desirable.

[0084] Generally speaking, the more open the COD optics are, the more the thickness of the confinement layer can be reduced. A water height around 10-15 cm induces a low D / 2f aperture, which is an advantage because a large aperture can damage the optics, as they will be close to the target (ejection of water and metal particles, by the plasma for example). In addition, with a water height typically of at least 10 cm, projections on the lens are non-existent. The D and f parameters of the system are typically a laser beam with a diameter on the COD between 15 and 30 mm and a focusing distance of approximately 20-30 cm.

[0085] Thus, a laser shock system 10 designed according to the invention makes it possible to avoid a breakdown at the surface of the confinement medium (the laser is not yet focused on the surface, so the laser intensity there is locally too low to initiate a breakdown). This breakdown is shifted in volume to the heart of the confinement medium, at a higher threshold intensity. Thus, the maximum intensity that can irradiate the target (for the same set of laser parameters) is increased, so the maximum pressure generated is also increased.

[0086] With a laser system according to the invention, the generated pressures are significantly increased (approximately +50%). This system is relatively simple to implement. It uses existing lasers and concentration devices and a tank filled with typically 10-15 cm of a containment liquid in which the target is immersed.

[0087] There figure 6illustrates the pressure P applied via the confinement plasma as a function of Imax for the two previous cases (cross points 2 mm water layer, circle points 15 cm water layer). Curve 60 is a numerical simulation based on a calculation made with a 1D laser-matter interaction code which does not take into account breakdown phenomena (we just calculate the resulting pressure of a given incident laser pulse, absorbed by the target).

[0088] For a thin layer (cross), it can be seen that beyond an intensity of about 10 GW / cm 2< (corresponding to Ibk / s), the generated pressure stagnates at about 8 GPa and then decreases. Surface breakdown limits the intensity illuminating the target. For a thicker layer (circle), the generated pressure follows the increase in Imax at least up to 22 GW / cm 2< and reaches a value of 12 GPa, an increase of more than 40% compared to the thin layer. In addition, the evolution follows the theoretical curve for a longer time without breakdown, which clearly shows that the problems related to breakdown are deferred to higher intensities.

[0089] According to one embodiment, the system 10 further comprises a BH element configured to homogenize the beam and arranged on the optical path of said beam. The presence of a beam homogenizer (typically a DOE) makes it possible to ensure that there are no overcurrents in the spatial profile of the laser, and therefore to avoid local breakdowns (which would lead to a local loss of intensity transmitted to the target).

[0090] In order to achieve a laser shock, the laser energy E (per pulse) and the optical concentration device are configured so that the laser intensity at the target surface Ist is between 0.1 GW / cm 2< and 25 GW / cm 2< and the Dst value is between 0.3 and 10 mm.

[0091] According to one embodiment, the pair (λ, liquid) is chosen so that the liquid Liq has an absorption coefficient α(λ) less than or equal to 0.1 / m 2< , that is to say, it uses a wavelength that is little absorbed by the confinement medium.

[0092] Preferably the liquid is water and the wavelength λ of the laser is in the range [350 nm; 600 nm]. The wavelength of 532 nm is preferred, compared to the wavelength of 1064 nm frequently used in laser shock with a thin layer of water.

[0093] According to another aspect, the invention relates to a method for treating a Tar target by laser shock in confinement mode in a liquid Liq comprising: having a tank filled with said liquid Liq and containing the target Tar, generating a beam B having a pulse duration τ of between 1 ns and 30 ns with a pulsed laser, concentrating the beam B on the surface of the immersed target with an optical concentration device COD of focal length f, the incident beam on the optical concentration device having a diameter D. positioning the target in the tank then illuminating the surface with the beam, so that the beam passes through a thickness e of liquid chosen so that a laser intensity at the surface of the liquid Isl is then strictly lower than a laser intensity at the surface of the target Ist divided by 2.

Claims

1. A system (10) for treating a target (Tar) by laser shock in a regime of confinement in a liquid (Liq), the system comprising: - a pulsed laser (L) generating a beam (B) having a pulse duration τ of between 1 ns and 30 ns and a wavelength λ, - a concentrating optical device (COD) having a focal length f and configured to concentrate the beam (B) on the surface (St) of the target, the incident laser beam on the concentrating device having a diameter D, the system being characterised by : - a tank (TK) filled with said liquid having a refractive index n, a desired value of the diameter of the beam on a surface (St) of the target being predetermined and named Dst, a thickness e of liquid passed through by the beam before reaching the surface of the target being chosen such that a laser intensity on the surface of the liquid (Isl) is less than or equal to a laser intensity on the surface of the target (Ist) divided by 2, the thickness e being chosen to be greater than or equal to a minimum thickness emin defined by: e min = D st 2 − 1 2 tan arcsin arctan D 2 f n n being the refractive index of said liquid.

2. The system according to one of the preceding claims, further comprising an element (BH) configured to homogenise the beam and disposed on the optical path of said beam.

3. The system according to one of the preceding claims, wherein an energy E of the laser and the concentrating optical device are configured such that the laser intensity on the surface of the target (Ist) is between 0.1 GW / cm2 and 25 GW / cm2 and said predetermined value Dst is between 0.3 and 10 mm.

4. The system according to one of the preceding claims, wherein the liquid has an absorption coefficient (α) at said wavelength λ of less than or equal to 0.1 / m2.

5. The system according to one of the preceding claims, wherein the liquid is water and the wavelength λ of the laser lies within the range [350 nm; 600 nm].

6. A method for treating a target (Tar) by laser shock in a regime of confinement in a liquid (Liq) comprising: - having a tank filled with said liquid and containing the target, - generating a beam (B) having a pulse duration τ of between 1 ns and 30 ns with a pulsed laser, - concentrating the beam (B) on the surface of the immersed target with a concentrating optical device (COD) of focal length f, the incident beam on the concentrating optical device having a diameter D, the method being characterised by: - positioning the target in the tank then illuminating the surface with the beam, such that the beam passes through a thickness e of liquid at least equal to a minimum thickness emin before reaching the surface of the target and such that the diameter of the beam on the surface of the target (St) is equal to a predetermined value Dst, the minimum thickness of liquid emin being defined by: e min = D st 2 − 1 2 tan arcsin arctan D 2 f n n being the refractive index of said liquid. a laser intensity on the surface of the liquid (Isl) then being strictly less than a laser intensity on the surface of the target (Ist) divided by 2.