System and method for processing a material by laser shock under liquid confinement

Abstract

The invention relates to a system (10) for treating a target (Tar) by laser shock under the constraint of a liquid (Liq), said system comprising: a pulsed laser generating a light beam having a pulse duration of between 1 ns and 30 ns and a wavelength, focusing optics (COD) having a focal length and configured to focus said light beam (B) on a surface (St) of said target (Tar), the incident light beam (B) on said focusing device having a diameter (D) filling a tank (TK) of said liquid (Liq) having a refractive index n, the expected value of the diameter (D) of the light beam on the target surface (St) being predetermined and called Dst, the thickness (e) of the liquid (Liq) crossed by the light beam (B) before reaching said surface (St) of the target being chosen so that the laser intensity (Isl) on the surface of the liquid is less than or equal to the laser intensity (Ist) on the surface of the target divided by 2.

Description

Technical Field

[0001] This invention relates to the field of material treatment by laser shock based on generating plasma confined to the surface of a target material and generating shock waves in the material. Background Technology

[0002] Laser shock spectroscopy is a laser method that allows energy to be rapidly applied to a target material (usually metal or made of composite materials) to create a plasma with extremely high pressure. This method generates extremely powerful shock waves (at pressures on the order of GPa), enabling a wide range of applications.

[0003] An example of a system 5 known in the prior art for implementing laser shock treatment is shown. Figure 1 The device comprises a pulsed laser L that generates a beam B in the form of a pulsed LP, and a focusing optics COD with a focal length f, configured to focus the beam B onto the surface of a target material Tar. Typically, the target material is not placed in the focal plane of the COD because, for the aforementioned application, the beam diameter is located on the target surface at the interface with the confinement medium, and is on the order of millimeters (typically between 0.3 mm and 10 mm).

[0004] The laser generates an extremely high-pressure plasma (PLconf) through laser ablation. A confinement medium is placed on the surface to be ablated by the laser. The most common and industrially practical confinement is a thin layer (CL) of a laser-transparent medium (water, other laser-transparent liquids, quartz, polymer tape, etc.), typically 1 to several millimeters thick. The confinement condition allows for a significant increase in plasma pressure and the duration of its application to the target. Optionally, a thermally protective coating (HPC) is deposited on the target. Using this system, extremely strong shock waves (OC) are generated, with pressures on the order of GPa, enabling the performance of various applications.

[0005] For example, laser / material interaction and laser shock treatment are described in the following publications: -Sollier et al., “Laser-matter interaction in laser shock processing”, First international symposium on High power laser macroprocessing, SPIE n° 4831, pp. 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, pp. 7-14 (2015).

[0006] In order to generate plasma under favorable conditions and thus generate a shock wave for application, a laser with a pulse duration τ typically between 1 ns and 30 ns and an energy E between 0.5 and 10 J, and focused on a target material with a size between 0.3 and 10 mm, should be used. These different parameters are selected according to the target application.

[0007] The main applications are: - Laser Shock Adhesion and Disassembly Test (LASAT): Two shock waves are generated by two laser pulses, which are staggered in time and converge at the joint of the component to be tested or disassembled. Strong tensile strain is required (Reference: Berthe et al., "State-of-the-art laser adhesion test (LASAT)," Nondestructive Testing and Evaluation, Vol. 26, Nos. 3–4, pp. 303–317 (2011)). - Surface strengthening by laser peening (LSP); high pressure is applied by plasma and transmitted to the target material via shock waves, which can plasticize the target material and enhance its properties (strength, life, etc.) (Reference: Montross et al., “Laser shock processing and its effects on microstructure and properties of metal alloys: a review”, International Journal of Fatigue 24, pp. 1021–1036 (2002)). - Characterization of materials under high pressure.

[0008] These applications primarily involve different areas of scientific research and industrial activities, such as aviation, nuclear energy, or maritime.

[0009] For these systems, the important parameter is GW / cm 2 The power density or intensity I of the irradiated target is expressed in units, because the pressure generated is proportional to the square root of the laser intensity (see, for example, Fabbro et al., “Physical study of laser produced plasma in confined geometry”, Journal of Applied Physics, 68(2), pp. 775–784 (1990)). Laser intensity I (GW / cm) is defined according to the following formula. 2 ): (1) Where E is the laser energy per pulse (J), τ is the laser pulse duration (ns), and S is the surface area irradiated by the laser (cm²). 2 ).

[0010] However, it is impossible to infinitely increase the generated pressure by increasing the laser intensity of the irradiated target, because the laser intensity transmitted to the target is saturated by the breakdown mechanism occurring on the confined surface. In fact, the laser / material interaction scheme in laser shock involves two different types of plasma: confined plasma PLconf and breakdown plasma PLbk / s. The confined plasma develops at the target surface, i.e., at the target-confined liquid interface, such as... Figure 1 As shown, breakdown plasma occurs at the surface of the liquid, such as Figure 2 As shown in the figure, this occurs because of the ionization of the confining medium.

[0011] Such breakdown plasma has been studied, for example, in the publication of Sollier et al.: “Numerical modeling of the transmission of breakdown plasma generated in water during laser shock processing”, Eur. Phys. AP, Vol. 16, pp. 131-139 (2001).

[0012] The presence of breakdown plasma PLbk / s makes it opaque to laser radiation, thus absorbing the remaining energy contained in the laser pulse. This breakdown phenomenon causes the maximum intensity of a confined laser shock irradiation target to be limited by the breakdown threshold intensity in the confining medium. This phenomenon can be observed in the aforementioned publications. Figure 5 The data shows that the incident intensity exceeding the threshold (which will be referred to as lbk) on the surface of the confined medium (here, water) is approximately 8 GW / cm². 2 (For example, λ = 1064 nm and τ = 25 ns), the intensity of the pulse transmitted through the confined medium saturates as the incident intensity increases.

[0013] Therefore, the maximum pressure that can be generated by laser shock is limited. Since the thickness of the water layer is very small compared to the focal length used (e.g., 1 to 3 mm thickness, 300 to 500 mm focal length), the intensity Ist on the target surface can be considered approximately equal to the intensity Isl on the water surface. Therefore, the maximum intensity that can be applied to the target surface is also 8 GW / cm². 2 Using this applied strength of 8 Gw / cm 2 Within the pulse duration range of 5-15 ns, the maximum pressure obtained by laser shock is approximately 8 GPa.

[0014] The pressure achieved by current laser peening is still too low to disassemble components of a specific strength or thickness. Similarly, a pressure typically 2.5 times greater than the yield strength is required to optimally strengthen the target material through laser peening; therefore, current pressures cannot handle all materials, especially the strongest ones.

[0015] Therefore, increasing the maximum pressure value will enable the resolution of existing unmet needs.

[0016] Several approaches are now available that could potentially increase the pressure on the target material:

[0017] Reduce the duration of the laser pulse. Literature (experimental and theoretical) confirms that, in a given confined medium, the breakdown threshold laser intensity Ipk is inversely proportional to the square root of the laser pulse duration. Therefore, in order to increase the maximum laser intensity (and thus the maximum pressure) of the irradiated target, one approach is to reduce the pulse duration of the laser system used, since the breakdown threshold intensity will increase. However, while this approach enables the increase of maximum plasma generation pressure, it introduces visibility and practicality issues: in reality, the duration of the shock wave depends on the duration of the laser pulse (roughly twice its value), and the damping of the shock wave increases more as its duration decreases. The resulting wave will decay more rapidly in the target material being treated (because the laser pulse duration has been reduced), and the "useful" pressure (at the core of the material, not on the surface) will therefore not increase, or may even decrease.

[0018] Using the so-called direct approach: The second approach consists of a constrained feeding method and a direct irradiation method: thus, there is no longer a constraint around the target, increasing its pressure. However, the direct irradiation method allows for pressure similar to that of the constrained method by using laser intensities 10 to 100 times higher. On the other hand, a high vacuum must be generated around the target to prevent any breakdown in the air, which is very likely to occur at these intensity levels. This second approach is difficult to apply industrially because, on the one hand, it requires establishing a vacuum around the part to be treated, and on the other hand, it requires an extremely high-energy laser system, making it expensive and bulky.

[0019] Magnetic or electric fields are used when establishing plasma (patents N° CN201210571521 and N° US10745776) to apply additional energy to the plasma (by transmission). However, these schemes have failed to produce convincing results and have not significantly increased the generated pressure level, or are too complex to implement and calibrate.

[0020] One object of the present invention is to remedy the aforementioned drawbacks by providing a system that enables an increase in the maximum intensity at the target surface, thereby increasing the pressure of the laser shock transmitted to the target. Furthermore, the system according to the invention is cost-effective because it does not require modification of the laser used in existing laser shock systems. Summary of the Invention

[0021] The subject of this invention is a system for laser shock treatment of a target material under liquid confinement conditions, the system comprising: - A pulsed laser that generates a beam with a pulse duration τ and wavelength λ between 1 ns and 30 ns. - A focusing optical device having a focal length f and configured to focus the laser beam onto the surface of the target material, wherein the incident laser beam on the focusing device has a diameter D. - A tank, the tank being filled with the liquid having a refractive index n, The desired diameter of the beam on the surface of the target is predetermined and referred to as Dst. The thickness e of the liquid through which the light beam passes before reaching the surface of the target is selected such that the laser intensity on the surface of the liquid is less than or equal to the laser intensity on the surface of the target divided by 2. According to a preferred embodiment, the thickness e is selected to be greater than or equal to the minimum thickness e defined by the following formula. min : According to one embodiment, the system according to the invention further includes elements configured to homogenize the light beam and disposed in the optical path of the light beam. According to one embodiment, the laser energy E and the focusing optics are configured such that the laser intensity on the surface of the target material is between 0.1 GW / cm². 2 and 25 GW / cm 2 The predetermined value Dst is between 0.3 and 10 mm. According to one embodiment, the liquid has a concentration of less than or equal to 0.1 / m at the wavelength. 2 The absorption coefficient. According to one embodiment, the liquid is water and the wavelength λ of the laser is in the range of [350 nm; 600 nm]. According to another aspect, the present invention relates to a method for treating a target material by laser shock under liquid confinement conditions, comprising: - Fill the tank with the liquid and contain the target material. -Use pulsed lasers to generate beams with pulse durations τ between 1 ns and 30 ns. - A focusing optical device with a focal length of f is used to focus a light beam onto the surface of the impregnated target material. The incident light beam on the focusing optical device has a diameter D. - Position the target material in the groove, and then irradiate the surface with the light beam such that the light beam passes through a liquid at least equal to the minimum thickness e before reaching the surface of the target material. min The thickness e, and such that the diameter of the beam on the surface of the target material is equal to a predetermined value Dst, The minimum thickness e of the liquid min Defined by the following formula: The laser intensity on the surface of the liquid is strictly less than the laser intensity on the surface of the target material divided by 2. The following description provides several exemplary embodiments of the apparatus of the present invention: these examples do not limit the scope of the invention. These exemplary embodiments illustrate essential features of the invention and additional features associated with the embodiments considered. Attached Figure Description

[0022] The invention will be better understood from the following detailed description, and in consideration of the accompanying drawings, which are given as non-limiting examples, and other features, objects, and advantages thereof will emerge:

[0023] Figure 1 (As mentioned) A laser shock characterization system based on the prior art is shown.

[0024] Figure 2 (As mentioned) Two types of plasma involved in the laser shock mechanism are shown.

[0025] Figure 3 The measurement protocol used to demonstrate the existence of the volumetric breakdown mechanism is shown.

[0026] Figure 4 The trend of transmittance as a function of the maximum strength Imax reached in the confined medium is shown for two cases (cross point is 2 mm water layer, circle point is 15 cm water layer).

[0027] Figure 5 A system for treating a target material by laser shock according to the present invention is shown.

[0028] Figure 6 The pressure applied via the confined plasma is shown as a function of the maximum intensity achieved in the confined medium for the two cases mentioned above (the cross point represents a 2 mm water layer, and the circle point represents a 15 cm water layer). Detailed Implementation

[0029] This invention was discovered by the inventors while studying the breakdown mechanism in laser shock systems.

[0030] First, the inventors' work enabled the first demonstration that there are actually two breakdown modes: breakdown via the surface of the confining medium, as known in the prior art, and breakdown within the volume of the confining medium. This volumetric breakdown has not been studied in laser shock architectures because the confining layer used is always very thin, too thin for such breakdown to occur. This volumetric breakdown becomes visible as the thickness of the confining medium increases.

[0031] The inventor, through experiments, Figure 3 The measurement protocol schematically illustrated in the diagram reveals the existence of a breakdown mechanism in this volume. Part A corresponds to the condition according to the prior art (small water thickness, here 2 mm), and Part B corresponds to the measurement performed using a large water thickness (15 cm).

[0032] In both cases, the transmittance T = Et / Ei is measured, where Ei is the energy incident on the outer surface of the confined medium, and Et is the energy transmitted after passing through the thickness of the liquid. The energy Ei is known, and the measurement of Et is performed using a calorimeter CAL to recover the beam transmitted through window W and focused at the bottom of the groove TK.

[0033] For this measurement, the wavelength is 532 nm, the pulse duration τ is 7.2 ns, the beam has an initial diameter of 20 mm, and the focusing device has a focal length f = 80 mm. Here, an element BH (usually DOE, meaning "diffractive optical element") is used, which is configured to homogenize the beam and positioned in the beam's optical path.

[0034] Figure 4 The trend of transmittance T as a function of the maximum intensity Imax reached in the confined medium is shown for a given intensity Isl on the liquid surface (which is varied (via changes in laser energy)). The intensity Isl can be readily derived from Ei using E1 and knowledge of the beam diameter on the surface.

[0035] In case A, because the liquid is very thin, the intensity is essentially the same at the liquid surface or throughout the depth of the tank. Therefore, the intensity Imax is considered to be the incident intensity Isl at the liquid surface: Imax ≌ Isl. When a target is present, this intensity Imax will also be equal to the intensity on the target surface in contact with the confining medium, i.e., Ist.

[0036] For case B, the system COD combined with element BH is configured to focus the light based on a known minimum diameter of 1.5 mm in the liquid volume. Knowing the incident energy and the minimum diameter, the associated intensity Imax is derived from them.

[0037] then, Figure 4 The x-axis Imax corresponds to the maximum strength obtained in the liquid (either on a surface or in a volume).

[0038] Therefore, the intensity Imax obtained in the liquid may be the maximum intensity that can be obtained on the surface of the target material for generating shock waves.

[0039] exist Figure 4 In the diagram, the crosshair corresponds to the value obtained using the measurement based on A, and the circle corresponds to the value obtained using the measurement based on B.

[0040] For the cross, surface breakdown, a phenomenon known in the prior art, was observed again, with an experimental threshold (to be referred to as Ibk / s) of 8 GW / cm. 2Around 4 GW / cm². This result is consistent with that obtained by Arnaud Sollier (see the publication mentioned above), who describes the results as a function of the laser parameters used at 4 and 10 GW / cm². 2 Between the breakdown strength thresholds, breakdown plasma appears on the surface of the confined medium.

[0041] The innovation lies in utilizing the variation of T obtained from a circle, which reveals a new threshold (to be called Ibk / v) corresponding to breakdown in the liquid volume, occurring at the point of highest intensity. This threshold is approximately 20-21 GW / cm². 2 That is, greater than Ibk / s.

[0042] Therefore, using this measurement, the inventors proved that there are two breakdown thresholds, not just one: the surface breakdown threshold, to be called Ibk / s, and the volume breakdown threshold, Ibk / v. Furthermore, these measurements have made it possible to determine the values ​​of these two thresholds for the same laser pulse duration and the same wavelength, and from this, it is deduced that the volume threshold is higher than the surface threshold: Ibk / v > Ibk / s.

[0043] Define the ratio R= .

[0044] For pairing (τ = 7.2 ns; λ = 532 nm), R 2.5.

[0045] The existence of this ratio R, combined with the fact that it is greater than 1, yields a significant result. This means that when using a thicker confinement material, a greater strength Ist can be obtained on the target than before breakdown, compared to using a thinner material.

[0046] This highlights the two breakdown thresholds and experimentally demonstrates that the parameter R>1, which links the two thresholds together, is a true discovery that has not been revealed until now. One breakdown threshold is on the surface and plays a dominant role when the constraint layer is thin, while the other is in the volume and occurs when the constraint layer becomes thicker.

[0047] In other words, this work is the first to demonstrate that, given constant laser parameters, the breakdown threshold in the confined medium is larger if breakdown occurs within the confined medium volume rather than on its surface (typically 8-10 GW / cm² for a 7.2 ns pulse using water confinement). 2 Up to 20-25 GW / cm 2 ).

[0048] When the thickness *e* of the confining medium is sufficiently large, breakdown at the surface of the confining medium is avoided (since the laser is not focused solely on the surface, where the laser intensity is locally low). This breakdown is transferred to the volume at the core of the confining medium, where the breakdown threshold intensity is higher than that at the surface. This increases the maximum intensity capable of irradiating the target (with the same set of laser parameters), and therefore also increases the maximum pressure generated. In the aforementioned example with breakdown in the volume, compared to 8 GPa (8-10 Gw / cm²) for conventional surface breakdown... 2 Compared to this, it can deliver a pressure of 12 GPa to the target material (corresponding to a threshold intensity of 20-22 GW / cm). 2 ).

[0049] Several other experimental measurements and physical derivations show that the value of this ratio R depends on the confinement material considered and remains relatively stable within the pulse duration range [1-30 ns]. For water, R is between 2.5 and 3. More generally, for the laser conditions and confinement materials of interest in laser shock, the inventors have determined that R is typically between 2 and 4.

[0050] From the fact that R>1, it can be deduced that if there is a breakdown on the surface, the maximum strength that will be reached will be And the strength on the target is at most equal to Similarly, for volumetric breakdown, the maximum target strength is at most equal to In order to maximize the maximum strength on the target, it must be in a condition with volumetric breakdown, and this breakdown will occur at the target by positioning the target at the point where the laser is most focused (maximum intensity).

[0051] In other words, the laser shock system must therefore be designed so that when the intensity (Ist) on the target material has a value At that time, on the surface of the constraining medium, less than (Ist). In fact, if this were not the case, it would mean, for example, when the target material has At that time, it had already exceeded the time limit on the surface. Therefore, there is already penetration on the surface, so it is actually impossible for there to be penetration on the target material. .

[0052] Therefore, this means that the following conditions must be met: (2)

[0053] The fulfillment of condition (2) ensures that when there is surface breakdown ( Then the strength on the target material is at least equal to Therefore, in reality, there is breakdown in the volume before breakdown on the surface: the possible strength on the target has been maximized (because the threshold in the volume is greater than on the surface, the conditions must make breakdown occur in the volume first, which is not the case for a 1 mm water thickness).

[0054] The minimum value of R has been determined through experiments, Rmin = 2.

[0055] Therefore, the laser shock system satisfies: (3)

[0056] These intensities in the liquid on the surface of the liquid and on the surface of the target material can be measured using, for example, a Joule meter or a photodiode.

[0057] Considering the above, there will always be volumetric breakdown for both the laser of interest and the confined medium.

[0058] The compliance with condition (3) is a result obtained by the inventors, which enabled the design of the laser shock system according to the invention, which prioritizes breakdown in volume. The laser shock system according to the invention utilizes experimental evidence that there exists a higher breakdown threshold in volume than on the surface of the confined medium.

[0059] This invention relates to a kind of Figure 5 The system 10 illustrates a system for treating a target material Tar by laser shock under liquid Liq confinement. The system includes a pulsed laser L generating a beam B and a focusing optics COD, the beam B having a pulse duration τ between 1 ns and 30 ns and a wavelength λ. The COD has a focal length f and is configured to focus the beam B onto the surface St of the target material. The incident laser beam on the focusing device COD has a diameter D. The system also includes a tank TK filled with the liquid, which has a refractive index n.

[0060] In a laser shock blasting system, the beam diameter Dst on the surface St of the target material irradiated by the beam constitutes the input diameter, which is a function of the application and properties of the material being treated. In practice, this desired diameter Dst varies between 0.3 mm and 10 mm, preferably between 0.8 mm and 5 mm.

[0061] From the input parameters (D, f, n, Dst), it can be seen that the target is set in the groove so that the beam passes through the thickness e of the liquid and then reaches the surface St of the target. The thickness e is selected so that the laser intensity (Isl) on the liquid surface is less than or equal to the laser intensity (Ist) on the target surface divided by 2 (condition (3)).

[0062] With the input parameters set, the observation condition (3) enables the determination of the minimum thickness e of the liquid to be observed.min .

[0063] The following equations apply: (4) and (See) Figure 5 )

[0064] And using the law of folding ratio: (5) n is the refractive index of the liquid.

[0065] also, (6)

[0066] Intensity I is defined as follows: Where E is the laser energy (J) of each pulse. Where S is the pulse duration (ns), and S is the area of ​​the irradiated surface (cm²). 2 ).

[0067] therefore , where DE is the diameter of the irradiated surface.

[0068] From relation (2), derive the following: therefore

[0069] From relation (6), derive the following:

[0070] (7)

[0071] Using relations (4) and (5), tan(θr) can be expressed as a function of parameters D, f, and n, ending at: (8)

[0072] By taking the minimum value of R, i.e., Rmin=2, we can derive e from it. min The value of is sufficiently large for all systems of interest: (9)

[0073] The laser system must be configured such that the thickness e of the liquid that will pass through before the beam reaches the target surface is chosen to be greater than or equal to e. min In this case, the laser intensity Isl on the liquid surface is less than or equal to the laser intensity on the target surface divided by 2.

[0074] Thickness emin This depends on the parameters D, f, n, and Dst of system 10. As an example, for D = 20 mm, f = 500 mm, n = 1.33 (water), and Dst = 4 mm, e min = 55 mm.

[0075] In practice, the scale of system 10 will be determined by covering all cases of interest with, for example, 10-15 cm of water, while ensuring that condition (3) is observed.

[0076] It should be noted that the above calculations are valid for Gaussian laser beams if a far-field mode is used (i.e., the distance from the "beam waist" to the lens focal point is very far, and the laser beam at the beam waist has a diameter of several micrometers). In laser shock systems, these conditions always hold (laser spot Dst ≥ 300µm).

[0077] For small angles, formula (9) is simplified: (10) Where N = f / D.

[0078] According to the dimensional setting of the laser shock system of the present invention, the numerical aperture (ON=D / 2f) of the system is linked to the liquid thickness used to move the breakdown on the constraint surface into the constraint volume.

[0079] From formula (9), it can be seen that the system / water thickness depends on the spot size Dst. That is, if 3 mm is needed to process titanium and 1 mm is needed to process aluminum, then two different minimum slot thicknesses are determined. In practice, manufacturers designing laser shock systems will use the same slot with a thickness greater than the maximum e. min Furthermore, its grooves will work for both materials.

[0080] Depending on the requirements, the laser system according to the invention can be adjusted for the numerical aperture (D / 2f) of the system in use by determining the minimum thickness of the liquid required to transfer breakdown from the surface to the volume.

[0081] It should be noted that, assuming e satisfies the condition e ≥ e min Therefore, the system 10 according to the invention is compatible with an architecture in which e=f, ​​which corresponds to the device COD in the immersion tank.

[0082] In other extreme cases, assuming a very open COD optical system, system 10 is compatible with smaller water layer thicknesses, which ensures... However, this configuration has a drawback: it brings the optical system closer to the target, which is undesirable.

[0083] Generally, the more open the optical system of a COD (Catalyst Optical Deposition) system, the greater the reduction in the thickness of the confinement layer can be. A water height of approximately 10-15 cm includes a small aperture D / 2f, which is advantageous because large apertures can damage the optical system as they would be very close to the target material (e.g., plasma ejection water and metal particles). Additionally, the water height is typically at least 10 cm, so there is no splashing on the lens. The system parameters D and f are typically for a laser beam with a diameter between 15 and 30 mm on the COD and a focusing distance of approximately 20-30 cm.

[0084] Therefore, the laser shock system 10 designed according to the present invention enables the avoidance of breakdown on the surface of the confining medium (where the laser is not focused on the surface, and therefore the laser intensity thereon is locally very small and cannot initiate breakdown). This breakdown is moved to the volume at the core of the confining medium, where the threshold intensity is higher. Thus, the maximum intensity that can irradiate the target material (with the same set of laser parameters) is increased, and therefore, the maximum pressure generated is also increased.

[0085] The laser system according to the invention significantly increases the generated pressure (approximately +50%). This system is relatively simple to implement. It uses a laser and existing focusing devices, as well as a tank typically filled with a 10-15 cm thick confinement fluid in which the target material is impregnated.

[0086] Figure 6 The pressure P applied via confined plasma as a function of Imax is shown for the two cases mentioned above (crosspoint represents a 2 mm water layer, and circle represents a 15 cm water layer). Curve 60 is a numerical simulation based on calculations performed using laser / material interaction 1D code, which does not consider breakdown phenomena (only the pressure caused by a given incident laser pulse absorbed by the target is calculated).

[0087] For the thin-layer (cross-shaped) case, it was observed that the efficiency exceeded approximately 10 GW / cm². 2 At an intensity (corresponding to Ibk / s), the generated pressure stagnates at approximately 8 GPa and then decreases. Surface breakdown limits the intensity of the irradiated target. For thicker layers (circular), the generated pressure follows an increase in Imax, reaching at least 22 GW / cm². 2 The value reached 12 GPa, which is more than 40% higher than that of a thin layer. Furthermore, the trend followed the theoretical curve without breakdown over a longer period of time, which clearly indicates that the problems associated with breakdown were transferred to higher strengths.

[0088] According to one embodiment, system 10 further includes an element BH, which is configured to homogenize the beam and is positioned in the optical path of the beam. The presence of the beam homogenizer (typically a DOE) ensures that there is no excessive intensity in the spatial distribution of the laser, and thus avoids local breakdown (which would result in a local loss of intensity transmitted to the target).

[0089] To generate laser shock, the laser energy E (per pulse) and the focusing optics are preferably configured such that the laser intensity Ist on the target surface is between 0.1 GW / cm². 2 and 25 GW / cm 2 The values ​​are between 0.3 and 10 mm, and the Dst value is between 0.3 and 10 mm.

[0090] According to one embodiment, a pair (λ, liquid) is selected such that the liquid Liq has a value less than or equal to 0.1 / m. 2 The absorption coefficient α(λ) means that the wavelengths absorbed by the confined medium are very small.

[0091] Preferably, the liquid is water, and the wavelength λ of the laser is in the range of [350 nm; 600 nm]. A wavelength of 532 nm is preferred compared to the 1064 nm wavelength commonly used in laser shock with a thin layer of water.

[0092] According to another aspect, the present invention relates to a method for laser shock treatment of a target material Tar under liquid Liq confinement conditions, comprising: - Fill the tank with the liquid Liq and contain the target material Tar. - A light beam B is generated using a pulsed laser, with a pulse duration τ between 1 ns and 30 ns. - A focusing optics device COD with a focal length of f focuses a beam B onto the surface of the impregnated target. The incident beam on the focusing optics has a diameter D. - Position the target material in the groove, and then irradiate the surface with the light beam, such that the light beam passes through the thickness e of the liquid, and the laser intensity on the surface of the liquid Liq is strictly less than the laser intensity Ist on the surface of the target material divided by 2.

Claims

1. A system (10) for laser shock treatment of a target (Tar) under liquid (Liq) confinement conditions, the system comprising: - A pulsed laser (L) generates a beam (B) with a pulse duration τ between 1 ns and 30 ns and a wavelength λ. - A focusing optics (COD) having a focal length f and configured to focus the light beam (B) onto the surface (St) of the target material, wherein the incident laser beam on the focusing optics has a diameter D. - A tank (TK), said tank being filled with said liquid having a refractive index n, The desired diameter of the beam on the surface (St) of the target material is predetermined and referred to as Dst. The thickness e of the liquid through which the light beam passes before reaching the surface of the target is selected such that the laser intensity on the surface of the liquid is less than or equal to the laser intensity on the surface of the target divided by 2. The thickness e is selected to be greater than or equal to the minimum thickness e defined by the following formula. min : 。 2. The system according to claim 1, further comprising: An element (BH) is configured to homogenize the light beam and is positioned in the optical path of the light beam.

3. The system according to any one of claims 1-2, wherein, The laser energy E and the focusing optics are configured such that the laser intensity on the surface of the target is between 0.1 GW / cm². 2 and 25 GW / cm 2 The predetermined value Dst is between 0.3mm and 10mm.

4. The system according to any one of claims 1-2, wherein, The liquid is water, and the wavelength λ of the laser is in the range of [350 nm; 600 nm].

5. A method for laser shock treatment of a target (Tar) under liquid (Liq) confinement conditions, comprising: - Fill the tank with the liquid and contain the target material. - A beam with a pulse duration τ between 1 ns and 30 ns is generated using a pulsed laser (B). - The beam (B) is focused onto the surface of the impregnated target using a focusing optics device (COD) with a focal length of f, wherein the incident beam on the focusing optics device has a diameter D. - Position the target material in the groove, and then irradiate the surface with the light beam such that the light beam passes through a liquid at least equal to a minimum thickness e before reaching the surface of the target material. min The thickness e is such that the diameter of the beam on the surface (St) of the target material is equal to a predetermined value Dst. The minimum thickness e of the liquid min Defined by the following formula: The laser intensity on the surface of the liquid is less than the laser intensity on the surface of the target material divided by 2.

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