Arnaud Sollier

Experimental determination by PVDF and EMV techniques of shock amplitudes induced by 0.6-3 ns laser pulses in a confined regime with water

With the objective to envisage short pulses for laser-shock hardening of materials, this paper reports experiments where laser-shock amplitudes P generated with 0.6-3 ns laser pulses at = 1.06 µm in a confined regime with water have been compared with those achieved with the usual 10-30 ns configuration. First, the experimental characterization of shock waves with polyvinylidene fluoride (PVDF) and electromagnetic (EMV) gauges shows that the short durations allow the generation of higher shock amplitudes than longer duration pulses (10 GPa versus 5 GPa) because of an increase of the pressure saturation intensity threshold Ith with short pulses (up to 100 GW cm-1 at 0.6 ns). Above Ith , a pressure pulse shortening accompanies the saturation. The P = f (I ) curves have been confirmed by surface deformation measurements induced on a Al12Si alloy. Second, the use of 10 µm aluminium coatings on 316L steel targets impacted at 40 GW cm-2 irradiance was shown to provoke a 25% maximum increase of the peak pressures by type mismatch acoustic impedance effects. Lastly, the PVDF technique is shown to be an accurate method to measure laser shock wave profiles in the 0-200 GW cm-2 regime, whereas the EMV technique is limited to intensity values of less than 20 GW cm-2 .

The generation of laser shock waves in a water-confinement regime with 50 ns and 150 ns XeCl excimer laser pulses

The generation of shock waves by laser-induced plasma in a water confinement regime has been investigated with 0.308 µm/50 ns and 0.308 µm/150 ns excimer laser pulses. Shock-wave profiles have been characterized by numerical simulations of rear surface velocity measurements behind Al foils with the use of a velocity interferometer system. The results are compared with those obtained with the third harmonic of a Nd:Yag. Above 1-2 GW cm-2, pressure levels saturate at 2-2.5 GPa and the pressure duration is much shortened by laser plasma breakdown which occurs in water. Therefore we suggest that this parasistic plasma generation is favoured by long pulse durations and a laser with short wavelengths.

Microjetting from grooved surfaces in metallic samples subjected to laser driven shocks

When a shock wave propagating in a solid sample reflects from a free surface, geometrical effects predominantly governed by the roughness and defects of that surface may lead to the ejection of tiny jets that may breakup into high velocity, approximately micrometer-size fragments. This process referred to as microjetting is a major safety issue for engineering applications such as pyrotechnics or armour design. Thus, it has been widely studied both experimentally, under explosive and impact loading, and theoretically. In this paper, microjetting is investigated in the specific loading conditions associated to laser shocks: very short duration of pressure application, very high strain rates, small spatial scales. Material ejection from triangular grooves in the free surface of various metallic samples is studied by combining transverse optical shadowgraphy and time-resolved velocity measurements. The influences of the main parameters (groove angle, shock pressure, nature of the metal) on jet formation and ejection velocity are quantified, and the results are compared to theoretical estimates.

Skew photonic Doppler velocimetry to investigate the expansion of a cloud of droplets created by micro-spalling of laser shock-melted metal foils

Dynamic fragmentation in the liquid state after shock-induced melting, usually referred to as micro-spallation, is an issue of great interest for both basic and applied sciences. Recent efforts have been devoted to the characterization of the resulting ejecta, which consist in a cloud of fine molten droplets. Major difficulties arise from the loss of free surface reflectivity at shock breakout and from the wide distribution of particle velocities within this cloud. We present laser shock experiments on tin and aluminium, to pressures ranging from about 70 to 160 GPa, with complementary diagnostics including a photonic Doppler velocimeter set at a small tilt angle from the normal to the free surface, which enables probing the whole cloud of ejecta. The records are roughly consistent with a one-dimensional theoretical description accounting for laser shock loading, wave propagation, phase transformations, and fragmentation. The main discrepancies between measured and calculated velocity profiles are discussed in terms of edge effects evidenced by transverse shadowgraphy.

Probing local and electronic structure in Warm Dense Matter: single pulse synchrotron x-ray absorption spectroscopy on shocked Fe

Understanding Warm Dense Matter (WDM), the state of planetary interiors, is a new frontier in scientific research. There exists very little experimental data probing WDM states at the atomic level to test current models and those performed up to now are limited in quality. Here, we report a proof-of-principle experiment that makes microscopic investigations of materials under dynamic compression easily accessible to users and with data quality close to that achievable at ambient. Using a single 100 ps synchrotron x-ray pulse, we have measured, by K-edge absorption spectroscopy, ns-lived equilibrium states of WDM Fe. Structural and electronic changes in Fe are clearly observed for the first time at such extreme conditions. The amplitude of the EXAFS oscillations persists up to 500 GPa and 17000 K, suggesting an enduring local order. Moreover, a discrepancy exists with respect to theoretical calculations in the value of the energy shift of the absorption onset and so this comparison should help to refine the approximations used in models.

PDV MEASUREMENTS OF NS AND FS LASER DRIVEN SHOCK EXPERIMENTS ON SOLID TARGETS

We present a new heterodyne velocimeter setup embedding a second low‐power frequency‐tunable laser acting as a local oscillator. We thus double the overall bandwidth of the system and we make the tuning of the laser power levels easier, to achieve good matching between the electric signal matching and the dynamics of the detector. Recently, we used this velocimeter onto metallic target shock driven by high power laser. The aim is to test the ability of this means to reveal shock propagation and effects into materials under extremely high strain rate with fast variations into the loading evolution. Spallation and fragmentation experiments carried out on aluminum samples were performed on the LULI2000 laser (800 J, 3 ns) and on the 100 TW laser (30 J, 300 fs) of the Ecole Polytechnique, with both VISAR and HV diagnostics. Comparisons reveal a very good consistency of experimental results.

Laser-matter interaction in laser shock processing

Laser shock processing (LSP) is an emerging industrial process in the field of surface treatment with particular application to the improvement of fatigue and corrosion properties. In the standard configuration, the metal sample is coated with a sacrificial layer in order to protect it from detrimental thermal effects, and a water overlay is used to improve the mechanical coupling by a confining like effect. Whereas the induced mechanical effects are now well understood, very few studies have been realized concerning the thermal effects. For this purpose, the knowledge of the confined plasma microscopic parameters has a great importance. A complete model describing the laser-liquid-metal interaction is presented. The model predicts the time evolution of the plasma parmmeters (temperature, density, ionization) and allows us to compute the induced pressure and temperature in the metal sample. By comparing the numerical results with various experimental measurements, predictions can be made concerning the best laser irradiation conditions for LSP.

Observation of the shock wave propagation induced by a high-power laser irradiation into an epoxy material

The propagation of laser-induced shock waves in a transparent epoxy sample is investigated by optical shadowgraphy. The shock waves are generated by a focused laser (3?ns pulse duration?1.2 to 3.4?TW?cm?2) producing pressure from 44 to 98.9?GPa. It is observed that the shock wave and the release wave created by the shock reverberation at the rear face are both followed by a dark zone in the pictures. This corresponds to the creation of a tensile zone resulting from the crossing on the loading axis of the release waves coming from the edge of the impact area (2D effects). After the laser shock experiment, the residual stresses in the targets are identified and quantified through a photoelasticimetry analysis of the recovered samples. This work results in a new set of original data which can be directly used to validate numerical models implemented to reproduce the behaviour of epoxy under extreme strain rate loading. The residual stresses observed prove that the high-pressure shocks can modify the pure epoxy properties, which could have an influence on the use made of these materials.

Experimental and numerical investigations of shock and shear wave propagation induced by femtosecond laser irradiation in epoxy resins

In this work, original shock experiments are presented. Laser-induced shock and shear wave propagations have been observed in an epoxy resin, in the case of femtosecond laser irradiation. A specific time-resolved shadowgraphy setup has been developed using the photoelasticimetry principle to enhance the shear wave observation. Shear waves have been observed in epoxy resin after laser irradiation. Their propagation has been quantified in comparison with the main shock propagation. A discussion, hinging on numerical results, is finally given to improve understanding of the phenomenon.

Experimental study of microjetting from triangular grooves in laser shock-loaded samples

When a shock wave interacts with a free surface, geometrical defects such as scratches, pits or grooves can lead to the production of high velocity, ∼μm-size debris. Because their ballistic properties are a key safety issue for various applications involving high pressure dynamic loading, and because these debris may inhibit surface measurements commonly used in shock physics, this process usually referred to as ’material ejection’ or ’microjetting’ has motivated extensive research work for many years. Recently, we have started a systematic investigation of microjetting under laser driven shock loading of thin metallic samples with calibrated grooves in their free surface. Transverse shadowgraphy (complemented with Photonic Doppler Velocimetry) provides jet velocities for different metals, various groove angles, over a range of shock pressure, both below and above shock-induced melting. Besides, the short duration of pressure application allows partial sample recovery, which provides original insight into...