Patrick Mercier

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.

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.

Influence of edge conditions on material ejection from periodic grooves in laser shock-loaded tin

In a material subjected to high dynamic compression, the breakout of a shock wave at a rough free surface can lead to the ejection of high velocity debris. Anticipating the ballistic properties of such debris is a key safety issue in many applications involving shock loading, including pyrotechnics and inertial confinement fusion experiments. In this paper, we use laser driven shocks to investigate particle ejection from calibrated grooves of micrometric dimensions and approximately sinusoidal profile in tin samples, with various boundary conditions at the groove edges, including single groove and periodic patterns. Fast transverse shadowgraphy provides ejection velocities after shock breakout. They are found to depend not only on the groove depth and wavelength, as predicted theoretically and already observed in the past, but also, unexpectedly, on the edge conditions, with a jet tip velocity significantly lower in the case of a single groove than behind a periodic pattern.

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 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...

Doppler laser interferometry improvements in detonics

For more than 30 years now, Doppler Laser Interferometry has been used in detonics to measure velocity versus time accurately. The means is composed of: a laser source, an optical fiber bringing light to the moving target, another which collects back-reflected light, a device built around a Fabry-Perot interferometer to create the rings pattern, a streak camera. In the beginning, the source was a CW singlemode argon laser with a 7 W output power. Later, it was replaced by a wide spectrum rhodamine dye laser requiring two twin Fabry-Perot interferometers, the first one modulating the spectrum and the second analyzing the Doppler shifted light; the available power was 1 kW for a 40 μs pulse. Recently, we have improved the means by increasing the channel number with sufficient output power, replacing the dye with a solid amplifier, increasing pulse width, decreasing the velocity and time uncertainties and reducing the volume and the cost of the equipment. To achieve this quality we acquired a long-pulse singlemode Yag laser. It has 15 channels, each providing 1 kW with a 70 μs rectangular pulse at 532 nm. The analysis bench uses only 1 Fabry-Perot interferometer for 5 channels. The laser, target and analysis bench are connected through a 3 optical fibers bundle; one for lighting, one to measure velocity and a third one to record the photometric curve and determine shock breakout time accurately. To improve accuracy, we worked on two areas: (1) the equipment with the anamorphic optical device, the value of the fringe constant and the number of rings lit on the camera slit, (2) the building of a chart where we write all influent parameters in order of importance: in order to decrease the uncertainty of velocities less than 1000 m/s, we need to evaluate the influence of electronic streak camera distortion.

New optical instantaneous imaging laser technique in detonic experiments

Dealing with dynamic behavior of solids, detonator initiation, shock and detonation waves and other fast processes implies a number of new techniques. We are working on wave propagation at velocities of several km/s, with states existing for only a few microseconds or even nanoseconds. In this case performances of our fastest rotating mirror framing cameras are not high enough to observe states of surface or large discontinuity zones (problem of dynamic blur). We have developed a new laser technique called Instantaneous Image (I.I.). This technique consists in recording a single image in a short exposure time to minimize the dynamic blur of our fast phenomena. We use a Q-switched Nd:YAG laser made of an oscillator, a pre-amplifier, a 16 mm diameter amplifier and a KDP crystal. The available energy is in the order of 200 mJ at 532 nm for a ten nanoseconds pulse duration. A large amount of work has been done to minimize the non uniformity of the delivered light, to eliminate speckle defects and to collect the most illumination light by an optimized optic device. Under these conditions a large diameter field image (D equals 200 mm) can be achieved with a resolution better than 15 line pairs/mm. With a double proximity focused microchannel plate image intensifier (M.C.P.) it is possible to obtain faster shuttered times (a few nanoseconds) with a higher gain to observe poor reflective surfaces. But under these conditions the resolution decreases drastically to some line pairs per millimeter.

PDV-based estimation of high-speed ejecta particles density from shock-loaded tin plate

In the context of ejecta particle experiments created from shock-wave loading a metallic surface with machining grooves, we present a method to estimate the areal mass Ms (mg.cm–2) of the particle cloud from non-invasive PDV velocimetry diagnostic. Through the statistical properties of the time-velocity PDV spectrogram, the set of parameters most likely to have generated experimental PDV data is estimated. This approach is detailed on an experiment consisting of a 1-mm thick tin surface with 2D triangular known surface patterns (period of 60 µm and peak-to-peak amplitude of 8 µm) explosively driven by an electro-detonator with a peak breakout pressure of approximately 29 GPa at the metal-vacuum interface.

Embedded optical fibers for PDV measurements in shock-loaded, light and heavy water

In order to study the shock-detonation transition, we propose to characterize the shock loading of a high explosive plane wave generator into a nitromethane cell. To eliminate the reactive behaviour, we replace the nitromethane by an inert liquid compound. Light water (H2O) has been first employed; eventually heavy water (D2O) has been chosen for its better infrared spectral properties. We present the PDV results of different embedded optical fibers which sense the medium with two different approaches: a non intrusive optical observation of phenomena coming in front of them (interface, shock wave, detonation wave) followed by their mechanical interaction with the fiber.