J. P. Romain

Shock profile induced by short laser pulses

Standard 25‐μm‐thick polyvinilydene fluoride (PVDF) piezoelectric gauges and new 450‐μm‐thick P(VDF 70%, TrFE 30%) piezoelectric copolymer have been used to record shock profiles at the back face of metallic targets irradiated by laser pulses of 2.5 and 0.6 ns duration at a 1.06 μm wavelength. The records are fully explained with simplified space–time diagram analysis. The pressure profile applied at the front face of the target has been determined from these records combined with numerical simulations of wave propagation through the target. A numerical code describing the interaction of laser with matter (FILM) has also been used for computing the applied pressure. Both methods lead to very close results. The peak pressure dependence on incident laser intensity is determined up to 30 GPa at 1012 W/cm2.

Shock waves and acceleration of thin foils by laser pulses in confined plasma interaction

The process of shock wave generation and shock evolution in a solid target by confined laser plasma, in the range 108–1010 W/cm2 incident intensity, is analyzed with the use of computer simulations. Predicted variations of plasma pressure are consistent with theoretical results from simple analytical models and with experimental data. The simulations also provide a complete description of various effects such as target or confinement thickness and acceleration of thin foils.

Experimental study of laser acceleration of planar targets at the wavelength 0.26 μm

The main characteristics of accelerated aluminum targets, which are the target velocity, the uniformity of the acceleration and the backside temperature have been studied in laser experiments performed at wavelength 0.26 μm with an absorbed flux of a few 1013 W/cm2, in 400‐ps pulse duration by using the double‐foil technique and an optical pyrometry diagnostic: The ablation pressure was inferred from the velocity measurements. The uniformity of the acceleration was shown to be controlled by the hot spots in the focal spot, and the importance of studying the smoothing of laser inhomogeneities for accelerated targets with large ablated fractions was emphasized. The observed dependence of the backside temperature as a function of the initial foil thickness is discussed in the light of shock wave heating and radiative heating.

Shock pressure and free surface velocity measurements in confined interaction — Response of new VF 2 /VF 3 piezoelectric gauges

To determine the peak pressure induced versus the incident intensity of a neodymium (Nd) glass pulsed laser, with a duration of 25 ns in glass confined geometry, two methods have been comparatively used. Free surface velocity measurements have been performed using an electromagnetic gauge. The results are compared with pressure measurements realized at the back of irradiated aluminum targets with the use of polyvinylidene fluoride (PVDF) gauges. Both diagnostics provide consistent results. The measurements of peak pressure as a function of laser irradiance are used to determine the calibration curve (current density versus loading pressure) for new VF 2 /VF 3 copolymer shock gauges used in this laser-matter interaction configuration. These experimental set-up deliver time resolved measurements that are interpreted by the shock-propagation phenomena.

Measurements of laser shock pressure and estimate of energy lost at 1.05‐μm wavelength

Laser‐driven shock pressures at 1.05‐μm wavelength have been evaluated from measurements of shock transit time through aluminum foils by streak camera records of shock luminosity at the back face of the foil. An ablation pressure of 0.3 TPa is obtained for 1.2×1014 W/cm2 laser pulses focused on 300‐μm spot diameter and 0.55 TPa for 3.5×1015 W/cm2 laser pulses focused on 60‐μm spot diameter. These results, compared with theoretical values, show an important loss of energy, attributed to two‐dimensional effects. The ratio of effective energy for compression to incident energy is estimated to be 12% for 1.2×1014 W/cm2 experiments and only 1% for 3.5×1015 W/cm2 experiments.

Acceleration–deceleration process of thin foils confined in water and submitted to laser driven shocks

An experimental, numerical, and analytical study of the acceleration and deceleration process of thin metallic foils immersed in water and submitted to laser driven shocks is presented. Aluminum and copper foils of 20 to 120 μm thickness, confined on both sides by water, have been irradiated at 1.06 μm wavelength by laser pulses of ∼20 ns duration, ∼17 J energy, and ∼4 GW/cm2 incident intensity. Time resolved velocity measurements have been made, using an electromagnetic velocity gauge. The recorded velocity profiles reveal an acceleration–deceleration process, with a peak velocity up to 650 m/s. Predicted profiles from numerical simulations reproduce all experimental features, such as wave reverberations, rate of increase and decrease of velocity, peak velocity, effects of nature, and thickness of the foils. A shock pressure of about 2.5 GPa is inferred from the velocity measurements. Experimental points on the evolution of plasma pressure are derived from the measurements of peak velocities. An analytic...

Two‐dimensional study of shock breakout at the rear face of laser irradiated metallic targets

The two‐dimensional propagation dynamics of laser‐driven shock waves in solids is studied through the analysis of the shock breakout at the rear face of the target for a set of materials and laser intensities. The laser shock simulations were carried out by means of a two‐dimensional hydrodynamics code in which the laser‐ablation pressure is replaced by an equivalent pressure pulse. It is shown that the two‐dimensional code is a very useful tool to analyze laser‐shock experiments where two‐dimensional effects arise from a finite laser‐spot size or a heterogeneous energy deposition.

Some applications of laser-induced shocks on the dynamic behavior of materials

High-power pulsed lasers are used widely nowadays as shock generators. They settle as a complementary technique to the conventional shock generators by the high peak pressure and short duration shocks they deliver. They are used to investigate the feasibility of new industrial processes and to get information on the behavior of matter in specific conditions of extremely high strain rate. In this paper, some studies about typical applications of laser shocks are presented : surface densification of porous materials, spallation in ductile and brittle materials, α-e phase change in iron, and an estimate of the relaxation time for this transition. Laser shock experiments provide additional data on the dynamic behavior of materials at the nanosecond time scale.

Investigation of the response of a porous steel to laser driven shocks

Abstract. Laser-driven shocks have been performed to investigate the dynamic response of a sintered porous steel to uniaxial compressive loading at very high strain rates. The water-confinement technique has been used to increase both amplitude and duration of the laser shocks. Two steels of different initial porosities have been studied. Time-resolved wave profiles have been measured with thick piezoelectric transducers stuck at the back of the steel targets. The residual porosity has been evaluated by post-shock examination of the recovered samples. A simple constitutive material model, based on a macroscopic description involving the equation of state of the compact steel and a traditional P-

SPALLING DUE TO A STRONG SHOCK WAVE DECAY PROCESS IN SOLID TARGETS IRRADIATED BY A PULSED LASER

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