Study of physical processes involved in laser shock processing of materials
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The generation of shock waves by laser-plasma in Water Confinement Regime (WCR) has been investigated for the first, second and third harmonics of Nd:Glass laser (1,064, 0,532, 0,355 μn) with 0.6, 10 and 25 ns laser pulse durations. Pressure measurements have been mainly performed using a Velocimetry Interferometer System for Any Reflector (VTSAR). It appears that, depending on laser parameters, above a given laser intensity threshold, the peak pressure is saturated and the pressure duration is reduced due to laser-induced breakdown plasma in the confining water. The observation of the interaction zone (confined plasma and confining water) with a fast intensified camera shows that this parasitic breakdown occurs exclusively at the surface of water and limits the efficiency of the process. The time-resolved transitivity of this plasma has been measured with a continuous Argon laser beam for a 1.064 μm/25 ns incident laser irradiation. Above 10 GW/cm2, the peak power density transmitted through the breakdown plasma saturates and the laser pulse transmitted is reduced in agreement with pressure measurements. The relative influence of main physical mechanisms occurring during the generation of the laser breakdown at the surface of water have been discussed. According to the wavelength effect which tends to authorize higher pressure with longer wavelengths, the influence of multiphotoionic processes seem to dominate the effect of avalanche ionization. The protective coatings and laser spot size influences have been also investigated regarding the stress levels induced in the targets. About 30 % stresses increase in the material is shown to occur with adapted coating. On the contrary, the laser spot size has no specific effects on the pressure induced in WCR. Residual Stresses (RS) field have been determined for different laser conditions.Keywords:
Plasma channel
Abstract A secondary concentric cylinders cell was used to measure the thermal conductivity of binary mixtures of argon‐nitrogen, argon‐helium, and argon‐neon approximately 20, 40, 60, and 80 vol. % at 75°C, and pressures to 2,800 atm. Measurements were made also on the pure gases argon and nitrogen.
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When shock waves induced by pulsed electrical discharges in dielectric liquids are widely applied in industrial fields, it is necessary to improve the energy transfer efficiency from electrical energy to mechanical energy to improve the shock wave intensity. In order to investigate the effect of the plasma channel length created by the liquid electrical discharge on the shock wave intensity, a test stand of dielectric liquid pulsed electrical discharge is designed and constructed. The main capacitor is 3 μF, and the charging voltage is 0–30 kV. Based on the needle-needle electrode geometry with different gap distances, the intensities of shock waves corresponding to the electrical parameters, the relationship between the plasma channel length and the deposited energy, and the time-resolved observation of the plasma channel development by a high speed camera are presented and compared. The shock wave intensity is closely related to the power and energy dissipated into the plasma channel. The longer plasma channel and the quicker arc expansion can lead to a higher power and energy deposited into the plasma channel, which can activate a stronger shock wave.
Plasma channel
Intensity
Pulsed Power
Electric potential energy
Electric shock
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Plasma cleaning
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It is a well known but puzzling result that zones within star formation regions sometimes show molecular hydrogen emission at very high (∼100 km s−1) velocities. These kinds of observations are somewhat difficult to explain because non-magnetized, J-type shock waves of velocities above ∼20 km s−1 mostly dissociate the molecules present in the preshock medium, and therefore produce almost no H2 emission. We quantify this result by presenting models of steady shock waves moving into a molecular environment, which show that the H2 molecules are indeed dissociated in the immediate postshock region for higher shock velocities. We argue that the total destruction of molecules by high-velocity shocks is a direct result of the assumption of an instantaneous ‘turning on’ of the flow that is generally done in computing shock models. We present models in which a shock wave gradually accelerates over a period of ∼1000 yr as would be expected, for example, from the ‘turning on’ of an outflow from a young star. We find that such shock waves are indeed able to accelerate significant masses of molecular material to velocities of ∼100 km s−1, and are a plausible explanation for widely observed high-velocity H2 emission.
Outflow
Hydrogen molecule
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Argon gas
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Low pressure broadening and shift of four spectral lines of argon, 591.2, 687.1, 693.7 and 737.2 nm have been investigated. The values of the pressure broadening and shift coefficients for argon-argon, argon-neon and argon-helium interactions in the glow discharge are determined. For all lines in the pure argon a red shift and in the argon-helium mixture a blue shift has been found. In the argon-neon mixture the shifts of these lines are small and have different signs.
Blueshift
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We have observed the structure and velocity of laser-driven shock waves in aluminum foils. We have measured shock velocities as high as 13 km/s and shock luminosity rise-times less than 50 ps, and we have inferred pressures of 200 GPa and shock-front thicknesses 0.7 \ensuremath{\mu}m. These results suggest that such techniques may be used for measuring equation-of-state parameters and studying the detailed structure of shock fronts.
Shock front
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Isotopes of argon
Argon gas
Gravimetric analysis
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