D. Dufresne

Pulsed CO2 laser-induced effects on water droplets

Pulsed high-power CO2 laser beams propagating through the atmosphere can be affected by such different linear or nonlinear phenomena as aerosol and molecular absorption, scattering, turbulence, thermal blooming, or optical breakdown, depending on the atmospheric characteristics and laser parameters. An experimental investigation was carried out to study the pulsed CO2 laser-induced effects on water droplets. Water droplets with radii of 8-400 /im were irradiated. The average fluences used for the interaction were between 1 and 4 J/cm2 (1.6-6.4 J/cm2 on the droplet) for a pulse duration adjustable between 2 and 6 /is. Most measurements were made using an Imacon 790 converter camera allowing framing rates of between 104 and 2 x 107 frames/s with fast framing and a scanning rate of between 1 ns/mm and 1 /is/mm with streak photography. The experimental results presented in this paper identify four main mechanisms that can occur sequentially throughout the laser interaction, without specifying which dominate over the others: vaporization, deformation, shattering, and propulsion. In particular, with time, evolution of the hot vapor and shock wave produced by the vaporization of a single water droplet was observed. Evolutions of the droplet deformation with time is shown. Threshold values and characteristic shattering times of the liquid particles are given, as well as the drop velocities and ejection directions across the area illuminated by the laser pulse.

Experimental study of CO2‐laser‐induced air breakdown over long distances

Results of an experimental study on air breakdown produced by radiation from a high‐power CO2 laser are presented. From these measurements, the breakdown threshold over a flux range (1.5×108 <φ<7×108 W cm−2) is obtained as a function of the focal length varying from 1 to 55 m. The first of these experiments was carried out in the laboratory atmosphere (10<f/D<100), whereas the second part (f/D=135) was done outdoors under typical atmosphere conditions. The temporal evolution of the energy transmitted through the breakdown region for different laser parameters (5<Ei<220 J) was found.

Pressure and impulse on an aluminium target from pulsed laser irradiation at reduced ambient pressure

An experimental investigation of the time‐dependent pressure induced on an aluminium target from a pulsed CO2 laser irradiation at reduced ambient density has been made. As the pressure of the air surrounding the target decreases from 760 to 0.5 Torr, the width of the pressure pulse on the target becomes narrower, and the amplitude of the pressure increases. A pressure as high as 1.7 kbar and a mechanical coupling of 20 dyn sec J are obtained.

Propagation of Pulsed Laser Energy Through the Atmosphere

An experimental investigation has been conducted to study pulsed laser energy propagation over long distance through the atmosphere. Typical gain switched pulse of the CO2 cold cathode electron gun laser (160 J, 2.5 /AS) shows an initial high-power spike (5.6 x 108 W, 50 ns FWHM) followed by a high-energy tail (130 J). The laser beam is focused by means of of a 5 X telescope outside the laboratory. Spatial and temporal distributions of intensity and energy density are measured in the breakdown area by burning of thermosensiti ve paper (envelope of the focused laser beam) and by use of calibrated Kalvar film (focal spot). Experimental determination of the air breakdown average threshold (l-2x!08 W/cm2, 3-5 J/cm2) leads to the conclusion that such a phenomenon is initiated on the spike of the pulse by > 4-^m-diam aerosol particles. A particle optical counter is used for in situ measurements of aerosol distribution (axially scattering spectrometer probe, calibrated in the 0.5 ftm

Atmospheric Propagation of Two C02 Laser Pulses

An experimental investigation has been conducted to study the propagation of a high-energy laser beam (0£ >10 W/cm; FB >10 J/cm) in the atmosphere. At these intensities and fluence levels air breakdown can occur because of the interaction of the intense radiation with the aerosol particles naturally suspended in the path of the beam. An air plasma is created, which expands rapidly and consequently can have a detrimental effect on the energy propagation. This paper reports, first, the energy transmitted through the breakdown plasma as a function of the incident average energy density (ET/Ej 300 J/cm) and second, the possibility of increasing the transmission of the incident energy by the application of a precursor pulse as a function of the double-pulse separation time (controllable from a few microseconds to 1/10 s). In our particular experiment, the maximum increase (by a factor of three) is observed for At= 100-200 j*s, an effect due to the cleaning effect of the first pulse by vaporizing aqueous aerosol particles.

Measurement of high pressure induced in water by a CO2 laser pulse

The vaporization and thermoelastic pressures induced in water by a high‐energy CO2 laser pulse are measured as a function of the laser intensity. Below the air/water interface breakdown threshold, the vaporization peak pressure increases as the (6)/(5) power of the laser intensity and as the (2)/(3) power of the laser intensity above the breakdown threshold. The thermoelastic peak pressure is proportional to the laser power.

Study Of The Cavitation Induced By A High Power Laser Beam

This paper is concerned with a basic study of the cavitation induced by a high intensity laser beam. The present work is dealt in a first approach with the model of the initially only one spherical bubble, then followed by exploring the collective effects between several bubbles. The experimental setups producing one or several cavitation bubbles in the liquid (water) are at once described. The experimental apparatus (shadowgraph or Schlieren type) used to visualize bubbles and shock waves at high speed are also presented. Bubbles dynamics in infinite medium and near a solid wall are showed on sequential photographs (framing and streak). Oscillations, shock waves radiated upon collapse and jets developped towards the wall are also displayed. The comparison between experimental dynamics and numerical calculations prove that the evolution of optical cavitation bubbles is in good agreement with classical laws. This study of cavitation is extended to interacting effects when four bubbles are induced independently and simultaneously. The achieved visualizations show such interesting results as strong asymmetries, deformations, attraction or repulsion and premature collapses.

Laser-driven shock waves in an aerosol-induced breakdown in air

Aerosol-induced breakdown strongly limits the propagation of high-power laser pulses through the atmosphere. Indeed, when air breaks down due to an intense laser pulse, the high-pressure, high-density plasma generated is highly absorbing to the incident laser radiation. Plasma expands, fills the laser beam, and subsequently blocks off the laser energy. The purpose of this paper is to present a simple hydrodynamical model to simulate the development of the plasma resulting from the breakdown initiated by the aerosols hanging in the air. This model describes the expansion of an unsteady spherical shock wave. Experimental investigations (timeintegrated, framing, streak, and schlieren photographs recording the plasma luminosity and shock wave) have been conducted to evaluate the accuracy of the model.

Atmospheric Propagation of Pulsed High-Power Laser Beam Increase in Energy Transmission Using a Precursor Pulse

The propagation of pulsed high-power laser beams through the atmosphere is generally limited by different linear or nonlinear phenomena such as aerosol and molecular absorption and scattering, atmospheric turbulence, thermal blooming and optical breakdown. This phenomenon of breakdown due to the presence of small-size particles suspended in the path of the beam plays a preponderant role for relatively short and high-power pulses. In fact, optical breakdown leads to the formation of a plasma which expands, greatly absorbing the incident laser radiation. Energy blocking by the plasma depends principally on the density of the incident radiation and the concentration and size of the airborne particles. Consequently, by modifying the aerosol distribution along the beam path, it seems possible to increase the transmission of laser energy. Previously published experiments on pulses show the existence of a clearing effect of the atmosphere using a precursor pulse. We present an experimental investigation conducted inside and outside the laboratory to study this clearing effect, characterized by increases in air- breakdown threshold values and in the transmitted energy of the second pulse through the cleaned zone as a function of the precursor fluence (6-56 J/cm2), In- terpulse time (0.1-103-ms), concentration and size of the airborne particles and meteorological conditions (air temperature, relative humidity).

Surface Treatment by Laser Generated Shock Waves

A study of surface treatment by laser generated shock waves was carried out on various materials. A plasma trapping device enabled intense shock waves to be obtained with attenuated thermal effects. A hardening of 50 % over 1 mm of thickness was obtained on aluminium samples. Amongst the different surface treatment techniques which seem the most promising is that using a high power pulse laser. With the new technique it is possible to obtain intense shock waves over large surfaces of whatever form at the same time as attenuated thermal effects without creating bulk deformation. An experimental study of surface treatment by shock waves generated by an Nd-YAG pulse laser (30 J, 25 ns) was carried out on metals (aluminium, titanium, steel). To obtain high pressures (40 Kbar) with moderate energies and not very high laser intensities (109 W/cm2), a containment device for the plasma resulting from the interaction and the easily vaporizable materials was used. A analysis showed a hardening of more than 50 % over a surface of 0.6 cm2 throughout the thickness of an aluminium sample (1 mm). No hardening effect was observed for the other metals. These results were obtained without optimization of the trapping technique. Better control of this will enable a hardening effect to be obtained on other metals.