There are a number of industrial applications of laser ablation in a gas atmosphere. When an intense pulsed laser beam is irradiated on a solid surface in the gas atmosphere, the surface material is ablated and expands into the atmosphere. At the same time, a spherical shock wave is launched by the ablation jet to induce the unsteady flow around the target surface. The ablated materials, luminously working as tracer, exhibit strange unsteady motions depending on the experimental conditions. By using a high-speed video camera (HPV-X2), unsteady motion ablated materials are visualized at the frame rate more than 106 fps, and qualitatively characterized.
The aim of the present study is to clarify the time-dependent characteristics of the impulsive force generated by
irradiating a laser pulse onto metallic and polymer materials. A Velocity Interferometer System for Any Reflector
(VISAR) is employed to measure the acceleration driven by the laser ablation. The VISAR has two delay-lines that
enable the velocity measurement in the range from 10 m/s to 100 m/s. The ablation impulse is inferred from the
measured acceleration history. The influence of the ambient air on the ablation pressure is investigated for aluminum
using a Nd:YAG laser (wavelength: 1064 nm, pulse energy < 1 J, pulse duration ~ 10 ns) and for a polymer material
using a CO2 laser (wavelength: 1.06 μm, pulse energy < 10 J, pulse duration ~ 2 μs). The results of the preliminary
experiments revealed the promising potential of the VISAR measurement.
For the purpose of constructing the design rules of air- breathing laser launcher, the expansion of plasma, which was produced from air by focusing pulse laser, was investigated. A 10-J-pulse TEA CO2 laser was used for these experiments. First, plasma was formed in a quiescent atmospheric air. Photographs and shadowgraphs were taken using an ICCD camera with high-speed gating. From the photographs, the propagation velocity along the laser axis was measured. The velocity was found around 104 m/s much higher than that of a detonation wave calculated using LSD model. Since laser intensity at the plasma front was below the threshold for LSD regime, absorption/expansion mechanism other than LSD might predominate under the experimental condition. Shadowgraphs were taken to measure the expansion velocity of plasma and that of shock wave around plasma. The flow facility, in which plasma is formed in Mach 2 stream, was presented.
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