An interferometric investigation of laser−supported absorption waves

Abstract

Laser−supported detonation waves, other laser−supported absorption waves, and plumes of vaporized material were the subject of interferograms. Both instantaneous ruby laser interferometry and time−resolved argon laser interferometry were used to study these events. Qualitative behavior is seen in the interferograms which was not evident in photographic studies of these phenomena, such as the progress of radial shock waves and structure in the absorption zone. Abel inversion is used to obtain quantitative radial behavior of the change in the index of refraction. The differences between the laser−supported absorption waves ignited at the surfaces of several materials are discussed. Alumina specimens produced vapor plumes under these conditions whose temporal motion can be followed because of AlO absorption of the argon laser wavelength. The changes in the index of refraction were measured for similar events at the different wavelengths of the instantaneous and time−resolved interferometers and were used to determine the density profiles of electrons and neutrals. Compared to STP air densities, an ionization of a few percent is seen near a silica surface at laser beam intensities slightly below the ignition threshold for laser−supported detonation waves. An analysis of observed radial shock wave expansion rates predicts a neutral density which, when compared with the density determined from the Abel inversion technique, indicates that the shock front ionization is also a few precent.