A Study of Strong Thermal Interactions between a Laser Beam and an Absorbing Gas

Abstract

A recent study of temperature-dependent absorption, nonsteady beam propagation, and laminar to turbulent transition in a laser-induced convection column is described. The column is generated by passing a CO2-N2 laser beam vertically through a CCh absorption cell. The radial intensity distribution of the transmitted beam is scanned during the transient and quasi-steady-state absorption periods using a Ge:Au detector. During the transient period, relaxation and beam divergence effects are observed. In the quasi-steady period, a flickering of the beam profile from the initially Gaussian shape due to induced turbulent convection is observed at sufficiently high pressure. The onset of turbulence is measured with a fine-wire resistance thermometer and laser power meter. The transition boundary in terms of cell pressure p and incident laser power P0 at a fixed beam diameter and cell pressure is found to be given approximately by pPo = const. This can be shown to imply a constant transition Reynolds number for the induced flow under certain restrictive conditions. The transition Reynolds number so determined is found to be surprisingly low. Local gas temperatures measured near the edge of the laser beam indicate very strong absorption heating within the convection column at the lower cell pressures.