Brillouin scattering from a gas subject to a temperature gradient : optical considerations

Abstract

This thesis describes an attempt to use the Brillouin light scattering technique to observe nonequilibrium effects in gaseous sulphur hexafluoride subject to a temperature gradient. Due to the inherent geometry of the experimental setup, an imaging fibre optic probe was investigated as a means of scattered light collection. The SF₆ gas was housed in a moderate pressure gas cell. The scattered light collected was analyzed with a Fabry-Perot interferometer. A computer simulation of the behaviour of temperature, density and refractive index for gaseous SF₆ subject to a temperature gradient was performed using a 2 dimensional nonconvective heat flow model. A computer simulation of the scattered ray paths from the gas cell to the optical fibre in the imaging fibre optic probe was also performed. The sound speed in SF₆ at a pressure of 1.28 MPa and a temperature of 20°C was measured to be 133 ± 6 m.s⁻¹. Sound speeds in SF₆ for pressures from 0.44 MPa to 1.83 MPa were also measured. These sound speeds showed a larger nonlinearity in its changed with different pressures than that calculated from theory. Nonequilibrium Brillouin spectra for SF₆ were observed at pressures of 0.891 MPa and 1.35 MPa with temperature gradients of about 15.3 K cm⁻¹ and 18.9 K cm⁻¹, respectively. The Brillouin components at nonequilibrium showed an intensity increase of about 16% when compared to their counterparts at equilibrium.