Advancements In Laser Rayleigh Scattering Diagnostics For Selected Gas Properties

Abstract

Novel approaches and applications for laser Rayleigh scattering (LRS) and filtered Rayleigh scattering (FRS) are introduced. FRS combines a tunable, narrow linewidth laser to induce Rayleigh-Brillouin scattering from the molecules of a gaseous flow or mixture, with a narrowband notch filter that is based on an absorption line of an atomic or molecular gas vapor. The scattered light is passed through this filter, and the Rayleigh scattering signal is resolved while the stray scattering is significantly reduced. The Rayleigh scattering signal becomes modified as a result, and transforms a measurement initially dependent on number density to one that is dependent on gas temperature, pressure, and velocity, among other experimental parameters. Here, a model of FRS sensitivity, the first derivative(s) of the signal, is theoretically developed and analyzed across a range of parameters such as observation angle and filter conditions in Chapter 2. The sensitivity model is discussed in the context of a direct pressure measurement capability, i.e. a measurement that is inferred from pressure alone. Experimental demonstrations accompany these modeling efforts in Chapter 3.

LRS is used for flow visualization of cold atmospheric plasma jets in order to determine the flow regime transition in Chapter 4. FRS is also applied for the novel application of identifying and discerning two gases with similar scattering cross-sections, argon and molecular nitrogen. Applications for high-speed flows are also considered for the measurement of density in the presence of turbulent fluctuations.

Lastly, LRS is explored as a diagnostic for the measurement of number density of neutral species near the edge of a fusion device, a tokamak in Chapter 5. To achieve this, a narrowband optical bandpass filter is used to significantly reduce the accompanying laser Thomson scattering (LTS). LRS and LTS can then be used to extract information on both the neutral species and the electron background based on a diagnostic called two-color scattering. Simulations of the Rayleigh scattering lineshape under electron background environments and calculations for the signals are performed to evaluate the range of optical and tokamak property parameters under which these measurements may be possible.