Flow field imaging through sharp-edged atomic and molecular 'notch' filters
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abstract
Sharp cut-off atomic and molecular notch filters simultaneously provide high spectral resolution and allow imaging by collecting light over a wide field of view. Many important properties of flow fields can be observed by imaging light elastically scattered from small particles, molecules or electrons. In order to extract information about the flow field from elastic scattering, the spectrum of the scattering must be resolved and the background scattering must be suppressed. Very high resolution, on the order of a few tens of megahertz, is usually required. The spectrum of the scattered light is broadened and shifted by the motion of the scatterers. For particles, which have relatively little thermal or acoustic motion, the spectral shift is only a function of the velocity. For molecules, the scattering spectrum is a function of the temperature, velocity and pressure of the gas as well as its composition. For electrons, the spectrum is a function of the electron temperature and electron number density in a plasma. In this paper, sharp edged notch filters made of rubidium, iodine or mercury vapour are used to image shock wave and boundary layer structure by Rayleigh scattering from particles, to image gas pressure, velocity and temperature by molecular. Rayleigh scattering, and to measure electron temperature and electron number density by Thomson scattering. For molecular scattering, filter transmission is generally a function of velocity, temperature and pressure, but, under some circumstances, it is a function of only one or two variables, so a notch filter can provide single-pulse images of a specific flow field parameter.
