Demonstration of low angle Thomson scattering for background interference suppression

In weakly ionized plasmas such as those associated with hypersonic flows, Thomson scattering (TS) measurements often suffer from background interference from Rotational Raman scattering (RRS) and plasma luminosity. To overcome this background interference, we propose a new concept, Filtered low-angle Thomson scattering (FLATS), the principle of which is based on the frequency narrowing of the Thomson scattering collected in the forward direction in contrast to the collection angle independent Rotational Raman scattering and plasma luminosity. In this work, we explain the principles of FLATS and present experimental observations of TS and RRS at multiple collecting angles, which confirm the narrowing TS and the frozen RRS at such angles. Additionally, successful spectral separation of TS from background scattering at a low-angle of 22 deg observation, which leads to improved measurement accuracy, is demonstrated. Combined with an atomic vapor prism that can serve as a dispersion-based spectrometer, this technique is expected to enable Thomson measurement in low-temperature, partially ionized plasmas in air/hypersonic environments.