Cavity enhanced Thomson scattering for plasma diagnostics
Date
2019
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Abstract
Measurements of electron number density (nₑ) and electron energy distribution function (EEDF) are of great importance to the study of weakly ionized plasmas, such as those used in laser preionization, semiconductor processing and fabrication, electric propulsion devices, and atmospheric pressure plasmas. Currently, these parameters can be measured by physical probes, e.g. Langmuir probes, or with the use of non-intrusive Laser Thomson Scattering (LTS). While physical probe measurements have been an indispensable tool of the plasma physics community, they affect plasma source operation and result in unwanted plasma perturbation. LTS measurements are appealing due to the non-perturbing nature of the technique, but suffer from low signal levels and optical interference, making application to low-density plasma systems very challenging. This dissertation describes the development of a novel cavity enhanced Thomson scattering (CETS) diagnostic that enables sensitive, non-perturbing measurements of plasma properties. The technique is based upon frequency locking a high-power, narrow-linewidth continuous wave (CW) laser source to a high-finesse optical cavity to build-up intra-cavity power to a level where it can serve as an interrogation laser source. In this way, intra-cavity powers as high as ~12 kW have been generated from a ~5 W laser source and sensitive measurements on a plasma source and gas samples placed within the optical cavity were performed. Despite the CETS technique being widely applicable to a variety of plasma sources, this work focused on the measurement of electric propulsion devices, such as hollow cathodes and Hall effect thrusters. These devices are used as in-space propulsion systems on satellites and scientific probes and may be used as the primary in-space propulsion systems for exploration of the Moon, Mars, and beyond. This work describes the development of the CETS diagnostic including the cavity locking approach, creation of a gas and plasma scattering model, and the development of both a low- and high-power experimental instrument. CETS is demonstrated by performing rotational Raman and Rayleigh scattering measurements on a variety of gases and by performing Thomson scattering measurements in the plume of a hollow cathode. The cathode measurement campaign was conducted over a range of operating conditions, and electron densities and temperatures in the range of ~10¹² cm⁻³ and ~3 eV were measured. Finally, a mobile fiber coupled version of the CETS setup designed for use in large vacuum facilities is presented, and Thomson scattering measurements made with the mobile instrument in the plume of a hollow cathode are discussed.
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Subject
optics
propulsion
Thomson
plasma
laser
scattering