Other
Scientific paper
Jan 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990phdt.......113s&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, BERKELEY, 1990.Source: Dissertation Abstracts International, Volume: 52-04, Section: B
Other
Scientific paper
The objective of this work has been to develop laser spectroscopic methods to diagnose partially ionized plasmas. It is motivated by the need to increase understanding of the role of flow non-uniformities and plasma/wall interactions in plasma devices by making in-situ measurements of electric field strength, ion mobilities, concentrations and temperatures in a non-intrusive fashion that allows point-, one-, and two-dimensional imaging. We have focused on methods that are based on observing the Doppler shift in ionic spectra due to an ion drift velocity. Two particular methods we have been working with are Velocity Modulation Laser Spectroscopy (VMLS) and Two-Beam Doppler Shift Laser Spectroscopy (TBDSLS). The scientific approach has been to use conventional laser induced fluorescence (LIF) to probe ion absorption line profiles. If there is an electric field present, the ions will experience a net force and undergo drift, resulting in a shift in the position of the line profile. This shift can be related to the ion mobility or the electric field component along the probe direction if one or the other is known. Temperature and concentration can be recovered by conventional laser spectroscopic means. A well characterized, flame assisted plasma has been developed to use for investigation of the laser based spectroscopic methods. The Lawton-Weinberg theory has been used to describe the plasma. This theory predicted the shape of the plasma I/V characteristic curves from which an estimate of the ion number density and mobility could be obtained. An experimental investigation of the I/V curves of the barium seeded, flame assisted plasma, helped in establishing the optimum conditions for the laser measurements. LIF has been used to measure the absorption line profile of the barium ion. VMLS and TBDSLS were then investigated. The experimental results were used to determine a detectability limit for the LIF signal and line shift. The diagnostic is limited by signal to noise ratio rather than Doppler shift resolution. Spatially resolved LIF measurements have been made to study the ion density profiles under the action of applied electric field. The plasma temperature distribution was obtained from the LIF data using a new method of fitting single spectral line data to a Voigt profile. Both the ion density and the temperature profiles featured a parabolic distribution characterstic of a developed boundary layer. Finally, Langmuir probe measurements have been performed to estimate the plasma electric field distribution. The results of these measurements showed that a field of the order of few tens of V/cm existed in the plasma. This field is sufficient to cause a Doppler shift of the order of the line width, however, the drop in ion number density was limiting.
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