Paper PS1-ThM1
Plasma Diagnostics in Microdischarges Using Laser Scattering

Thursday, October 23, 2008, 8:00 am, Room 304

Laser scattering experiments were performed in high pressure (100s of Torr) DC microdischarges operating in argon or nitrogen. Laser Thomson Scattering (LTS) and Rotational Raman Scattering were employed in a novel, backscattering, confocal configuration to measure important plasma parameters. LTS allows direct and simultaneous measurement of both electron density (n_e) and electron temperature (T_e). LTS experiments in microdischarges are challenging because of the low signal and excessive stray light. Measurements were performed at the center of the gap of a parallel plate slot-type microdischarge with plate separation of 600 microns. This location corresponded to the positive column of the DC microdischarge. For 50 mA current and over the pressure range of 300 – 700 Torr, measurements yielded T_e = 0.9 ± 0.3 eV and n_e = (6 ± 3)∙10¹³ cm^-3, in reasonable agreement with the predictions of a mathematical model. In order to obtain absolute values of the electron density, calibration of the Thomson scattered intensity was carried out using Raman scattering in nitrogen. This Rotational Raman spectroscopy was also employed to measure the gas temperature (T_g) in nitrogen DC microdischarges. Gas temperatures were determined by matching experimental spectra to synthetic spectra obtained by convolution of theoretical line intensities with the apparatus spectral resolution, with T_g as the adjustable parameter. Measurements were performed for a set of N₂ pressures (P = 400 – 600 Torr) and over the current range of 5 – 30 mA. In the center of the interelectrode gap, T_g changed from 450 ± 40 K at 5 mA to 740 ± 40 K at 30 mA. The gas temperature was nearly independent of pressure within the error of the experiment. Advantages and limitations of the laser scattering techniques employed will also be discussed.