Characterization Of Moderate Pressure DBDs For CH₃I Decomposition And Iodine Atom Formation

The thesis aims exploring a novel method to produce iodine atoms via iodine-containing compound decomposition induced by DBD jet-like plasmas. Three discharge schemes were proposed. By comparing the breakdown threshold, temporal stability, spatial homogeneity and power consumption of the discharges among the schemes, a pin to ring DBD configuration was chosen due to its best performance in stability and homogeneity to study the effect of discharge condition parameters (such as discharge voltage, frequency and gas flow rate or pressure) on the production and yield of iodine atoms.By using Ar, N2and He as carrier gas, CH3I was mixed and introduced into the discharge gap as iodine atom precursors and a stable DBD state with uniform glow can be achieved in the chosen configuration. With mixing ratio from5%to100%of CH3I, stable DBD discharges can be sustained with a volume of0.5cm3for more than30minutes under moderate pressure (500-11000Pa) by average cost power from3watts to25watts. The mixed CH3I molecules were decomposed with high efficiency by the discharge plasma and the resultant iodine atoms were carried heavily by the plasma jet through a Laval nozzle into the downstream chamber full of stable afterglow with pressure of200-1000Pa. The gas temperature of the afterglow in downstream was determined about400-450K by simulating the second positive band of N2from the afterglow and was found almost constant even if the discharge power was raised.The emission spectra from300nm to950nm was measured for the DBD plasma and the afterglow and used to probe the trends of the plasma parameters and iodine atom production. In all the measured spectra,4lines of iodine atom were identified and attributed to the transition6P-6S of I and a lot of feature bands or lines of N2、 He and Ar also appeared. As an indicator of the iodine atom formation, the emission intensity of I905.83nm was used to present the yield of iodine atom The normalized intensity of I905.83nm to the strongest feature line of the used carrier gases was characterized to the discharge conditions for the trends of iodine atom production, based on which the production and loss rate were traced in the DBD region and the afterglow with the discharge conditions.