Time-resolved measurements of OH radicals in pulsed barrier discharge

Abstract

Summary form only given. Hydroxyl (OH) radicals generated within a pulsed barrier discharge in an open air type reactor were measured using a gated ICCD camera and LIF system. The barrier discharge is widely used to generate high concentration of ozone for conventional water treatment systems. However, the persistent materials that have higher oxidation potential than ozone must be treated using OH radicals generated by a plasma reactor or a reaction of O3 with ultraviolet irradiation. The time resolved spatial distribution of OH is quite important because a reacting method of OH with water is strongly affected by the lifetime and the distribution of OH. The barrier discharge driven by AC voltage source consists of many micro discharges where the current density is too high to generate OH radicals efficiently. Therefore the pulsed barrier discharge was used for generating uniform plasma that has no constricted portion within the plasma. The electrode system consists of one glass plate and a perforated metal electrode. The effective diameter of electrode is 70 mm and the gap is 2.5 mm. Pulsed voltage of 1 microsecond width 15 kV peak was applied by switching a ceramic capacitor of 2 nF at the frequency of 10 Hz. The dry or humid gas such as Ar, N2, Ne, Xe, He, O2 was fed from the center hole of the metal electrode. The emission intensity of OH radicals strongly depends on the gaseous species. With Ar feed, the highest OH emission of 309 nm was obtained. The nitrogen molecules in ambient air influenced the spectra when the mass of feed gas was smaller than that of nitrogen. Visible light pictures showed that a uniform but a weak discharge formed just after the pulsed voltage application and then tuned into bright micro discharges. The temporal and spatial distribution of OH was measured by ICCD camera with band-pass filter of 310 nm. Uniform OH distribution was observed during the initial period but not observed after the generation of bright micro discharges. The intensity of LIF showed the similar result. These observations support that a uniform discharge not a constricted discharge is suited for effective generation of OH radicals