Optimization of ultraviolet emission and chemical species generation from a pulsed dielectric barrier discharge at atmospheric pressure

Abstract

One of the attractive features of nonthermal atmospheric pressure plasmas is the ability to achieve enhanced gas phase chemistry without the need for elevated gas temperatures. This attractive characteristic recently led to their extensive use in applications that require low temperatures, such as material processing and biomedical applications. The agents responsible for the efficient plasma reactivity are the ultraviolet (UV) photons and the chemically reactive species. In this paper, in order to optimize the UV radiation and reactive species generation efficiency, the plasma was generated by a dielectric barrier discharge driven by unipolar submicrosecond square pulses. To keep the discharge diffuse and to maintain low operating temperatures, helium (He) was used as a carrier gas. Mixed with helium, varying amounts of nitrogen (N2) with the presence of trace amounts of air were used. The gas temperature was determined to be about 350K at a 1-kHz pulse frequency for all cases and only slightly increased with frequency. The UV emission power density, PUV, reached its highest level when 5% to 10% of N2 is mixed to a balance of He. A maximum PUV of about 0.8mW∕cm2 at 10-kHz pulse frequency for a He(90%)+N2(10%) mixture was measured. This was more than four times higher than that when He or N2 alone was used. Furthermore, the emission spectra showed that most of the UV was emitted by excited NO radicals, where the oxygen atoms came from residual trace amounts of air. In addition to NO, NO2, and excited N2, N2+, OH, and He were also present in the plasma.