Numerical Simulation of Plasma Kinetics in Low-Pressure Discharge in Mixtures of Helium and Xenon with Iodine Vapours

Abstract

Due to ecological problems related to the utilization of gas-discharge ultraviolet (UV) mercury vapour lamps widely used in lighting technology, photochemistry, and photomedicine, there arises a need for developing new mercury-free sources of UV radiation using electron bands of rare-gas monohalides and halogen molecules as well as spectral lines of their atoms (Lomaev et al., 2003). The most high-power mercury-free lamps are UV emitters with “chlorine noble gas” active media emitting on transitions of the excimer molecules XeCl (308 nm) and KrCl (222 nm). The use of aggressive chlorine molecules in the gas mixtures of such emitters results in a comparatively low mixture life (1100 hours), which impedes their wide application in various optical technologies. It is mainly due to the absorption of chlorine by open metal electrodes (especially strongly heated cathode) and its heterophase chemical reaction with a quartz discharge tube accompanied by the formation of polymer compounds (chlorosiloxanes). That is why the replacement of chlorine molecules by less aggressive iodine ones in the working media of excilamps represents an urgent task. Ultraviolet radiation of glow discharge plasma in mixtures of helium with iodine vapours that is still transparent to air is mainly concentrated in the spectral ranges 175-210 nm and 320-360 nm. The mixture life of such lamps reaches 103 hours (Lomaev & Tarasenko, 2002; Shuaibov et al., 2005a, 2005b). The use of the 〉е-Хе-I2 active medium in a glow discharge lamp also allows one to obtain emission of the excimer molecule XeI(B-X) (253 nm) (Shuaibov, 2004 et al.). Moreover, of special interest is the fact that the wavelength of this transition is close to that of the most intense spectral line of the mercury atom in low-pressure gas-discharge lamps, which is used in a number of optical technologies. The basic spectral lines of the iodine atom (183.0, 184.5, 187.6, and 206.2 nm) (Liuti & Mentall, 1968) are also close to those of the mercury atom (184.9, 194.2, 202.7, and 205.3 nm) now used in the corresponding low pressure UV emitters. In this connection, it is important to optimize output characteristics of helium-iodine and xenon-iodine gas discharge emitters. The kinetics of plasmachemical processes in the gas discharge plasma in mixtures of noble gases with iodine molecules was till now studied only for high-pressure emitters excited by a barrier discharge (BD) in krypton-iodine and xenon-iodine mixtures (Zhang & Boyd, 1998, 2000). The conditions of BD plasmachemical