Study of the dark phase in the initial stage of the positive column formation in a neon glow discharge
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MPERATURE PLASMA
Study of the Dark Phase in the Initial Stage of the Positive Column Formation in a Neon Glow Discharge N. A. Dyatko*, F. E. Latyshev**, A. S. Mel’nikov**, and A. P. Napartovich* *Troitsk Institute for Innovation and Thermonuclear Research, Troitsk, Moscow oblast, 142090 Russia **Fock Research Institute of Physics, St. Petersburg State University, Ul’yanovskaya ul. 1, Petrodvorets, St. Petersburg, 198504 Russia Received May 19, 2005
Abstract—The initial stage of the positive column formation in a neon glow discharge is investigated both experimentally and theoretically. A decrease in the plasma radiation intensity (the so-called “dark phase”) was observed experimentally over a time period of about 1 ms. A similar dip was also observed in the time dependence of the electric field strength. The measured population of the lower metastable states of Ne was found to have a maximum at the beginning of the dark phase. A relevant theoretical model has been developed and used to perform calculations for the actual experimental conditions. A comparison between the numerical and experimental results shows that the model adequately describes the processes that occur during the formation of the positive column in a neon glow discharge. Experimental and theoretical studies show that the dark-phase effect is related to the excessive amount of metastable Ne atoms at the beginning of a discharge and, accordingly, to the high rates of stepwise ionization and chemionization. PACS numbers: 52.80.Hc DOI: 10.1134/S1063780X06020140
1. INTRODUCTION The term “dark phase” (DP) is used to describe a peculiar effect observed in the initial stage of the positive column formation in a glow discharge, when the plasma radiation is almost absent though the discharge current has nearly reached its steady state value. This effect was first observed in [1, 2], where glow discharges in pure helium and its mixtures with small additives of nitrogen or carbon oxide were investigated. It was found that, when the output voltage of the power supply was high enough, i.e., when the discharge current was mainly determined by the ballast resistance, the discharge displayed a number of specific features. In particular, after a short and very intense emission peak at the beginning of the discharge current pulse, the emission intensity from all the spectral lines and bands dropped and remained almost zero over a certain period of time. The DP lasted from a few tens of microseconds to a few milliseconds. The emission intensity then rapidly increased and reached a steady-state level (generally, after a few oscillations). In this case, the discharge current, which was controlled by the ballast resistance, remained almost constant throughout the discharge pulse. Note that a similar effect was observed in a continuous discharge after additional excitation of the plasma by a high-voltage nanosecond pulse [3, 4]. In [2], this effect was explained as follows. In discharges excited in mixtures of helium with a small additive of a molecular gas, the main io
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