Eur. Phys. J. Appl. Phys. 30, 201–204 (2005) DOI: 10.1051/epjap:2005022 T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS The impedance of the discharge positive column at medium pressures under different metastable level decay schemes S. Sobhanian 1, 2, a , S. Khorram 1, 2 , and A. Muradov 1, 3 1 Research Institute For Fundamental Sciences, Tabriz 51664, Iran 2 Centre For Applied Physics Research, University of Tabriz, Tabriz 51664, Iran 3 Dept. of Physical Electronics, Baku State University, Baku 370148, Azerbaijan Republic Received: 3 February 2004 / Received in final form: 5 September 2004 / Accepted: 21 December 2004 Published online: 11 March 2005 – c  EDP Sciences Abstract. The impedance of a positive column has been calculated at medium pressures i.e. at the regime of step ionization-ambipolar diffusion. Calculations were carried out for two cases: (a) metastable level decay by electron impact; (b) metastable level decay on the walls as a result of diffusion. In both cases the impedance has generally an inductive character. In the case of metastable level decay by electron impact, the impedance curve starts from the coordinate origin at ω = 0. At low frequencies the PC has a negatron property, while the static volt-ampere characteristics of the stationary PC becomes parallel to the current axis. The module |Z| reaches its maximum value at frequencies of about the order of  τ -1 a αmn0. At very high frequencies ω -→ ∞, the impedance reaches the real value of Z = R0. In the case when metastable levels decay as a result of diffusion, the impedance curve starts from the negative real values at ω = 0. The volt-ampere characteristics of the stationary PC drops. The module |Z| reaches its maximum value at frequencies of about the order of  τ -1 a τ -1 m . At very high frequencies ω -→ ∞, the impedance reaches the real value of Z = R0. In both cases, impedance curves have single structured shapes in spite of existence of several characteristic time scales of different order. PACS. 51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.) – 52.20.-j Elementary processes in plasmas 1 Introduction The dynamical properties of a positive column (PC) may be described by the help of its impedance (i.e. dy- namical complex resistance). Dynamical characteristics or impedance curves provide more complete information about plasma processes and they represent in this context important application interests. The shape of the dynam- ical characteristics permits the determination of different production and loss mechanisms of charged particles, their lifetimes, and discharge equivalent circuits [1–4]. So it is possible to study self oscillations, instabilities and their elimination in a plasma using the dynamical characteris- tics method [5–7]. To calculate the impedance curves it is necessary to know the rates of direct and stepwise ionizations and ex- citations, their derivatives with respect to the longitudinal electric field intensity. It is also indispensable to know the populations of excited levels, their lifetimes and rates of elementary processes. It is possible to obtain such detailed and extensive reliable data only for plasma parameters in the range obtainable by combined probe and spectroscopic a e-mail: sobhanian@tabrizu.ac.ir measurements. To obtain outside this range the deriva- tives of reaction rates with respect to E, one should change the plasma parameters over a wide range and this leads to qualitative changes of the PC and doubtful results. At low pressures charged particles appear as a result of direct ionization and disappear on the walls due to am- bipolar diffusion or free fall. At high pressures the main way of producing charged particles is stepwise ionization, whereas the main loss mechanism is recombination. There is a quite wide range of medium pressures between these two conditions, where charged particles appear as a result of stepwise ionization and disappear on the walls due to ambipolar diffusion. In the present work the impedance of PC at medium pressures, i.e. in the regime of step ionization-ambipolar diffusion, has been calculated. Calculations were per- formed under conditions when the probe measurements of electron energy distribution function (DF ) and other pa- rameters are possible. Under the above mentioned regime reasonable assumptions could be fulfilled. The availability of much experimental data at this conditions [8] permits Article published by EDP Sciences and available at http://www.edpsciences.org/epjap or http://dx.doi.org/10.1051/ep