ISSN 1063-780X, Plasma Physics Reports, 2013, Vol. 39, No. 1, pp. 51–61. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.A. Gurashvili, N.A. Dyatko, I.V. Kochetov, A.P. Napartovich, D.I. Spitsyn, M.D. Taran, 2013, published in Fizika Plazmy, 2013, Vol. 39, No. 1, pp. 60– 70. 51 1. INTRODUCTION Non-self-sustained discharge sustained by fast electron beams is often used to pump the gain media of fast-flow CO lasers [1, 2]. There are two schemes of electron-beam-sustained gas-discharged CO lasers, with subsonic and supersonic gas circulation [3]. In the subsonic scheme, the gas velocity is relatively low and, accordingly, the residence time of the gas in the discharge region is relatively long. In this case, the required value of the specific energy deposited in the gas mixture is reached at relatively low current densi- ties of the fast electron beam. In the supersonic scheme, the specific deposited energy can be increased by increasing the current density of the fast electron beam and/or the voltage applied to the dis- charge gap. The maximum beam current density is limited by the thermal resistance of the foil that sepa- rates the vacuum volume of the ionizer from the gas- discharge chamber (GDC). The increase in the dis- charge voltage results in a streamer breakdown of the discharge gap, which may lead to the destruction of the foil if it is used as an electrode [4]. Therefore, an additional electrode—the so-called antistreamer grid—is usually used in the GDC [5]. It is placed in parallel to the plane of the foil at a certain distance from the output window of the ionizer and serves to protect the foil from destruction when streamer break- down of the gap occurs [6]. The electrode grid intercepts a fraction of the fast electron beam, which leads to a decrease in the rate of gas ionization in the gap. Therefore, it is desirable that the coefficient of transmission of fast electrons through the grid be as large as possible. On the other hand, even in the presence of the grid, a fraction of the discharge current flows through the foil. The higher the coefficient of transmission through the grid, the higher this fraction and the higher the probability that, in the case of breakdown, the streamer will be closed through the foil, rather than through the grid. There- fore, the fraction of the discharge current flowing onto the foil can serve (at least qualitatively) as a criterion of the GDC reliability with respect to the destruction of the foil during a streamer breakdown. In the present work, we calculated the spatial distri- butions of the rate of gas ionization by the electron beam and the electric field and discharge current den- sity in a GDC with an electrode grid. The simulations were carried out for the conditions typical of a CO laser with supersonic circulation of the gas mixture [7–9]. The fraction of the discharge current flowing onto the foil was found. The voltage drop near the grid surface that occurs due to the inhomogeneous distri- bution of the discharge current density was deter- mined. The dependence of the calculated values on the geometric parameters of the electrode grid and its position with respect to the ionizer was investigated. 2. FORMULATION OF THE PROBLEM Figure 1 shows the scheme of the GDC under study. Such a scheme is typical of electron-beam-sus- tained gas-discharge CO lasers [10]. The fast electron beam is formed in the ionizer. The output window of DIAGNOSTICS OF PULSED SYSTEMS Calculation of the Field and Current Density Distributions in a Non-Self-Sustained Discharge in the Discharge Chamber of a CO Laser V. A. Gurashvili, N. A. Dyatko, I. V. Kochetov, A. P. Napartovich, D. I. Spitsyn, and M. D. Taran Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow, 142190 Russia e-mail: dyatko@triniti.ru Received June 8, 2012 Abstract—Two-dimensional spatial distributions of the electric field and current density in a non-self-sus- tained discharge controlled by a fast electron beam were calculated in the quasineutral plasma approxima- tion. The calculations were carried out for a gas-discharge chamber with an antistreamer electrode grid placed in parallel to the output window of the ionizer. The voltage drop near the grid surface that appears due to the inhomogeneity of the spatial distribution of the current density was calculated. The fraction of the dis- charge current that passes the grid and flows onto the foil separating the vacuum volume of the ionizer from the gas-discharge chamber was estimated. The dependence of the calculated values on the geometric param- eters of the electrode grid and its position with respect to the output of the ionizer was analyzed. DOI: 10.1134/S1063780X12120033