ISSN 1063-780X, Plasma Physics Reports, 2010, Vol. 36, No. 11, pp. 1000–1011. © Pleiades Publishing, Ltd., 2010. Original Russian Text © O.V. Bolotov, V.I. Golota, B.B. Kadolin, V.I. Karas’, V.N. Ostroushko, L.M. Zavada, A.Yu. Shulika, 2010, published in Fizika Plazmy, 2010, Vol. 36, No. 11, pp. 1059–1072. 1000 1. INTRODUCTION At present, considerable attention is paid to inves- tigation of gas discharges at atmospheric pressure, because they are widely used in many plasmachemical technologies, such as ozone synthesis, cleaning of flue gases from NO x and SO x , pumping of high-power technological gas lasers, disinfection, and plasma- chemical synthesis. Such a broad application of atmo- spheric-pressure discharge is related to the unique properties of the low-temperature nonequilibrium plasma that forms in the discharge gap and which is an efficient source of excited particles and active radicals. In recent years, discharges in short discharge gaps with a highly inhomogeneous electric field typical of point–plane electrode systems have received wide application. A barrier-free atmospheric-pressure gas discharge in a point–plane electrode system can oper- ate in different modes, depending on the electrode geometry and the polarity and amplitude of the voltage applied to the point electrode. In particular, for the positive polarity at the point electrode, the discharge can operate in a nonsteady mode in which the oscillo- gram of the discharge current represents a sequence of quasi-periodic pulses. The pulses are generated due to the presence of a space charge and the propagation of cathode-directed streamers in the discharge gap. The streamers have the form of bright filaments rapidly propagating between high-voltage electrodes in the gas-filled gap. The radius of these filaments is much smaller than their length. Classical studies by Loeb and Meek [1], as well as by Raether [2], demonstrated that the streamer is a highly nonlinear two-dimen- sional (2D) object. Streamer discharges are a striking example of highly nonlinear self-consistent phenom- ena. The nature of streamers is not yet quite clear, and the unified self-consistent theory of streamer genera- tion and propagation is still lacking. The absence of a satisfactory theory of streamers is related, first of all, to the complexity of relevant mathematical simulations, which require a large amount of computer resources. Therefore, in studying streamer discharges, one has to often use similarity laws based on dimensional rela- tions. Similarity invariants are very helpful in both car- rying out laboratory experiments and constructing theoretical models. As a rule, instead of calculating all the quantities characterizing the discharge, only invariants are calculated (or measured), which sub- stantially simplifies the problem. The similarity laws allow one to use the known properties of a discharge at a particular gas pressure to predict the discharge char- acteristics at other pressures of interest. It is worth not- ing that, in the physics of weakly ionized plasma, sim- ilarity laws are well established and used to identify the parameters of the discharge plasma at different gas pressures. However, although there are a large number of papers devoted to similarity laws in gas discharges, the ideas of the discharge similarity are still rather con- LOW-TEMPERATURE PLASMA Similarity Laws for Cathode-Directed Streamers in Gaps with an Inhomogeneous Field at Elevated Air Pressures O. V. Bolotov, V. I. Golota, B. B. Kadolin, V. I. Karas’, V. N. Ostroushko, L. M. Zavada, and A. Yu. Shulika National Scientific Center Kharkov Institute of Physics and Technology, vul. Akademichna 1, Kharkiv, 61108 Ukraine Received July 21, 2009; in final form, April 14, 2010 Abstract—Results are presented from experimental studies of cathode-directed streamers in the gap closure regime without a transition into spark breakdown. Spatiotemporal, electrodynamic, and spectroscopic char- acteristics of streamer discharges in air at different pressures were studied. Similarity laws for streamer dis- charges were formulated. These laws allow one to compare the discharge current characteristics and streamer propagation dynamics at different pressures. Substantial influence of gas photoionization on the deviations from the similarity laws was revealed. The existence of a pressure range in which the discharges develop in a similar way was demonstrated experimentally. In particular, for fixed values of the product pd and discharge voltage U, the average streamer velocity is also fixed. It is found that, although the similarity laws are violated in the interstreamer pause of the discharge, the average discharge current and the product of the pressure and the streamer repetition period remain the same at different pressures. The radiation spectra of the second pos- itive system of nitrogen (the C 3 Π u –B 3 Π g transitions) in a wavelength range of 300–400 nm at air pressures of 1–3 atm were recorded. It is shown that, in the entire pressure range under study, the profiles of the observed radiation bands practically remain unchanged and the relative intensities of the spectral lines corresponding to the C 3 Π u –B 3 Π g transitions are preserved. DOI: 10.1134/S1063780X10110097