920 ISSN 1063-7761, Journal of Experimental and Theoretical Physics, 2017, Vol. 125, No. 5, pp. 920–925. © Pleiades Publishing, Inc., 2017. Original Russian Text © E.A. Sosnin, G.V. Naidis, V.F. Tarasenko, V.S. Skakun, V.A. Panarin, N.Yu. Babaeva, 2017, published in Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2017, Vol. 152, No. 5, pp. 1081–1087. On the Physical Nature of Apokampic Discharge E. A. Sosnin a,b, *, G. V. Naidis c , V. F. Tarasenko a,b , V. S. Skakun a , V. A. Panarin a , and N. Yu. Babaeva c a Institute of High Current Electronics, Siberian Branch, Russian Academy of Science, pr. Akademicheskii 2/3, Tomsk, 634055 Russia b National Research Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia c Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 13, str. 2, Moscow, 125412 Russia *e-mail: badik@loi.hcei.tsc.ru Received May 14, 2017 Abstract—Experimental and theoretical investigations of a diffuse jet (streamer) of apokampic discharge are carried out for various values of air pressure. It is established experimentally that this regime of pulse-periodic discharge is formed stage by stage. At the first stage, in a microsecond discharge of a voltage pulse of positive polarity, a potential spark channel formed during the first pulses between two needle electrodes is trans- formed into a diffuse channel. At the second stage, a weakly glowing halo is formed near the discharge chan- nel, and a bright offshoot arises near the bending point. Finally, at the third stage of discharge in the steady- state mode, for frequencies of a few to tens of kilohertz in each pulse, the offshoot becomes a source of plasma bullets (streamers) moving with a velocity of up to 200 km/s. As a result of simulation of a streamer in atmo- spheric pressure air under conditions corresponding to the experimental data, a propagation velocity of up to 400 km/s is obtained for the streamer. It is shown that the formation of a jet significantly depends on the air temperature. DOI: 10.1134/S1063776117100168 1. INTRODUCTION Today, a large variety of types of discharge in air are known [1], including discharges at high altitudes in the Earth’s atmosphere [2–6]. Variations in the gas environment (composition, gas pressure, impurities, etc.) and the excitation conditions (various parameters of voltage and current pulses, geometry of electrodes and material of which they are made, and so on) allow one to change the shape of discharge and the charac- teristics of discharge plasma. The most common types of discharges are described in detail in [1]. A transition to nanosecond high-voltage pulses allows one to form diffuse discharges at high pressures of air and other gases without using an additional source of preioniza- tion [7] and to obtain runaway electron beams of sub- nanosecond and picosecond durations [8]. Recently, during the study of discharge in air and nitrogen at atmospheric pressure, as well as in other gases, a new discharge mode has been implemented with increasing pulse repetition rate [9–16]. The phenomenon consists in the following: if microsecond high-voltage pulses with leading edge width of 300–500 ns are applied to one electrode of the discharge gap, while the second electrode (just as the discharge channel itself) are left at a floating potential, for example the circuit is closed through a capacitor C, then a long visually observed plasma jet is formed between the electrodes about perpendicular to the bending region of the discharge channel (Fig. 1a). In fact, both the discharge channel and the electrodes in such a system are at high potential (of a few kilovolt) with respect to the ground. The phenomenon was named an apokampic dis- charge, or an apokamp (from Greek απó (off) and καμπη (bend)), i.e., a discharge formed at a bending point. The term was given with regard to the formation rules of new scientific terms [17]. Figure 1a shows that, in the mode with an apokamp, the discharge channel 1 serves as a source of a halo 4, a bright offshoot 5, and an apokamp proper 6. The phenomenon is sensitive to the energy introduced into the discharge gap. For example, when the capacitance increases by a factor of 12 (Fig. 1b), the apokampic discharge turns into an ordinary pulsed spark: bright spots appear on the elec- trodes, the size of the thermal halo 4 significantly increases, but the apokamp is not formed. Accord- ingly, the plasma spectrum contains wide continua characteristic of arc discharges. A similar metamor- phosis occurs when the peak value of the voltage U a on electrode 2 increases. The discharge mode with an apokamp was imple- mented not only in air and nitrogen, but also in noble STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS