1257 Russian Physics Journal, Vol. 48, No. 12, 2005 ON THE FORMATION OF NANOSECOND VOLUME DISCHARGES, SUBNANOSECOND RUNAWAY ELECTRON BEAMS, AND X-RAY RADIATION IN GASES AT ELEVATED PRESSURE V. F. Tarasenko and I. D. Kostyrya UDC 537.5 Properties of runaway electron beams and x-ray radiation in a nanosecond volume discharge in air at atmospheric pressure are investigated, and results of recent studies in this direction are analyzed. A physical nature of forming runaway electron beams and x-ray radiation in the nanosecond volume discharge is discussed. INTRODUCTION The first studies in which x-ray radiation was observed in a gas discharge at atmospheric pressure were carried out late in the 60s [1–4]. Gas diodes with the potential cathode having a small radius of curvature and the flat anode were used in the experiments. High-voltage short pulses were applied to the gap. X-ray radiation was registered in helium [1, 3] and air [2, 4]. Early in the 70s, a runaway electron beam was recorded behind the anode (foil) of a gas diode filled with air at atmospheric pressure [5]. These results gave impetus to the study of conditions of forming electron beams and x-ray radiation in various gases. Investigations were carried out at low and intermediate pressures (see reviews [6, 7] and the references therein) as well as at elevated pressures (see review [8] and the references therein). Investigations of forming runaway electrons in discharges in the atmosphere are summarized in review [9]. However, up to 2002 inclusive, the amplitudes of beam current recorded behind the foil at high pressures were very low. No more than 10 9 electrons [5, 8] were registered with the facilities analogous to those used in [4, 5] when the gas diode was filled by air at atmospheric pressure. For current pulse duration at half maximum of 1 ns, this corresponded to a beam current amplitude of several fractions of an ampere. Therefore, runaway electron beams registered in gas diodes have not found practical application. On the other hand, in [10–15] it was demonstrated that large dozes of soft x-ray radiation could be obtained in a nonuniform electric field in discharges in working mixes of CO 2 , exciplex lasers, and in air. The maximum energy of x-ray radiation was in the region of 5–20 keV, and the expose doze in the active volume was as great as 60–300 R [14]. X-ray radiation from the discharge was successfully used for preionization of lasers with transverse discharge, including wide-aperture lasers [12]. In recent studies [16–29] (see also reviews [30, 31]), supershort avalanche electron beams (SAEB [19]) were obtained in gas-filled diodes behind the metal foil. Their amplitude varied from several tens to several hundreds of amperes, and the pulse duration at half maximum was several hundreds of picoseconds. Moreover, as measurements [26–29] demonstrated, the duration of the beam current at half maximum behind the foil did not exceed 100 ps. In 2003–2005, SAEB were used to form a volume discharge in a CO 2 laser at atmospheric pressure [32] and to excite cathode luminescence in different crystals [33, 34]. The maximum SAEB amplitudes were registered in a volume discharge in a gap [19–26]. The duration of the current pulse running through the volume discharge plasma significantly exceeded the SAEB duration. The volume discharge was formed without additional preionization source and was observed in various gases [19–29]. In [19], this mode of the discharge was called VADIEB (volume avalanche discharge initiated by the electron beam). The VADIEB was used to excite generation of an IR laser on Ar-Xe mixture [35, 36]. The volume character of the discharge under these conditions High-Current Electronics Institute of the Siberian Branch of the Russian Academy of Sciences, e-mail: VFT@loi.hcei.tsc.ru. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 40–51, December, 2005. Original article submitted September 29, 2005. 1064-8887/05/4812-1257 ©2005 Springer Science+Business Media, Inc.