Modification of Material Properties 334 Modification of the Near-Surface Layers of a Copper Foil under the Action of a Volume Gas Discharge in Air at Atmospheric Pressure M.A. Shulepov , V.F. Tarasenko, I.M. Goncharenko, N.N. Koval, and I.D. Kostyrya Institute of High-Current Electronics SB RAS, 2/3, Academichesky ave., Tomsk, 634055, Russia Phone: +8(3822) 49-14-43, Fax: +8(3822) 49-24-10, E-mail: MixShlp@yandex.ru Abstract – We have studied the modification of the near-surface layers of a copper foil under the ac- tion of a volume gas discharge, which was generated in air at atmospheric pressure by nanosecond high- voltage pulses of both negative and positive polarity applied between the foil and an electrode with a small radius of curvature. It is established that the surface layer of the discharge-treated copper foil in the central region is cleaned from carbon contami- nations, while oxygen atoms penetrate in depth of the foil. The depth of a cleaned layer depends on the polarity of voltage pulses. For the positive voltage polarity on the foil, the cleaning takes place up to a depth exceeding 50 nm, while oxygen penetrates up to a depth of about 25 nm. 1. Introduction Electric discharges of different types as well as elec- tron beams are now widely used for the modification of near-surface layers of various materials [1]. It was reported that a high-voltage diffuse discharge at a low gas pressure and a low power could be used for the inactivation of microbiological cultures [2]. As is known [3], a volume discharge can be generated using inhomogeneous electric field in gases at atmospheric pressure. For this purpose, high-voltage (~100 kV) pulses of nanosecond duration are applied to a gas- filled interelectrode gap. A specific feature of such discharges in air at atmospheric pressure is the ac- companying X-ray emission [3–5] and the formation of runaway electron beams [5]. In recent years, it was established that a volume discharge can be generated in air at atmospheric pres- sure in an inhomogeneous electric field for both po- larities of the high-voltage pulses applied to an elec- trode with a small radius of curvature [6, 7]. Another feature of volume discharges in inhomogeneous elec- tric fields, which are initiated by ultrashort avalanche electron beams (UAEBs), is the possibility of reaching high levels of specific power (up to 800 MW/cm 3 [8]) deposited in the discharge. In such regimes, runaway electron beams emitted from the discharge plasma are characterized by the beam current amplitudes reaching tens and hundreds of amperes (behind the foil), while the current pulse duration (defined as the pulse full width at half maximum, FWHM) does not exceed 100 ps [9]. Such discharges in inert gases are also accom- panied by high-intensity vacuum ultraviolet (VUV) emission [10]. Thus, the generation of UAEB-induced volume discharge in gases at high pressures leads to a com- plex action of the high-density nanosecond discharge plasma (with a specific deposited power of several hundred megawatt per cubic centimeter), UAEBs (with a broad energy spectrum), and radiation in various spectral ranges (including UV and VUV emitted from the discharge plasma) on the anode. Thus, the generation of UAEB-induced volume discharge in gases at high pressures leads to a com- plex action of the high-density nanosecond discharge plasma (with a specific deposited power of several hundred megawatt per cubic centimeter), UAEBs (with a broad energy spectrum), and radiation in various spectral ranges (including UV and VUV emitted from the discharge plasma) on the anode. This Letter presets the results of an experimental investigation of the modified near-surface layers of a copper foil upon the action of plasma of a UAEB- induced volume discharge in air at atmospheric pressure. 2. Setup and methods of experiment The experiments were performed using a discharge chamber, which is schematically depicted in Fig. 1. A distance from a flat copper foil to a tubular elec- trode could be varied within 8–16 mm. The interelec- trode voltage was supplied from an oscillator of the RADAN-220 type, which generated voltage pulses with an amplitude of ~230 kV (in the open-circuit regime), a FWHM of ~2 ns (on a matched load), and a leading front width of ~0.5 ns [11]. It was possible to change the polarity of generated voltage pulses ap- plied to the potential electrode. The discharge current was measured using a shunt composed of resistor chips connected between the foil anode and the dis- charge chamber housing. The results of measurements showed that the current pulse amplitude for both po- larities of the applied voltage pulse was ~3 kA and the total duration of the discharge current pulse was ~30 ns (the first half- period of the discharge current pulse had a duration of ~8 ns). With the negative po- larity of a voltage pulse applied to the electrode with a small curvature radius, the gas-filled diode generated a beam of runaway electrons. Under these conditions,