Influence of Long- ived Species on EEDF in Afterglow Plasma

Experimental studies of Electron Energy Distribution Function (EEDF) in well-defined conditions in Flowing Afterglow plasma using Langmuir probe are reported. EEDF was measured in pure helium at medium pressures (1600 Pa) during late afterglow (35 to 95 ms after discharge) in nearly relaxed thermal plasma (Te ~ 300–600 K). The influence of fast electrons produced in interactions of helium metastable on body and tail of EEDF was observed. The experimental data are compared with simple model. Introduction It is well known fact that under broad range of experimental conditions in discharges in pure helium the number densities of helium metastable atoms (2S and 2S) can be appreciably higher than the electron number density. These “high energy” long-lived particles influence the property and the behaviour of the plasma [Wenig et al., 2006]. There are many measurements of Electron Energy Distribution Function (EEDF) in discharges (see e.g. publications of N.B. Kolokolov from St. Petersburg State University [Kolokolov et al., 1994]) and others plasmas [Maresca et al., 2002; Biberman et al., 1982; Kortshagen et al., 2002]. In general in these studies electron temperature was several electron-volts and usually only the high energy part (tail) of EEDF was precisely measured, partly because of the used modulation technique and partly because of difficulties in EEDF measurements in thermal plasma with electron temperature below 1 eV. In several studies groups of fast electrons populated in Penning ionization or in superelastic processes were observed [Gorbunov et al., 2002], in another experiment depletion of fast electrons from EEDF was observed [Kortshagen et al., 2002]. In comparison with plasma in active discharges the situation in afterglow plasmas in inert gases is simpler. In general in an afterglow the electron energy is not sufficient for ionization by electron impact, but because of the presence of highly excited long-lived neutrals (metastables) the electron gas can be heated by their energy (by superelastic collisions and by production of fast electrons in chemoionization). In our experiments we are using well characterized afterglow plasma – Flowing Afterglow (FA) in FALP experiment (FA with Langmuir Probe) to study electron-ion recombination (for details see e.g. [Glosik et al., 2001; Glosik et al., 2003; Novotny et al., 2006]). If fast recombining molecular ions are dominant in the Flowing Afterglow plasma, than recombination and the diffusion govern plasma decay (e.g. H5, Kr2 with rate coefficient > 10 cm s [Pysanenko et al., 2002]). If slowly recombining ions prevail in the Flowing Afterglow plasma (with rate coefficient < 10 cm s e.g. He2, KrH, XeH etc. [Korolov et al., 2006]) the plasma decay is slow and other processes are influencing the decay and the recombination process. In order to clarify conditions in FA plasma used for recombination studies in FALP experiments, we measured the EEDF during the plasma decay in pure helium and in helium with small admixture of reactant gases (H2, Xe in present experiments). The studies were carried out in conditions typical for FALP experiments with helium buffer gas. In discussed experiments helium pressure was pHe = 1600 Pa. EEDF were measured in late afterglow for decay time τ ~ 35–95 ms. At these conditions the EEDF is nearly Maxwellian with electron temperature close to buffer gas temperature Te = 300–600 K (dependent on energy balance). The increase of electron temperature is connected with presence of long-lived helium metastables. The obtained experimental results are discussed and compared with data obtained from a numerical model. Experiment The 3D picture and the schematic of the Flowing Afterglow with Langmuir Probe (FALP) are shown in Figures 1 and 2 respectively. The apparatus and gas handling systems are UHV compatible. Helium buffer gas flows through the glass discharge tube (12 mm in diameter), where plasma is WDS'07 Proceedings of Contributed Papers, Part II, 163–168, 2007. ISBN 978-80-7378-024-1 © MATFYZPRESS