Eur. Phys. J. Appl. Phys. 49, 13104 (2010) DOI: 10.1051/epjap/2009195 Regular Article T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS Time varying afterglow emission and gas pressure in a pulsed N 2 gas microwave flowing discharge at reduced pressure A. Ricard a , F. Moser, S. Cousty, S. Villeger, and J.P. Sarrette LAPLACE, Universit´ e Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France Received: 30 June 2009 / Received in final form: 11 September 2009 / Accepted: 20 October 2009 Published online: 26 November 2009 – c  EDP Sciences Abstract. Flowing microwave N2 afterglows at reduced pressure are studied as a source of N-atoms. It is presently reported results obtained by pulsing the nitrogen gas flow with an electromagnetic (EM) valve before the plasma, starting with discharge conditions of continuous flow around 1 Torr. By keeping the microwave power tuned on as in continuous flow, it is obtained two successive plasmas: the first one at a pressure of 2–3 Torr when the flow gas is on and the second one at lower pressure (0.4-0.5 Torr) when the flow gas is off. The gas pressures were measured in a 5 L post-discharge chamber. The two plasmas are here analysed by emission spectroscopy and the transmitted powers are deduced from the N + 2 /N2 intensity ratio for both cases. It is obtained about the same power as for continuous flow when the flow gas is on with a negligible reflected power. The transmitted power is lower and thus the reflected power is important when the flow gas is off. The emission of the N2 1st positive system at 580 nm, signature of the N-atom recombination in the afterglow, is observed in the reactor after the gas pressure rise, with a maximum intensity obtained for a duty cycle of 0.5 at a period of 2 s, corresponding to a maximum N-atom density of about 10 15 cm -3 and a pressure variation Δp in the range 2.3-0.5 Torr. PACS. 82.33.Xj Plasma reactions – 52.70.-m Plasma diagnostic techniques and instrumentation 1 Introduction Production of N-atoms has been previously studied in con- tinuous flowing microwave afterglows at medium gas pres- sures (1-10 Torr) [1]. Bacteria inactivation by N-atoms in such post-discharge reactors has been largely studi- ed [1–3]. In the cited previous works, the N-atoms were pro- duced in continuous flow microwave plasmas with an in- jected microwave power of 100 W. The plasma is generated inside a quartz tube of int. dia. (i.d.) 5 mm and extends along a few centimeters downstream the cavity (surfatron) gap for a gas pressure between 1 and 10 Torr and a nitro- gen flow rate between 0.5 and 2.0 slm. In these conditions, the plasma gas temperature rea- ched up to 1000 K near the surfatron gap [4], needing an air cooling of the external quartz tube, and decreased to the room gas temperature after a few centimetres of af- terglow. It is presently reported results obtained by pulsing the N 2 gas flow with a Parker electromagnetic solenoid valve at frequency up to 15 Hz, having opening and closing limit times of 1-2 × 10 -2 s. As the injected microwave power is kept constant, pulsed plasmas are created, corresponding to the discharges generated during the flow on and off time periods. a e-mail: ricard@laplace.univ-tlse.fr The pulsed flow induces pressure variations in the af- terglow reactor at a repetition frequency of a few Hertz. In the present paper, the transmitted microwave power in the pulsed plasma and the time-varying pressure and pro- duction of N-atoms in N 2 are specifically studied in the afterglow reactor. 2 The experimental set-up The continuous flowing microwave afterglow reactor de- scribed in previous studies [1–3] is reproduced in Figure 1. A two-stage oil rotary pump with a pumping speed of 800 slm is setup after a 5 L afterglow reactor, giving a limit pressure of 0.15 Torr. The N 2 flow rate is controlled and fixed in continuous flow to 1 slm. The pressure inside the reactor can be adjusted by means of a throttle valve. It is always kept fully open in the present study. A 2.45 GHz microwave plasma was produced by a sur- fatron cavity in a quartz tube of 5 mm i.d. and 30 cm in length. The discharge tube is connected to a pyrex tube of 17 mm i.d. and 10 cm in length setup before the 5 L afterglow reactor in pyrex. Emission spectra are recorded at two different posi- tions: near the plasma gap (position 1) and through the afterglow reactor (position 2) with an optical fiber con- nected to a Jobin-Yvon spectrometer equipped with a Article published by EDP Sciences