Time varying afterglow emission and gas pressure in a pulsed N₂ gas microwave flowing discharge at reduced pressure

LAPLACE, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France

Corresponding author: ricard@laplace.univ-tlse.fr

Received: 30 June 2009
Revised: 11 September 2009
Accepted: 20 October 2009
Published online: 26 November 2009

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 N₂⁺/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 10¹⁵ cm^-3 and a pressure variation Δp in the range 2.3-0.5 Torr.