Time varying afterglow emission and gas pressure in a pulsed N

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