Role of Penning ionization in the enhancement of streamer channel conductivity and Ar(1s5) production in a He-Ar plasma jet

Abstract

Plasma jet devices that use a helium gas flow mixed with a small percentage of argon have been shown to operate with a larger discharge current and enhanced production of the Ar(1s5) metastable state, particularly in the discharge afterglow. In this experiment, time-resolved quantitative measurements of He(23S1) and Ar(1s5) metastable species were combined with current and spectrally resolved emission measurements to elucidate the role of Penning ionization in a helium plasma jet with a variable argon admixture. The plasma jet was enclosed in a glass chamber through which a flowing nitrogen background was maintained at 600 Torr. At 3%–5% Ar admixture, we observed a ∼50% increase in the peak circuit current and streamer velocity relative to a pure helium plasma jet for the same applied voltage. The streamer initiation delay also decreased by ∼20%. Penning ionization of ground-state argon was found to be the dominant quenching pathway for He(23S1) up to 2% Ar and was directly correlated with a sharp increase in both the circuit current and afterglow production of Ar(1s5) for Ar admixtures up to 1%, but not necessarily with the streamer velocity, which increased more gradually with Ar concentration. Ar(1s5) was produced in the afterglow through recombination of Ar+ and dissociative recombination of Ar2+ as the local mean electron energy decreased in the plasma channel behind the streamer head. The discharge current and argon metastable enhancement are contingent on the rapid production of He(23S1) near the streamer head, >5×1012 cm−3 in 30 ns under the conditions of this experiment.