Experimental and numerical investigation of time evolution of discharge current and optical emission in helium-nitrogen cryoplasmas

Cryoplasmas represent a class of non-equilibrium plasmas whose gas temperature can be controlled below room temperature. However, so far, the influence of the plasma gas temperature on the plasma chemical reactions has not yet been examined in detail. Here we investigated the time-dependent reaction dynamics related to optical emission in helium-nitrogen cryoplasmas. We acquired voltage and discharge current waveforms, optical emission spectra, and observed a temporal change of the emission intensity in helium-nitrogen cryoplasmas at two experimental conditions (temperatures of temperature detector: 5 K and plasma gas temperature: 28 K (condition A), 40 K and 54 K (condition B)). Two time-dependent phenomena were observed: the first was a longer duration of the discharge current compared to that of helium emission at both conditions A and B, and the second was nitrogen ion emission delayed by about 8 mu s with respect to the emissions of atomic helium and helium dimers at 40 K. The experimental observations could be reproduced qualitatively by a global reaction model, which took into account the effect of the plasma gas temperature on the reaction rate constants and the diffusion coefficients. The simulations suggested that the reactions related to metastable helium atom were the key reactions, and that the long lifetimes of metastable helium atoms at cryogenic gas temperatures are crucial for the appearance of the time-dependent phenomena. These results imply that the plasma gas temperature is one of the key parameters in non-equilibrium plasma chemistry.