Excitation and ionization of a diagnosis gas in front of the flexible μ tube plasma and in a diagnosis tube

The noble gases Helium, Argon, Krypton and even Xeon have been applied as discharge gases for the flexible μ tube plasma (FμTP) with comparable ionization efficiencies. It was assumed that a temporally and spatially limited potential plays a major role in the generation of reactant ions responsible for protonation in the surrounding air. However, the excited and ionized species of ambient air cannot be detected directly by emission spectroscopy because there is no known emission wavelength within the detectable range. In this work, a diagnosis gas was introduced as a substitute for the surrounding air to understand the excitation and ionization outside the discharge capillary. It was found that with Ne, Ar, Kr, Xe as plasma gases the diagnosis gas He can be excited during the positive half cycle. The emission of He 706 nm and N₂⁺ 391 nm propagate with and against diagnosis gas flow directions in case of He and Ne driven plasmas. In case of Ar, Kr, and Xe driven plasmas, the emission of both species mainly develops against the diagnosis gas and the shapes of He 706 nm are wide. Further on, the diagnosis gas He is also excited even though there is a glass wall between the plasma gas flow and diagnosis gas flow. These findings demonstrate that the transient potential is responsible for the excitation of the diagnosis gas, not penning ionization or charge transfer between discharge plasma and diagnosis gas.