Time Evolution of Metastable Helium Atoms in Atmospheric Pressure Plasma Jets

Atmospheric pressure plasma jets (APPJs) driven by kHz-ranged frequencies are well known as a highly periodic, stable discharge, consisting of repeated plasma generation and decay cycles. However, at the beginning of discharge, plasma jet is naturally transient because of unavoidable interactions with background neutral gases and sometimes liquids. Here, we investigate the time evolution of a metastable helium atom (He*) in APPJs during the ignition phase. Our plasma jets were operated using sinusoidal voltage waveforms with amplitudes of 3–4 kV at frequencies of 30–60 kHz in helium. To repeatedly monitor the ignition phase, the plasma jets were periodically turned on and off at intervals of 1–2 seconds, which were long enough to ignore memory effects from previous discharges. The plasma jets were characterized by He* with its number density measured using tunable diode laser absorption spectroscopy. The plasma jets stabilized completely after 100 discharge periods at most. Our results show that the ignition period decreased with increasing the amplitude and frequency of applied voltage. We noticed large fluctuations in laser absorption data were consistently observed during the ignition period in all cases, leading to the implementation of Schlieren imaging. The result reveals that the fluctuations were caused by perturbations in the neutral gas flow due to the electrohydrodynamic wind, resulting in air entrainment that complicates the plasma jet during the ignition period.