Nonlinear effect of Gas Flow on Helium Metastable Atoms in Weakly Ionized Gas Jet

Helium metastable atoms (He^*) play a crucial role in low-temperature plasmas, particularly in weakly ionized gas jets (plasma jets). However, limited experimental information regarding He^* in plasma jets has been available. To our knowledge, such lack of data can be attributed, in part, to the wide range of plasma characteristics with various control knobs and operating conditions. As a consequence, using uncharacterized plasma can result in poor performance and an underestimation of plasma technology, leading to impetuous decisions for certain applications. The primary objective of this study is to investigate the nonlinear effect of gas flow on the dynamics of He^* in a plasma jet. A single pin-type source was generated using a 50 kHz bipolar square voltage waveform with a duty cycle of 10%, and the spatiotemporal distribution of He^*(2³S₁) was measured using tunable diode laser absorption spectroscopy (TDLAS). Through our TDLAS data, we first observed a wave-like pattern of He* formation preceding the main ionization wave, which was not detected by a conventional intensified CMOS camera. To better understand this observation, optical emission spectroscopy and flow simulation using COMSOL were conducted. Our results conclude that penetration of ambient N₂ into the plasma channel somehow induces this phenomenon. This work provides deep insights into the complex interaction between ambient N₂ and He^* in plasma jet, contributing to the proper utilization of plasma jet in various applied fields of interest.