Spatio-Temporally Resolved Mapping of Helium Metastable Density in an Atmospheric Pressure Plasma Jet

Tunable diode laser absorption spectroscopy was used to measure the spatio-temporal evolution of helium metastable atoms He (2
³
S) in an atmospheric plasma jet. The production of these atoms is clearly correlated to the propagation of an ionization wave, the so-called plasma bullet. Whatever the applied voltage, the spatial structure of the density rapidly evolves from a donut shape to a conical form as the plasma propagates. For applied voltages higher than 4.5 kV, a complex transient structure appears at the tip of the jet.