Ar* density as a function of time, the detected peaks, and the mean of the (20) highest peaks are shown together with a voltage signal with an arbitrary amplitude and phase, and a frequency equal to the operating frequency of the plasma jet. The humidity of the feed gas, the curtain gas composition, and the distance to the nozzle z were varied: humidity: 879 ppm water, gas curtain: 5 slm O₂, z = 0.1 mm – see Fig 7 in the publication.