On-axis Molecular Beam Mass Spectrometer measurements of a cold atmospheric pressure plasma jet

The measured on-axis molecular species densities in the effluent of a cold plasma jet operated at atmospheric pressure were performed for different distances (3{\ldots}35 mm) with a molecular beam mass spectrometer (MBMS) (Hiden HPR 60). The investigated molecules are nitrogen (M = 28 u), oxygen (M = 32 u) and argon (M = 40 u). A stainless steel orifice with a diameter of 100$\mu $m in front of the first pump stage was used. After a calibration of the intensities of the mass spectrometer the absolute densities were calculated. These values are compared with a model of the gas flux and show excellent agreement. It is shown that because of the feed gas flow of the plasma jet the ambient air species (e.g. N$_{2}$ and O$_{2})$ are displaced in the case of small distances. For larger distances the diffusion of nitrogen and oxygen molecules into the effluent increases. Effects of composition distortion and pressure dependencies inside the MBMS were observed in the calibration curve and were taken into account.