Operation modes of the helium dielectric barrier discharge for soft ionization

•More insights into the ionization processes in a Helium-DBD are given.•A homogeneous and a filamentary plasma jet mode become obvious.•Modes can be distinguished by monitoring the plasma current.•Plasma modes have different rotational temperatures.•The rotational temperature is an indicator for Penning ionization or charge transfer.

Among different applications of dielectric barrier discharge (DBD) plasma, the soft ionization ability is certainly one of the most interesting. In this paper the helium plasma jet, produced by a capillary DBD, penetrating in the ambient atmosphere, has been spectroscopically investigated in dependence on applied voltage and helium flow. It was found that the change of the applied voltage leads to different discharge modes. Based on the measurements of the emission spectra of atomic He and N2+ and N2 molecules in the capillary and in the plasma jet with high spatial resolution, it can be assessed in which mode, i.e. under which conditions the plasma jet is expected to be most effective for soft ionization of molecules.