XXXIV ICPIG & ICRP-10, July 14-19, 2019, Sapporo, Hokkaido, Japan Understanding the evolution of a He and He/O2 capillary plasma jet C. Lazarou 1,2 , L. Wang 3 , A. Nikiforov 3 , C. Leys 3 , C. Anastassiou 1,2 , I. Topala 4 , A. S. Chiper 4 , I. Mihaila 5 , V. Pohoata 4 , P. Vogel 6 , A. Knodel 6 , B. Gilbert Lopez 7 , J. F. Garcia Reyes 7 , A. Molina-Diaz 7 , J. Franzke 6 and G. E. Georghiou 1,2 1 FOSS Research Centre for Sustainable Energy, Department of Electrical and Computer Engineering, University of Cyprus 2 ENAL Electromagnetics and Novel Applications Lab, Department of Electrical and Computer Engineering, University of Cyprus 3 Department of Applied Physics, Ghent University, Belgium 4 Iasi Plasma Advanced Research Center (IPARC), Faculty of Physics, Alexandru Ioan Cuza University of Iasi 5 Integrated Center of Environmental Science Studies in the North-Eastern Development Region (CERNESIM), Alexandru Ioan Cuza University of Iasi 6 ISAS—Leibniz Institut für Analytische Wissenschaften, Dortmund, Germany 7 Analytical Chemistry Research Group, Department of Physical and Analytical Chemistry, University of Jaén In this study, a two-dimensional axi-symmetric model has been developed in order to shed light into the evolution of a He and He/O2 capillary plasma jet. The plasma jet interacts with a dielectric surface placed normal to the jet axis to bring the model closer to applications. This work examines, from first principles, the shape and speed of the of the plasma bullet, the intensity of the induced electric (IEF) field on the dielectric surface and the underlying dominant chemical reactions of the plasma. Furthermore, the work provides insight and understanding into the mechanisms behind many experimental observations such as the torus (or donut) and sphere plasma bullet shapes for pure Helium and He/O2 plasma, respectively. Atmospheric pressure plasma jet devices have received a lot of attention in the last two decades, due to their low production cost and the wide range of potential applications, ranging from plasma medicine [1], to green analytical chemistry [2], material surface modification [3] etc. Helium plasma jet devices have recently shown very promising results in biomedical applications, such as wound healing, treatment of cancer, bacterial inactivation etc. Oxygen admixtures have been found to be very significant for biomedical applications of helium plasma jets as these increase the effectiveness of cancer treatment. In this study, a two-dimensional axi-symmetric model has been developed [4] to study the evolution of capillary helium plasma jets with and without oxygen, admixtures and their interaction with a dielectric surface placed normal to the jet axis. The model considers the gas mixing of helium and ambient air and the analytical chemistry between helium, nitrogen and oxygen species. In order to ensure the validity of the model, the simulation results are compared with experimental measurements. The simulation results show that the plasma bullet shape, speed and induced electric field on the dielectric surface are highly affected by the introduction of oxygen admixtures into the helium gas. In particular, the simulation results show that the shape of the plasma bullet during its propagation is controlled by the generation of seed electrons in front of the streamer head. For He/O2 plasma jets, the shape of the bullet remains sphere like because the seed electrons are mainly produced uniformly along the axis of symmetry in the helium channel, through Penning ionization of helium metastable species with the admixture of O2 molecules. In the case of the pure helium jet the bullet is torus shaped because seed electrons in the helium channel are mainly generated on the edges of the channel through Penning ionization of nitrogen and oxygen molecules by Hem and He2 m species (due to the higher N2 and O2 in this region from atmospheric air mixing). Furthermore, the simulations show that the He/O2 plasma jet generates a much stronger IEF compared to the one of the pure He plasma jet. The shape of the IEF on the dielectric surface being on the axis of symmetry for the He/O2 plasma jets and off the axis of symmetry for pure He plasma jet.