Kinetics of ions under discharge current and gas flow rate variation in the positive column of flowing nitrogen DC discharges. Role of electron density and residence time in ionic phenomena

Abstract The present study combines experimental and theoretical analysis of the kinetics related to charged species in the positive columns of flowing nitrogen direct current (DC) low pressure discharges for independent variation of the experimental parameters discharge current and gas flow rate. The analysis was carried out at a fixed position at the end of the positive columns. Optical emission spectroscopy (OES) and Langmuir double probes experimental techniques were employed in the characterization of important discharges parameters. These are the reduced electric field, electron density, gas temperature, degree of ionization, discharge residence time, and N 2 + (B 2 Σ + u , v = 0) relative density. We constructed a state-to-state kinetic numerical model to investigate the positive columns. From the model, we analyze the N + , N 2 + (X 2 Σ + g ), N 3 + , and N 4 + ionic species densities and their formation rates from 27 ionization and ion transfer mechanisms. The N 2 + (B 2 Σ + u ) densities are also calculated and compared to the measured excited ions densities from OES. The total cation calculated densities are compared to the experimentally determined electron densities. We study the role of several physicochemical mechanisms including electron collisions, Penning and associative ionization, and ion transfer reactions. This work brings original and detailed results concerning the ions kinetics for experimentally studied nitrogen DC discharges generated for different conditions of discharge current and gas flow rate. We have observed that while ionization reactions have a fundamental role in the formation of certain ions, the ion transfer reactions are very important in the formation of other ones.