Self-consistent modelling of charged and neutral particle dynamics in short-gap helium and hydrogen discharges

A self-consistent model of charged and neutral particle dynamics is developed for the case of high pressure short-gap discharges in helium (0.48 mm x 760 Torr) and hydrogen (0.48 mm x 760 Torr and 1.8 mm x 500 Torr). Boundary wail effects on the electron swarm parameters are first investigated by a Monte Carlo method in order to verify the validity of the classical local held approximation in short-gap discharges. The hydrodynamic transport equations of the self-consistent model are then described with an emphasis on the different terms involved in the close coupling between charged and neutral particles and the electric field. These equations are solved by powerful two-dimensional numerical schemes for both transport and electrical field equations. The discharges are studied from an initial electronic cloud to the first stages of breakdown. Cathode emission is discussed in terms of its prime importance in the spatio-temporal evolution of the short-gap discharges and it is shown that the principal difference between helium and hydrogen discharges is due to the mode of cathode emission. The particular observations in the luminosity in hydrogen are discussed in terms of ionization of the gas and secondary emission processes at the surface. A detailed analysis reveals a complex distribution of charged particles due to the superposition of ionization and transport effects. Furthermore, Joule heating of the neutral medium is evaluated in the entire time scale of the discharge and its influence on the discharge evolution is discussed.