A model study of propagation of the first ionization wave during breakdown in a straight tube containing argon

The mechanisms responsible for the propagation of the first anode directed ionization wave that occurs in a straight discharge tube during breakdown are studied by means of a fluid model. The discharge tube contains argon at a pressure of a few Torr and is operated at a dc voltage with the cathode heated to thermal electron emission temperatures. The two-dimensional model incorporates continuity and momentum equations for the electrons, for several effective excited states and for the ions, a balance equation for the electron energy and the Poisson equation. The model is capable of describing the first ionization front in a way that is qualitatively consistent with observations made in experiments. The mechanisms behind the breakdown evolution are investigated by considering the temporal and spatial evolution of the quantities described by the model. Previously, researchers have described this breakdown evolution in terms of an RC-line circuit. The validity of this picture is surveyed by considering the distribution of charges within the lamp. The effect of control parameters on the breakdown process and the assumptions that affect the validity of the model for later stages in breakdown are considered.