Ionizing shocks in argon. Part I: Collisional-radiative model and steady-state structure

Abstract

A detailed collisional-radiative model is developed and coupled with a single-fluid, two-temperature convection model for the transport of shock-heated argon. The model is used in a systematic approach to examine the effects of the collision cross sections on the shock structure, including the relaxation layer and subsequent radiative-cooling regime. We present a comparison with previous experimental results obtained at the University of Toronto’s Institute of Aerospace Studies and the Australian National University, which serve as benchmarks to the model. It is shown here that ionization proceeds via the ladder-climbing mechanism, in which the upper levels play a dominant role as compared to the metastable states. Taking this into account, the present model is able to accurately reproduce the metastable populations in the relaxation zone measured in previous experiments, which is not possible with a two-step model. Our numerical results of the radiative-cooling region are in close agreement with experiments and have been obtained without having to consider radiative transport. In particular, it found that spontaneous emission involving the upper levels together with Bremsstrahlung emission account for nearly all radiative losses; all other significant radiative processes, resulting in transitions into the ground-state, are mostly self-absorbed and have a lesser impact. The effects of electron heat conduction are also considered and shown to have a large impact on the electron-priming region immediately behind the shock front; however, the overall effect on the induction length, i.e., the distance between the shock front and the electron avalanche, is small.