Approach to equilibrium lonization behind strong shock waves in argon

Previous work on strong shock waves in argon (1–3) has shown that the degree of ionization reaches thermodynamic equilibrium for shock speeds greater than about ten times the speed of sound in room temperature argon. The degree of ionization reaches about 25% at the highest shock strengths available in an ordinary shock tube. This paper reports a detailed theoretical and experimental study of the approach of high-temperature argon to this equilibrium degree of ionization.The mechanism for the approach to equilibrium can be divided into two stages. The first, which accounts for about 10% of the final ionization, is apparently controlled by impurities present in the gas even after considerable care is taken to insure high purity. No reasonable explanation has been found for the unusual effectiveness of impurities in ionizing argon. In the second stage, ionization by electron-atom collisions becomes dominant. The rate of ionization by this process is limited by the rate of energy transfer to electrons by elastic collisions with the atoms and ions in the gas and is relatively independent of the inelastic ionization cross section. This rate is calculated and is in agreement with experimental measurements making use of (1) the visible continuum radiation from the gas, and (2) the electrostatic potentials in the gas due to the diffusion of electrons.