Approach to Ionization Equilibrium and Atomic Collision Cross-Section Measurement in a Helium Shock Wave

A theoretical and experimental study of the initial ionization phase of an ionizing helium shock wave has been accomplished. Ionization relaxation time and the electron density profile behind the shock translational front have been measured. Both of these parameters have strong dependence upon the atomic collisional excitation rate in the initial phase of the ionizing shock wave, and therefore, a comparison with theory led to an estimation of the atom–atom collisional excitation cross section. Theoretical predictions for the relaxation zone of the helium shock wave indicated only a weak coupling between the electron and atom temperature during the relaxation process. A plasma focus driven shock tube was used to create the appropriate helium shock waves. Simultaneous measurement of the relaxation time and the electron density profile was performed using a multipass Fabry–Perot laser interferometer. A close agreement between the theoretical and the experimental temporal electron density profile was obtained with our experimental conditions (pressure P∞ = 0.5–5 Torr, shock Mach number 16–26). The effective cross section for atom–atom excitation close to threshold is [Formula: see text].