Rate of Ionization behind Shock Waves in Air. I. Experimental Results

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Abstract
The electron density profile behind normal shock waves in air at initial pressures 0.02 ≤ p1 ≤ 0.2 mm Hg and in the shock‐Mach‐number range 14 ≤ Ms ≤ 20 has been measured, using microwave‐reflection and magnetic‐induction probes in a 24‐in.‐diameter shock tube. It was found that the electron density generally rises rapidly behind the shock front without much incubation to a transient peak value which is considerably higher (i.e., by a factor of between 2 and 3) than that corresponding to the final equilibrium value. At a fixed shock Mach number, the maximum electron density gradient behind the shock appears to vary with the square of the initial air density, indicating that all the important rate‐governing steps are due to binary collisions. Thus, at Ms = 20, the distance behind the shock required to reach 90% of the transient peak electron density was found to be about 10 times the viscosity mean free path of the undisturbed gas ahead of the shock. At Ms = 14, the corresponding distance was found to be approximately 40 times the initial mean free path.