Rate of ionization behind shock waves in air

Abstract

Knowledge of the ionization rate behind strong shock waves in air is essential not only for problems involving the flight of hypersonic vehicles at high altitudes, but it is also important for many interesting problems in atmospheric and geophysical research (e.g. the meteoric ionization problem). The main difficulty experienced in earlier studies (1) was the extremely fast rate observed at normal air densities, which kept the electron density profile behind strong shock waves beyond the spatial resolution of most ionization detectors that can be used for quantitative measurements. In the present investigation, the above-mentioned difficulty has been removed through the use of a 24 in. diameter, low-density shock tube, which allows experiments to be performed at densities lower than the ordinary shock tube operating density (2) by a factor of 100. By using such a shock tube, together with a microwave reflection probe, the electron density profile behind normal shock waves in air at density corresponding to an altitude of 250,000 ft (initial shock tube pressure, p 1 = 20 μ Hg) and in the velocity range 10,000 < U < 25,000 ft sec has been successfully measured. The preliminary results, which agreed grossly with a theoretical prediction by Lin and Teare, (3) indicated that at satellite velocity ( U ≈ 25,000 ft sec ), the thermal ionization process will be completed at a distance of about 1 cm behind the shock front; while at a velocity of 15,000 ft sec the corresponding distance will be about 10 cm. At other altitudes (up to about 300,000 ft, where atomic oxygen begins to appear in the atmosphere) the corresponding ionization distance may be scaled inversely to the atmospheric density since all the collision processes which govern the initial rise of the electron density profile are binary.