Chemistry of electrons in pure-air hypersonic wakes.

The chemistry that determines the electron concentration in the far wake of a hypersonic body has been investigated. It was found that the far-wake electron density is determined predominantly by the chemistry of negative ions. For this chemistry, which involves a complicated cycle of attachment of electrons to neutral species, ion exchange, and exothermic chemi-detachment reactions, most of the necessary rate constants for pure-air wakes are known. Using these rate constants, electron concentrations in the wake are calculated for nonablating spheres and found to be in good agreement with ballistic range measurements. Atomic oxygen is shown to play a key role in the air ion chemistry. Following the attachment of electrons to oxygen and ion exchange, the electrons are freed in exothermic chemi-detachment reactions such as O + O 2~—> O3 + e, O + O~— * Qz + e, and O + O3~ -*• 2O2 + e. In addition to atomic oxygen, small amounts of NO, Oa and NO2 in the far wake strongly affect the final electron density decay. The roles played by these species are discussed, and their rate constants are listed.