Plasma decay in Air and N$_{2}$:O$_{2}$:CO$_{2}$ mixtures at elevated gas temperatures

Plasma decay after a high-voltage nanosecond discharge has been studied experimentally and numerically behind an incident and reflected shock wave in high temperature (900 -- 3000 K) air and N$_{2}$:O$_{2}$:CO$_{2}$ mixtures for pressures between 0.1 and 2 atm. Time-resolved electron density history was measured by a microwave interferometer for initial electron densities in the range (1-3)x10$^{12}$ cm$^{-3}$. It was shown that the electron density varies in the air afterglow in the ``recombination manner'', 1/$n_{e}(t)$ = 1/$n_{e}$(0) + \textit{$\alpha $}$_{eff}t$, where \textit{$\alpha $}$_{eff}$ is the effective electron-ion recombination coefficient. A numerical simulation was carried out to describe the temporal evolution of the densities of charged particles under the conditions considered. A good agreement was obtained between the calculated and the measured electron density histories in the air afterglow when taking into account electron attachment to O$_{2}$ to form O$_{2}^{-}$ ions and electron detachment from them, as well as electron-ion and ion-ion recombination. In CO$_{2}$-containing mixtures, it was necessary to consider the formation of complex negative and positive ions. These ions were formed in three-body reactions; therefore, the rate of plasma decay increased with gas density.