Ambipolar Diffusion in an Isothermal Oxygen Plasma at Elevated Temperatures

Electron removal in an afterglow discharge in oxygen was measured using microwave diagnostic techniques. For pressures below 1 mm Hg it was found that ambipolar diffusion was the principal loss process in the late afterglow which, by microwave radiation temperature measurements, was proven to be isothermal at the ambient gas temperature. A marked difference in the rare of diffusion for the low- and high-current discharge afterglow at room temperature was demonstrated with $D_{a}p$ values equal to 110±10 and 216±20 ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$, respectively. The identity of the ions leading to the higher value is not well established since it is not clear how ${\mathrm{O}}^{+}$ ions could be predominant in the high-current case. It is perhaps more probable that the ion ${\mathrm{O}}_3^{+}$ is responsible based on some reported mass spectrometric studies of the oxygen afterglow. The ions responsible for the lower $D_{a}p$ value, however, are most probably ${\mathrm{O}}_2^{+}$ judging from previously published results. These ions display a strongly temperature-dependent diffusion rate in the range 300 to 900°K which on the mobility versus temperature plot resulted in a maximum of about 5.0 ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{V}}^{-1\mathrm{}}$ around 600°K after a steep climb from the value 2.8 ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{V}}^{-1\mathrm{}}$ at $T=300\mathrm{}°$K; between 600 and 900°K the decline in mobility is only slight. This behavior is discussed in terms of ion-molecule force fields and charge transfer.