The Effect of Gaseous Impurities on the Conductivity of an Alkaline Plasma

To determine the effect of polyatomic impurities on an alkaline plasma usable in closed-cycle magnetohydrodynamic conversion, the axial electric field and the electron temperature were measured in a positive column of gaseous caesium by means of probes. The caesium pressure was between 10-1 and 1 torr. The impurities introduced were nitrogen and hydrogen, at pressures varying between 10-4 and 1 torr. The following results were observed: For diatomic impurity pressures less than 10-2 torr, a rapid growth in the electric field was found to accompany an increase in the impurity pressure, with no marked change in the electron temperature. The electric field increased as the alkali pressure and current density decreased. This increase is due to a drop in the electron density, which in turn is due to a decrease in the number of fast ionizing electrons, lost through inelastic collisions on the vibration levels of the impurity molecules. For impurity pressures above 10-2 ton, a rapid decrease in the electric field in the discharge was observed when the impurity pressure was increased at constant current. At the same time there is a visible striction of the positive column, which assumes a thread-like structure.. The electron temperature also increases slightly. This phenomenon is explained as follows: in a region of low election density (< 1012 cm-3), the electron distribution tail is weak since the inelastic collisions on impurities absorb more fast electrons than the electron-electron collisions can supply. The plasma cannot therefore be sustained. For high electron densities (> 1015 cm-3), however, the electron-electron collisions constitute a very efficient source of fast electrons and compensate for the losses due to collisions on impurities. In these circumstances the discharge can only exist at high density, and this would explain the striction and high conductivity of the discharge. (author)