Energy Exchange between Electron and Ion Gases through Coulomb Collisions in Plasmas

Abstract

The general aspects of energy exchange among the electron, ion, and molecule constituent gases, and their energy transfer to the boundary, are examined for a partially ionized gas. Coulomb collisions between electrons and ions are identified to contribute significantly to thermal energy transfer from the electron gas, even in very weakly ionized gases.Experiments were performed to determine the time required for electron temperature deviations from equilibrium to return to equilibrium with the ion and molecule temperature. The method of guided microwave interaction within the plasma was employed for selectively heating the electron gas to increments above that of the ion and gas temperatures, and for sensing the subsequent electron temperature deviations through the electrical conductivity of the plasma. In addition, phototube detection of the plasma luminous intensity is employed to detect electron temperature deviations.The experimental results show that energy exchange between electron and ion gases is predominant in the experimental plasmas, under appropriate conditions, where the ionization is less than 0.001%. In a plasma produced in neon gas at 2.25 mm Hg, 300\ifmmode^\circ\else\textdegree\fi{}K, the characteristic time for equipartition of excess mean electron energy with the ion gas varies from 11 to 5 \ensuremath{\mu}sec as the ion concentration increases from 1.6 to 5.8\ifmmode\times\else\texttimes\fi{}${10}^{11}$ per ${\mathrm{cm}}^{3}$. These times are much shorter than the 160 \ensuremath{\mu}sec which would be required for the electron temperature to return to equilibrium in the absence of Coulomb collisions. The expected behavior with respect to ion concentration and ion mass is confirmed, but an anomaly with respect to electron temperature is found.