Thermal Diffusion in Ionized Gases

Abstract

Thermal diffusion in mixed gases depends on a `thermal diffusion factor ' α, which is less than unity in all known neutral gas mixtures (in which the particles have no electric charge). In simple ionized gases α exceeds unity, being of order Z+1, where Ze is the charge on the ions. However, in the steady state of such a gas, at non-uniform temperature, thermal diffusion does not produce a concentration gradient of the electrons relative to the ions; instead it sets up an electric field, which keeps the gas electrically neutral almost everywhere. In an ionized atmosphere subject to gravity, in which the temperature increases upwards, this electric field opposes and reduces the electric field that keeps the electrons and ions together, despite the great difference between their masses. The thermal diffusion factor seems to be greatest for the ions of different kinds in a mixed ionized gas, composed mainly of electrons and light ions of small charge Ze, with a small admixture of heavy ions of greater charge Z'e. In this case α is of order 2.5 (Z'/Z)2, and can amount to several hundred. In such a gas, at non-uniform temperature, thermal diffusion can much increase the (still small) proportion of the heavy multiple ions in the hotter regions. The electric field that keeps the electrons with the ions, so that the gas is neutral, reinforces this tendency of the multiple ions, in the cases here considered. This tendency may have some importance in the solar corona, if turbulence is not too great and temperature inequalities persist long enough. It seems likely also that thermal diffusion will influence the distribution of the multiple ions present in the zeta nuclear fusion apparatus.