Nonequilibrium ionization in wet alkali metal vapors

A theoretical model for the transport of electrons in a wet vapor has been formulated. The medium is considered to consist of electrons, neutral gas atoms, positively charged atomic ions, and charged vapor droplets. The degree of ionization of the gas atoms is assumed to be given by Saha's Law evaluated at the electron temperature, whereas the degree of charging of the vapor droplets is obtained by equating the random electron current in the gas to the thermionic emission current of the vapor droplet at its internal temperature. The electron temperature is determined by equating the Joule heating rate of the electrons to their rate of energy loss due to elastic electron-atom, electron-ion, and electron-droplet collisions and inelastic electron-droplet collisions. The droplet internal temperature is determined by equating the rate of droplet heating due to electron bombardment to the rate of cooling due to atom and ion bombardment. The preceding theory is applied to MHD generator considerations for which the predominate effect of the droplets is found to be the absorption of free electrons from the system. The depression of electron density is found to be most severe at high-percent moisture and for small droplets. IVrONEQUILIBRIUM ionization, as it shall be referred to J- ^ in this paper, is the production of a highly-ionized gas at low gas temperatures as a result of applied and/or induced electric fields in the gas, which serve to heat the electrons. These energetic electrons subsequently ionize the gas to a degree commensurable with their energy rather than that of the cooler gas. This phenomenon has been a long-observed fact in gaseous discharges1' 2 and has recently been considered as a means of producing high electrical conductivities in an MHD generator at gas temperatures compatible with present materials technology. 3' 4 In Ref. 4, nonequilibrium ionization, as it applies to MHD generators utilizing noble gases seeded with low concentrations of alkali metal vapors as working fluids, was extensively analyzed. An equation for the electron temperature relative to that of the other species in the gas was obtained by equating the rate at which energy is gained by the electrons in an electromagnetic field, j • (E + u X B) (where j is the electron current, E is the electric field, u is the gas-flow velocity, and B is the magnetic field), to the rate at which energy is given up by the electrons to other species in the gas due to collisions. This energy balance results in the equation