Lawson Parks Harris

Electrical Conductivity of Cesium‐Seeded Atmospheric Pressure Plasmas near Thermal Equilibrium

Measurements have been made of the electrical conductivities of gaseous mixtures formed by the addition of small fractions of cesium vapor to nitrogen, helium, neon, or argon. The mixtures were maintained near thermal equilibrium at temperatures in the 1500°–2000°K range and a total pressure of one atmosphere. The cesium vapor pressures ranged over two decades, from 0.1 to 10 Torr.The apparatus consists, in essence, of two heated zones connected by a slow flow. The first zone is a low‐temperature (200°–400°C) oven where the body‐gas flow picks up the cesium vapor. The second zone is a small electrically heated furnace (1250°–1850°C) containing a diode test section.The principal measurements taken were the seeding temperature, furnace temperature, and voltages and currents in the test section.The results exhibit variations with temperature, seeding pressure, and gas species that correlate reasonably well with simple theory and values for electron collision frequencies and cross sections taken from the lite...

Ionization and Recombination in Cesium‐Seeded Plasmas near Thermal Equilibrium

Measurements have been completed of electron‐density relaxation in cesium‐seeded plasmas perturbed from thermal equilibrium at temperatures in the 1500° to 2000°K range. The gases consisted of 0.1 to 20 Torr Cs vapor in atmospheric pressure argon or helium. Electron densities ranged over 1.9×1011 to 3.8×1013 cm−3 and relaxation times over 30×10−6 to 50×10−3 sec. The results indicate that charge production and loss are dominated by the atomic‐ion process Cs+e+3.89 eV↔Cs++2e at the higher temperatures, and by the molecular‐ion process Cs+Cs+2.82 eV↔Cs2 ++e at the lower temperatures and higher cesium pressures. At the lower temperatures and lower cesium pressures the dominant process is ambipolar diffusion. The results obtained when the atomic‐ion process dominates agree well with calculations based on the equilibrium properties of the gas and recent theory for two‐electron, three‐body recombination. The rate of the molecular‐ion process corresponds to a two‐body recombination coefficient of approximately 1×10−8 cm3/sec at temperatures of 1250° to 1400°C, a value small relative to that derived from earlier experiments. All evidence indicates that, despite their numerical preponderance, the argon and helium atoms play no significant roles beyond those of thermal bath and diffusion barrier.

Influence of Alkali‐Vapor Atmospheres on Pyrometrically Determined Temperatures: Cesium and Potassium

The conventional pyrometric measurement of wall, electrode, and probe temperatures through an alkali‐vapor atmosphere (which is, in part, cooler than the surface) has been found to lead to significant under‐estimation of the temperature. The error arises from optical absorption of the alkali‐vapor dimers in the region of the pyrometer passband. As an example, the observation through 6 cm of saturated potassium vapor of a surface whose brightness temperature is 2500°K results in an error of from 50° to 500°K as the vapor temperature varies from 300° to 370°C. Thus, substantial errors may be introduced into the determination of properties such as electrical conductivities and reaction rates, which are exponentially dependent upon temperature. The reciprocal‐temperature error was found to be roughly proportional to the dimer optical depth, and agreed with theoretical predictions utilizing dimer optical absorption coefficients measured during these experiments.