Electrode effects in a seeded plasma.

Current-voltage characteristics of stagnation point electrodes embedded in a cylindrical probe were obtained for a lightly ionized, atmospheric pressure plasma. The probe characteristics were determined as functions of electrode temperature and seed material. A seeded ethyl alcohol-air flame exhausting into the atmosphere served as the plasma source. Saturation currents were measured which were independent of the electrode temperature below 600 °K and increased rapidly when the electrode temperature was increased above 600°K. The properties of the combustion products are calculated using measured temperatures and including partial dissociation of the seed molecules and electron attachment to the hydroxyl radical. Resistances calculated from the freestream electrical conductivity are in good agreement with the measured values. The dependence of the saturation currents on the electrode temperature is explained by thermal emission of electrons from the electrodes. The magnitude of the saturation currents for temperatures greater than 600 °K is in agreement with the Richardson equation if the work function of the steel electrodes is taken to be 1.8 v.