Cathode Sheath Voltage Models for Hydrazine Arcjets

Abstract

We present two different approaches to predict the cathode sheath potential necessary to account for the total voltage in a 1-kW hydrazine arcjet operating at 10 A, 50 mg/s. The first approach is a jump condition cathode sheath model, which treats the cathode surface conditions and properties at the sheath edge. The input of the sheath model includes attachment area and the temperature at the cathode, and the near-cathode bulk plasma properties, which are obtained from a nonequilibrium arcjet model. The sheath potential from the sheath model is approximately -30 V for a pure tungsten cathode with 2 x 10 -8 m 2 attachment area, and 3680 K cathode tip temperature. The second approach is a detailed nonneutral plasma cathode sheath model, which employs Poisson's equation to calculate the charge distribution in the sheath and Saha's equilibrium equation to calculate species densities in the ionization region. The sheath voltage, electron temperature, and electron number density predictions from the two cathode models agree well. However, the cathode surface electric field differs by a factor of 10 or more