Cascade arc studies of nonequilibrium hydrogen/nitrogen plasmas

A cascade arc facility was developed to study the characteristics of nonequilibrium plasma propellants for electric propulsion applications. The cascade arc was operated with 50 A hydrogen arcs at 2.0 and 6.0 psi. Spatially resolved spectral emission data were collected using a two dimensional Optical Multichannel Analyzer (OMA). Electron density was determined by fitting theoretical line profiles to experimentally measured Abel inverted emission line profiles. Radial distributions of plasma temperature were estimated from Boltzmann plots of spectral line intensity. The measured electron densities and plasma temperatures were compared to values predicted by nonequilibrium cascade arc simulations using the UTSI Cascade Arc Plasma Simulation (CAPS) code. The simulations underpredicted the peak experimental electron densities by as much as an order of magnitude and over predicted peak plasma temperatures by as much as a factor of 2.5. The experimental electric field has been accurately predicted by varying chemical kinetics in the CAPS code. Kinetic models developed at the University of Illinois gave the best agreement with the Cascade Arc measurements for hydrogen. Electron number density profiles for simulated ammonia and hydrazine were also obtained at pressures of 2.0, 6.0, and 10.0 psi. These hydrogen/nitrogen mixture data can be used in comparisons with a hydrogen/nitrogen version of the CAPS code expected to be developed in the future.