Thermal nonequilibrium in a low-power arcjet nozzle

Emission spectroscopy measurements were made of the plasma flow inside the nozzle of a 1-kW-class arcjet thruster. The thruster propellant was a hydrogen-nitrogen mixture used to simulate fully decomposed hydrazine. Several 0.25-mm-diam holes were drilled into the 12-mm-long diverging section of the tungsten thruster nozzle to provide side-on optical access to the internal flow. Electron excitation for atomic species and molecular vibrational and rotational temperatures were determined for the expanding plasma using relative line ratio techniques. The atomic excitation temperature decreased from 18,000 K at a location 3-mm downstream of the constrictor to 9000 K at a location 9 mm from the constrictor, while the molecular vibrational and rotational temperatures decreased from 6500 to 3000 K and 8000 to 3000 K, respectively, between the same locations. The electron density, measured using H^ line Stark broadening, decreased from ~1021 m~3 to 2 x 10 20 m~3 during the expansion in the nozzle. The results show that the plasma is in a nonequilibrium state throughout most of the nozzle, with relaxation times close to or larger than the particle residence time.