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. Atomic electron-excitation, vibrational, and rotational temperatures were determined for the expanding plasma using relative line intensity 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 K to 2500 K and from 8000 K to 3000 K, respectively, between the same locations. The electron density, measured using hydrogen H-beta line Stark broadening, decreased from about 10 to the 15th cu cm to about 2 x 10 to the 14th cu cm during the expansion. The results show that the plasma is highly nonequilibrium throughout the nozzle, with most relaxation times equal or exceeding the particle residence time.