Emission spectroscopic measurement of ammonia or mixture of nitrogen and hydrogen plasma in a direct-current arc jet generator with an expansion nozzle

Spectroscopic measurement was carried out to understand the plasma features in a 10-kW-class water-cooled direct-current arcjet generator with a supersonic expansion nozzle. Ammonia or a mixture of nitrogen and hydrogen was used as the working gas. In the mixture of N₂+nH ₂, the H₂ mole fraction n was varied from 0.5-3.0, in which a H₂ mole fraction of 3.0 corresponded to that of simulated ammonia. The discharge voltage and the vacuum tank pressure for N₂+3H₂ were higher than those for NH₃ at a constant discharge current and a constant input power, respectively. These characteristics agreed with those of H-atom electronic excitation temperatures in the constrictor of the nozzle throat. The NH₃ and N₂+3H₂ plasmas in the constrictor were expected to be nearly in a temperature-equilibrium condition. On the other hand, the plasmas in the expansion nozzle were in thermodynamical nonequilibrium state because the electron number densities rapidly decreased downstream. As a result, the H-atom excitation temperature and the N₂ rotational excitation temperature decreased from 7000-11000 K in the constrictor to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit with mass flow rates of 0.1-0.2 g/s at input powers of 7-12 kW, although the NH rotational excitation temperature did not show a significant axial decrease