Raman Scattering Measurements of Molecular Hydrogen in AN Arcjet Thruster Plume

Arcjet thrusters are electrically powered rockets that offer higher exhaust velocities than conventional chemical rockets. There is a desire to increase both the thermal efficiency and exhaust velocity of current designs and to develop higher thrust versions. Such improvements will depend partly on a better understanding of the plasma and gas dynamic processes occurring in the arcjet nozzle. Measurements of density, temperature, and velocity at the exit plane are of particular interest. This work describes Raman scattering measurements of molecular hydrogen density and rotational temperature at the exit plane of a 1-kW class hydrogen arcjet. These measurements were compared to the results of continuum and Monte Carlo simulations. Initial studies were made in the plume of a cold -flowing arcjet thruster using continuous laser excitation. Exit plane profiles showed strong two-dimensionality in the flow and also a moderate degree of asymmetry. The measured rotational temperatures on the center-line were significantly higher than expected for isentropic flow, suggesting the flow was not in rotational-translational equilibrium. Comparison of the measured density and rotational temperature profiles with Monte-Carlo simulations yielded good agreement by using a rotational relaxation rate at the low range of the accepted value. Axial profiles of density measured at various back pressures showed shock structure consistent with gas dynamic theory and the simulations. Experiments were also performed in the arc-ignited thruster plume where the signals were less than 1 photon s^{-1}. Quantum-limited detection was achieved through the use of gated photon counting and high-power pulsed laser excitation. Radial profiles of rotational temperature and density measured at the exit plane for various specific power levels were all asymmetric about the arcjet centerline. The temperature profiles were compared with the translational temperatures of atomic hydrogen measured by laser-induced fluorescence studies and the bulk temperature from continuum simulations. The rotational temperatures were significantly lower, implying that the arc-heated flow is also not in translational-rotational equilibrium. Comparison of density profiles to the model predictions suggests that diffusion is an important mechanism for transporting molecular hydrogen into the center of the arcjet.