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Hydrogen atoms were probed in the near-e eld plume of a 1.8-kW e ight-type arcjet thruster, using two-photon laser-induced e uorescence. The arcjet was operated on an N 2/H2 gas mix simulating hydrazine decomposition products. Thevelocity distributionsof Hatoms were used to obtain theirtranslational temperatures. Comparisons of the laser-induced e uorescence signal intensity, radial linewidth, and derived density distribution with direct simulation Monte Carlo results were in good agreement except near the nozzle exit plane. A relatively weak signal was observed near the exit plane, while the maximum signal was unexpectedly obtained 0.8 cm downstream. Fluorescence quenching, linewidth variation, and self-absorption effects are proposed to account for most of the discrepancy in signal intensity. The results improve the knowledge of near-e eld e ow parameters for the arcjet and ultimately enhance the accuracy and understanding of predicted plume impingement torques and other arcjet plume phenomenology on spacecraft.

Density, velocity, and temperature data were obtained in the near-e eld plume of a 1.8-kW hydrazine arcjet thruster, using the NH molecule as a probe. This was the e rst laser spectroscopic study on a e ight-type arcjet. The rotational temperature of NH was obtained in v = 0;1 of the electronic ground state and the vibrational population in v = 1;2. Rotational and vibrational temperatures were similar, and their variation along the thrust axis was minor. Line shapes as well as radial and axial proe les of the peak line intensity were obtained. Maximum laser-induced e uorescence signal was obtained on-axis 1.4 cm downstream from the exit plane, suggesting rapid NH production in the plume. Axial velocity components were determined at various plume locations. The power distribution of the available channels was found, on summation, to be in good agreement with arcjet input power. Direct simulation Monte Carlo predictions have been generated for comparison with much of the experimental data and to provide additional e owe eld information. Good agreement was achieved in most cases.