Entry conditions in planetary atmospheres: Emission spectroscopy of molecular plasma arcjets

A stationary arcjet, operating at low pressure (0.13 mbar in the vacuum cha mber), is used for the simulation of the gas flow surrounding a vehicle dur ing its entry into the atmosphere of Titan and Mars. For the Titan atmosphe re the gas mixture 99% N-2 - 1% CH4 is used and 97% CO2 - 3% N-2 for the Ma rtian atmosphere. The respective plasma arcjets are analyzed by optical emi ssion spectroscopy for identifying the emitting molecules and atoms and als o deducing the temperatures associated with their different internal modes. For the N-2-CH4 plasma the vibrational temperatures T-v deduced from CN an d NH spectra are found in concordance with the measured electron temperatur e, that is, 8000 K, whereas, from CH spectra, T-v is obtained close to 3700 K. The rotational temperatures are found to be between 2500 and 2800 K for CH and NH and nearly 5000 K for both CN and N-2(+) spectra. For the CO2-N- 2 plasma no emission from the arcjet is detected in the UV-visible range; t he feasibility of the infrared analysis of the Deltav = 2 band of CO is dem onstrated in a stationary plasma discharge experiment.