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

A stationary arcjet, operating at low pressure (0.13 mbar in the vacuum chamber), is used for the simulation of the gas flow surrounding a vehicle during its entry into the atmosphere of Titan and Mars. For the Titan atmosphere the gas mixture 99% N(2) - 1% CH(4) is used and 97% CO(2) - 3% N(2) for the Martian atmosphere. The respective plasma arcjets are analyzed by optical emission spectroscopy for identifying the emitting molecules and atoms and also deducing the temperatures associated with their different internal modes. For the N(2)-CH(4) plasma the vibrational temperatures T(v) deduced from CN and NH spectra are found in concordance with the measured electron temperature, 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 CO(2)-N(2) plasma no emission from the arcjet is detected in the UV-visible range; the feasibility of the infrared analysis of the Deltav = 2 band of CO is demonstrated in a stationary plasma discharge experiment.