Spectroscopy measurements in rarefied plumes

In the DLR-high vacuum plume test facility STG experiments have been performed to study the expansion of plumes from small thrusters. The main feature of the chamber, a large liquid Helium driven cryopump, allows the undisturbed plume gas expansion into the highly rarefied regime including the backflow. Nitrogen was taken as test gas to generate a simple case without chemical reactions and a constant ratio of specific heats. The essential parameter was the nozzle flow Reynolds number, the quantities measured were the Pitot pressure at the nozzle exit, the molecule number flux in the plume, the rotational temperature and the density in the plume. The last two quantities were studied by REMPI-method (Resonance Enhanced Multi Photon Ionization) which delivers a precise determination of temperatures, density and the gas velocity. For the rotational temperature measurements in N₂ the 2+2 REMPI at 283 nm was selected. The test conditions are p_o=0.5 bar to 2.0 bar, T_o=300 K, d*=0.6 mm, Re at the exit of the conical nozzle with, 15° throat 800 and 3200. The transitions S(0),... S(4) were taken for the determination of the rotational temperatures freezes at about 8 K. The flow field which could be studied by REMPI is pretty big; the detection limit was 2.5 m downstream on-axis, that corresponds to a density of 10^-3 1/cm³ for N₂-molecules