Flow velocity measurements in nitrogen plumes stimulated by REMPI

In the DLR-high vacuum plume test facility STG, experiments have been performed to study the expansion of supersonic plumes into vacuum with N₂ as a test gas. The measured values were the Pitot pressure, the molecular number flux, the rotational temperature and the density. The last two quantities were measured by the 2+2 REMPI-technique with a stimulation wavelength of 283-284 nm (REMPI = resonance enhanced multi photon ionization). This method is also suitable to measure the absolute flow velocity by the ion time-of-flight (TOF) method. The apparatus for the combination of the REMPI and TOF technique is similar to that for the TOF experiments using electron beam bombardment, but a focused laser beam is used instead of an electron beam. The results of such laser measurements in highly rarefied plumes for several stagnation temperatures are presented with nitrogen as test gas. On-axis results fit well with the theoretical limiting velocity for perfect expansion; angular velocity profiles off-axis are discussed and compared to theory. This study is focused on the experimental procedure to measure the flow velocity of N₂ in rarefied plumes expanding from supersonic nozzles into vacuum.