Mass spectrometry in shock tunnels

Abstract

A continuously sampling mass spectrometer was designed and constructed to detect the onset of driver gas contamination in a free piston shock tunnel. A skimmer, which formed a molecular beam that was ionised and analysed by a quadrupole mass spectrometer, sampled the flow. The design and construction of the vacuum system, the ion source, and the mass spectrometer are discussed. The response time of the apparatus was estimated to be 10 x 10⁻⁶ sec. It was established that a mass spectrometer could be used to detect the onset of helium driver gas contamination in the flow. Experiments were carried out varying the test gas, the initial shock tube pressure, the nozzle throat diameter and the conditions in the compression tube. The results showed that there was a low level of helium contamination very early in the flow and that this was almost independent of the test gas and the nozzle throat diameter. Three different sampling skimmers were used and the results showed that provided that the shock waves were attached to the sides of the skimmer then the flow sampled represented the free stream conditions. Finally, a boundary layer dump was used in the shock tube and this supported the argument that the helium was being introduced into the reflected shock region by the boundary layer.