Electron-density and electron-temperature measurements in boundary layers

Thin-wire Langmuir probes haye been used to measure the electron-density and electrontemperature distributions in the nozzle-wall boundary layer of a hypersonic shock tunnel and in the boundary layer of a sharp flat plate located on the nozzle centerline. Experiments were performed in a nozzle-wall boundary layer, with nitrogen as the test gas, in order to obtain a comparison between the results obtained with voltage-swept probes aligned with the flow and constant bias-voltage probes perpendicular to the flow direction. These measurements were performed in the boundary-layer flow because that is the flow environment in which the const ant-volt age probes were to be used in flight applications. Only voltage-swept probes aligned with the flow were used in the flat-plate experiments which were performed in preparation for antenna admittance measurements to be made in the future. These experiments were conducted in a test gas composed of argon plus 0.12% nitrogen. The flow conditions for both experiments were such that free-molecular flow theories could be used to infer ion densities from the ion-current portion of the probe characteristics. Electron temperatures measured in these boundary layers were found to be substantially greater than the calculated heavy-particle translational temperature at the boundary-layer edge. For the flat-plate experiments, the .electron temperature remained relatively constant through the boundary layer and along the plate. However, the electron temperature in the nozzle-wall boundary layer decreased from the outer-flow value as the wall was approached. The positive iondensity distributions obtained in this latter boundary layer using the two probing techniques (aligned with and perpendicular to the flow) were found to be in good agreement. In addition, the calculated boundary-layer thickness was found to approximate the distance from the wall at which the measured electron density approached a constant value.