Ion Density Measurements in the Wake of a Hypersonic Sphere

Abstract

A report is presented of the results of a major research effort undertaken to determine experimentally, through the use of cylindrical electrostatic probes and existing probe theory, the levels of ion density in the turbulent wakes behind 2.7 in. diameter spheres flown at 14 500 ft/s in a ballistic range filled with nitrogen at 7.6 Torr and at 20 Torr. Measurements of the ion current distribution have been made by means of a transverse survey array of cylindrical electrostatic ion probes. Three static theories and three kinetic theories have been applied to the mean current data of the probes, and experimentally measured temperature and velocity distribution data for sphere wakes was used to construct radial distributions of ion density estimates for each theory. These radial distributions have been fitted with gaussian curves by the method of least mean squares. Two parameters of the ion density distributions are determined: the ion density on the axis of the wake and the ion density radius of the wake. These results have been compared with electron density levels in the wake deduced from microwave interferometer measurements and Langmuir probe instrumentation.