Electron Density Fluctuation Measurements in Projectile Wakes

To understand the scattering properties of hypersonic wakes, it is necessary to have spatially resolved measurements of wake electron density fluctuation spectra. Such measurements are provided herein in the wake of sphere projectiles moving at 6 kfps in a 20-torr krypton atmosphere. Under these Mach 9 conditions, a high degree of ionization was produced and recombination chemistry was essentially frozen for long distances in the turbulent wake. To obtain the electron density measurements, three Langmuir probes were located adjacent to the projectile path, thus providing correlations and convection velocity of the fluctuations in addition to spectra. Electron density was measured in the range of XjD from 10 to 10 using collection wires that were 0.0001 in. in diam. They were operated in the electron-saturation mode, the collisionless sheaths being large compared with the wire diameter. Using a large bias, the current was directly proportional to the local instantaneous electron density. The present measurements are believed to be the first direct electron density measurements of projectile wakes on a local and instantaneous basis that are available. Mean values of electron density near 10/cc with peaks approaching 10 were obtained. The rms fluctuations were 0.6 to 1.0 of the mean electron density. The spectrum of the electron density fluctuations agrees with the previously obtained anemometer data spectrum, which suggests that the electron fluctuations are acting as passive scalars in the velocity and mass-density fluctuation field.