Boundary-layer ion density profiles as measured by electrostatic probes.

Ion-density profiles in boundary layers associated with supersonic flow of shock-heated air over a flat plate have been measured by cylindrical and flush-mounted electrostatic probes. A method of cylindrical-probe data interpretation presented accounts for the effects of a plasma drift velocity. An appropriate method for flush-probe interpretation also is given. The experiments were carried out in a pressure-driven shock tube at 0.1 torr initial shock tube pressure, and ranged over five orders of magnitude in freestream charge density from 10 ions/ cm to 10 ions/cm. The experimental profiles were compared to theoretical profiles derived on the basis of a zero-order boundary-layer theory, assuming frozen flow. Good agreement with the zero-order theory is obtained below 10 ions/cm. Above 10 ions/cm^ experimental evidence indicates that the boundary layer begins to depart from the theoretical assumption of frozen flow.