Use of Langmuir probes for hypersonic turbulent wakes.

The direct measurement of electron density fluctuations, either relative or ahsolute, in the wakes of hypersonic projectiles has not yet been accomplished. The purpose of this article is to indicate the feasibility of using a cylindrical Langmuir probe, operating at electron saturation, to accomplish this. Typical test conditions considered correspond to a projectile 6.8 cm in diameter, at a speed of 4.72 km/sec in air at 10 torr. The probe diameter was selected much smaller than an electron mean-free path so that collisionless probe theory could be applied, which predicts that the probe current is sensitive to only the electron density. To avoid a decrease of probe collection current due to multiple collisions within the sheath, a probe radius of 0.02 to 0.025 mm is suggested. It is possible, however, that a few collisions in the sheath will give probe currents larger than that predicted by collisionless theory. The mechanical frequency and noise characteristics were investigated and found to be suitable for such measurements.