Effect of Angle of Incidence on the Response of Cylindrical Electrostatic Probes at Supersonic Speeds

The response of cylindrical probes at various angles of incidence has been studied in low-density hypersonic argon plasma. All mean free paths were larger than probe radii (rp) except the ion-ion mean free path which was smaller than rp in most of the tests. In all experiments, an ion current peak has been observed near the zero angle of incidence. The magnitude of this effect is shown to depend strongly on probe radius-to-Debye length ratio (rp/AD) and is significant for 0.5 < rp/lD < 2. Insensitivity of this effect to probe length appears to be closely related to the presence of ion collisions. A physical model is proposed which explains the effect in terms of ionneutral collisions in the presheath of the probes. A simple theory is presented in the paper which attempts to account for the measured data on the ion current peak. The readings obtained with aligned and transverse probes are interpreted using theories (Laframboise, Smetana, modified Langmuir) and the current vs angle-ofattack variation is used to infer collision-free value of the ion current to aligned probes. The experimental results appear to agree with Laframboise's theory in the orbital-motion-limited region. Disagreements with theory reported by several investigators can be explained either in terms of ion-neutral collisions associated with aligned probes (in nearly collisionless plasmas) or in terms of an end effect (in highly rarefied plasmas).