MEASUREMENT OF SHEATH CHARACTERISTICS IN THE PRESENCE OF CONVECTION AND IONIZATION

A flame plasma whose electron and (or) ion density can be varied over several orders of magnitude is constrained to flow perpendicular to a planar grid Langmuir probe. The probe is biased negative to the plasma , and the current-voltage characteristics and the thickness of the ion sheath formed at the probe are measured. The level of the electron an d (or) ion density is set within a range at which the probe current du e to thermal ionization throughout the sheath is comparable with the c urrent of ions convected into the sheath. The experimental results are compared with the predictions of a recent theoretical paper that calc ulates the effect of recombination upon the characteristics of planar, cylindrical, and spherical probes with boundary layer sheaths. The th eoretical predictions and experimental results for an idealized planar configuration show good agreement over wide ranges of variation of pr obe bias and plasma electron and (or) ion density. This verification o f the theoretical planar electrode - perpendicular-flow model, which i s the basis for all three boundary layer relations, is seen as providi ng strong backing for these relations, which have application to ioniz ation measurements in various forms of recombinant plasma.