COLLECTION OF IONS BY ELECTRIC PROBES IN COMBUSTION MHD PLASMAS: AN OVERVIEW.

The use of negatively biased (ion-collecting) Langmuir (or electric) probes under conditions typical of MHD combustion plasmas is discussed. The interpretation of the measured probe current in order to yield ion density is also discussed, and it is shown that for many situations an extremely simple model is satisfactory. In this model, the ion flux convected by the bulk hydrodynamic flow into the sheath edge is equated to the mobility-dominated space-charge current inside the sheath. Some of the practical problems of using these probes in the rather harsh environment of a combustion MHD plasma are also considered. Specifically, problems arising from probe heating, seed and/or slag deposition are discussed.