Probe theory for arbitrary shape in a large Debye length, stationary plasma

Abstract

The exact analytic current-voltage characteristic recently obtained by Fendell and by Chapkis and Baum for spherical probes in the continuum Laplace limit, is shown to be valid for arbitrary probe shape. This result has important diagnostic implications. The form of the characteristic suggests a basic modification to the usual ’’retarding potential’’ technique, in which the retarded current divided by probe potential is graphed logarithmically to measure electron temperature. An approximate extension to finite mean free paths is made. This leads to a prediction that a long cylindrical probe will be much more sensitive to departures from collisionless conditions than a sphere of equal diameter. Another extension of the theory includes negative ion current. The effects of departure from Laplace conditions are investigated by presenting self-consistent numerical calculations for spherical probes. Formulas for triple-probe use are presented. A widespread misuse of the Druyvesteyn method is critically examined.