On the theory of spherical probes and dust grains

In this paper a theoretical analysis for negatively biased single and double spherical probes in a plasma is presented. it is assumed that the motion of ions with respect to the probes is radial and that the plasma electron distribution function is Maxwellian. Photoemission from the probes and secondary electron emission are not taken into account. The analysis provides a tool which may be used to derive plasma parameters directly from experimentally obtained double probe characteristics. In addition the analysis is applied to the case of an isolated negatively charged dust grain in a plasma. The probe theory indicates that the charge on such a dust grain, in a hydrogen or argon plasma at floating potential, is approximately the same as the charge that the grain would have in a vacuum, for the case in which the grain's radius is less than or approximately equal to one tenth of the Debye length of the ambient plasma. At larger values of the grain radius the charge on the grain increases with the grain radius.