Perturbation of a Plasma by a Probe

Abstract

The theory of the electrostatic probe immersed in a plasma is discussed under the assumptions that particle mean free paths are comparable with or smaller than probe radius. The range of validity of the results is for ${\lambda }_{\mathrm{c}}{\mathrm{/r}}_{\mathrm{p}}\text{\hspace{0.17em}<\hspace{0.17em}10}$ , $r_{\mathrm{p}}{\text{/100λ}}_{\mathrm{D}}\text{\hspace{0.17em}>\hspace{0.17em}1}$ , ${\lambda }_{\mathrm{c}}{\mathrm{/\lambda }}_{\mathrm{D}}\text{\hspace{0.17em}>\hspace{0.17em}10}$ , where $r_{\mathrm{p}}$ is probe radius, ${\lambda }_{\mathrm{c}}$ is particle mean free path, and ${\lambda }_{\mathrm{D}}$ is the Debye length. In the limit of ${\lambda }_{\mathrm{c}}{\mathrm{/r}}_{\mathrm{p}}$ approaching infinity, the results converge toward the Langmuir result for the same probe. The probe-current vs voltage characteristic obtained when mean free paths are short is distorted because the degree of perturbation of plasma density and potential vary with probe voltage. An unexpected result is that the shape of the probe characteristic in the vicinity of the "knee" is sensitive to ion temperature, and may possibly be used to determine the ion temperature.