Langmuir probe characteristics in a positive-ion/negative-ion plasma

Cylindrical probe measurements have been made in an iodine vapour plasma containing a ratio of negative ions to electrons sufficiently large for electron effects to be neglected; that is, in a positive-ion/negative-ion plasma. The ion-ion plasma was produced by allowing plasma from the Faraday dark space of a DC iodine glow discharge to diffuse into a side-arm. Almost symmetrical current-voltage probe characteristics were obtained in the ion-ion plasma, and both the positive and negative current regions were found to follow a (current) 2 -voltage relationship, as would be expected if ion collection by the probe was orbital-limited. The characteristics were analysed using orbital-limited current theory to provide data on plasma potential, ion number density and Debye length. The results of the analysis are consistent with the dominant ion species in the ion-ion plasma being I 2 + and I 3 − .