Electrostatic probe measurement of the negative ion fraction in an SF₆ helicon discharge

The ratio of the negative ion density to the electron density has been determined using a novel two-probe technique in the diffusion chamber of an SF₆ helicon reactor. The Bohm flux (as modified by negative ions) was measured using a guarded planar probe, while the electron thermal current was obtained using a small cylindrical probe. The negative ion concentration was then determined from the ratio of these two currents. Results obtained with this simple technique show that the plasma in the diffusion chamber is divided radially into three regions. The central region contains hot electrons from the source that are confined by the magnetic field, positive ions being created through impact ionization, and a large proportion of negative ions (n_-/n_e≈5). The edge region is a positive ion-negative ion plasma having a negligible electron density. These two regions are separated by a transition layer with a potential drop of ≈3 V. This layer performs some of the functions of a sheath. Consequently, the sheath at the chamber walls may be quite small since the thermal fluxes of positive and negative ions are nearly equal. The negative ion temperature is found to be ≈0.5 eV, which is much higher than the neutral gas temperature.