Response of the low-pressure hot-filament discharge plasma to a positively biased auxiliary disk electrode

In a steady-state plasma, the loss rate of plasma particles to the chamber wall and surfaces in contact with plasma is balanced by the ionization rate of background neutrals in the hot-filament discharges. The balance between the loss rate and ionization rate of plasma particles (electrons and ions) maintains quasi-neutrality of the bulk plasma. In the presence of an external perturbation, it tries to retain its quasi-neutrality condition. In this work, we studied how the properties of bulk plasma are affected by an external DC potential perturbation. An auxiliary biased metal disk electrode was used to introduce a potential perturbation to the plasma medium. A single Langmuir probe and an emissive probe, placed in the line of the discharge axis, were used for the characterization of the bulk plasma. It is observed that only positive bias to the auxiliary metal disk increases the plasma potential, electron temperature, and plasma density but these plasma parameters remain unaltered when the disk is biased with a negative potential with respect to plasma potential. The observed plasma parameters for two different-sized, positively as well as negatively biased, metal disks are compared and found inconsistent with the existing theoretical model at large positive bias voltages. The role of the primary energetic electrons population in determining the plasma parameters is discussed. The experimentally observed results are qualitatively explained on the basis of electrostatic confinement arising due to the loss of electrons to a biased metal disk electrode.