  In steady-state, the plasma loss rate to the chamber wall is balanced by the
ionization rate in the hot-filament discharges. This balance in the loss rate
and ionization rate maintains the quasi--neutrality of the bulk plasma. In this
report, we have studied the properties of bulk plasma in the presence of an
auxiliary (additional) biased metal disk, which is working as a sink, in
low-pressure helium plasma. A single Langmuir probe and emissive probe are used
to characterize the plasma for various biases (positive and negative) to the
metal disk, which was placed along the discharge axis inside the plasma. It is
observed that only a positively biased disk increases the plasma potential,
electron temperature, and plasma density. Moreover, the plasma parameters
remain unaltered when the disk is negatively biased. The observed results for
two different-sized positively metal disks are compared with an available
theoretical model and found an opposite behavior of plasma density variation
with the disk bias voltages at given discharge condition. The role of the
primary energetic electron population in determining the plasma parameters is
discussed. These experimental results are qualitatively explained on the basis
of electrostatic confinement arising due to the loss of electrons to a biased
metal disk electrode.
