Experimental investigation on how the ion-neutral collisions modify the ion flux at the sheath in low temperature plasmas*

Intuitively, we guess that ion flux toward a plasma boundary would decrease if the ion-neutral collisions are enhanced in partially ionized low temperature plasmas, but evaluating how much it will decrease the flux is not a simple task. A recent numerical study [Beving et al., Plasma Sources Sci. Technol. 31 084009 (2022)] with particle-in-cell simulations has quantified ion flux reduction factors due to the collisions. As a counterpart of the numerical investigation, we have conducted a series of experiments to directly infer the ion-flux reduction factors due to the ion-neutral collisions. A DC multidipole chamber, MAXIMS [Lim et al., Plasma Source Sci. Technol. 29 115012 (2020)], is used to generate DC Ar or He discharges with the gas pressure ranging from 0.2 to 30 mTorr for Ar and from 1.0 to 65.0 mTorr for He, where the gas pressure functions as to vary the collisionality. To secure wide ranges of the plasma parameter space we have also varied the discharge current from 0.002 A to a few amperes. Ion flux is measured with a disk Langmuir probe. By taking into account the sheath edge expansion caused by large negative bias voltages on the probe with a recently developed power law parametrization technique [Lim et al., Plasma Sources Sci. Technol. 31 024001 (2022)], we obtain the ion flux reduction factors associated with the ion-neutral collisions. Our results are quantitatively similar to the Beving's numerical results for He discharges, while there exist discrepancies for Ar discharges in the higher collisional regime within the investigated experimental conditions. We attempt to explain the agreement and the discrepancy based on an argument of a constant collision frequency and a constant mean free path for modeling the simulations.