Obtaining the unperturbed plasma potential in low-density, low-temperature plasmas

A method is proposed to effectively obtain plasma parameters in low-density, low-temperature plasmas that are easily perturbed by Langmuir probes. The methodology is based on the perimeter sheath expansion method because it provides a simple, but effective interpretation of the Langmuir probe characteristics and, in particular, of the non-saturating trend of the ion saturation due to the expansion of the probe sheath. However, the perimeter sheath expansion method is proposed here in a modified version with a dual purpose: to permit the verification of the Child-Langmuir dependence of the sheath thickness from the potential and, most importantly, to properly assess the plasma potential. When probe perturbation is significant the proposed method provides an effective correction term for the plasma potential. The resulting sheath voltage is consistent in all analysed cases and in close agreement with the expected value for cylindrical probes in Ar. The method is applied to Ar plasmas here, but because no specific assumption was made on the atomic species, it may be applicable to other situations of interest.