Parametrization of electron sheath expansion for Langmuir probe I-V traces

Abstract

The Langmuir probe is a common plasma diagnostic widely utilized in various fields of plasma physics, and its associated methods of data analysis remains in active development. In this work, we present an attempt using a power law parametrization model similar to Sheridan's empirical formulation is to describe the sheath expansion effect on electron current in an argon plasma. The fitting results using the power law parametrization method show observably better consistency compared to those using conventional interception method and inflection method. It is also observed that when used to fit the electron saturation current at probe bias voltages well beyond the plasma potential plus the argon ionization energy, resultant curve fitted by power law parametrization, as with that by linear fitting, deviates significantly from the curve fitted at lower bias voltages, albeit the saturation obtained fitted using the power law method remained consistent with that fitted at lower probe bias voltages. Probable causes of this deviation include anode glow or spots near the probe and the changes of the global plasma parameters via electron absorption at very high probe bias. The fact that the resultant electron saturation current remained consistent hints that these non-trival effects can also be approximated by power law parametrization.