A proposal for a new methodology to analyse Langmuir probe data from Ar and He plasmas in GyM

A Langmuir probe is a minimally invasive diagnostics frequently used to infer and monitor certain plasma parameters, [1]. Typically it consists of a small conductor (metal wire, disc or sphere, for example) inserted in a plasma, and electrically biased with respect to a reference electrode, to collect ion and electron currents. The simplicity of the experimental apparatus should not lead to believe that Langmuir probe data analysis is trivial. Indeed, no general model is available to study and utilize the probe current-voltage characteristics: a vast literature exists on theoretical and experimental studies describing the various regimes and charge collection processes. The plasma in GyM is sustained by a 2.45 GHz microwave source, resulting in a low-tmperature, low-density and weakly ionized plasma, with ion temperature significantly smaller than the electron temperature. The scope of this work is to propose a methodology to analyse Langmuir probe data from the GyM plasma as an alternative to the existing method based on a four-parameter fit, referred to here as 4PF, based on reference [2]. The proposed methodology is based on the perimeter method described in reference [3], and will be referred to here as MPM (Modified Perimeter Method). The plasma parameters of interest are typically the electron temperature and density, and the plasma and floating potentials. The important point in the present study is how to include the effect of the sheath in the data analysis. The sheath may have a significant influence on the collecting area, especially in low-density scenarios. The MPM was tested on He and Ar plasmas in GyM. The proposed MPM is described in Section 2; considerations and comparison will be made to illustrate the validity and applicability of the method in Section 3; conclusion and future work will be summarized in Section 4.