Enhanced method for analyzing Langmuir probe data and characterizing the Electron Energy Distribution Function (EEDF)

Abstract

Langmuir probes (LPs) are a versatile diagnostic tool that can be used to calculate a multitude of plasma parameters including electron temperature, number density, and the electron energy distribution function (EEDF), especially when the probe theory is properly matched with the plasma regime. In a Hall-effect thruster (HET) plasma plume, where the flow is supersonic and mesothermal, conventional approaches attempt to avoid measuring the anisotropic components and apply classical LP theory for isotropic, Maxwellian plasmas. Not only does this result in large systematic error bars (20%–50%) but also gives no further insight into the dynamics of these plasmas. In this study, the probe axis is placed transverse to the flow direction. The resulting effect of this orientation on the sheath and wake was taken into account through the LP theory used to analyze the data and in fact was used to the advantage of composing a streamlined, nongraphical analysis. The high-speed dual LP system was used downstream of the H6 HET to take time-averaged measurements. A pre-analysis binning method was used instead of smoothing, along with the Druyvesteyn method to find the EEDF. Error bars were ≤12% for each plasma parameter. A chi-square fitting routine along with statistical parameters were used to obtain more information about the EEDF's form and to guide the interpretation of the fit. The EEDFs were mainly drifting Maxwellians with a velocity shift of ∝106 m/s. This drift could be due to a number of mechanisms, one being some combination of double layers and beam-plasma interactions.