Will shifted and truncated Maxwellian EVDF near the wall exhibit different electron currents?

Abstract

Experiments to explore the predicted consequences on the I-V characteristics of a Langmuir probe based on the fluid approach for electrons and the electron Bohm criterion are conducted and their results are presented. The predictions on the I-V characteristics of a Langmuir probe when electrons are highly collisional are shown in terms of the estimation of plasma potentials and electron saturation currents, primarily represented by a rounded knee of the curve. An edge-effect reduced and guard-ringed Langmuir probe is employed to minimize geometrical effects on the measured data. To characterize the rounded knee of the curve, various methods on defining critical points around the plasma potential are presented. Additionally, emissive and cutoff probes were utilized to obtain bulk plasma properties, providing accurate reference plasma parameters. The physically valid condition for electrons to exhibit a shifted Maxwellian velocity distribution near the wall, wherein the fluid approach is applicable, is established from theory of the ion-acoustic instability enhanced electron-electron collisions. In this work, we conducted experiments scanning from collisionless to collisional regimes for electrons with theoretical considerations. Our observation indicate that the Langmuir probe can be understood with the conventional wisdom represented by the model of truncated electron velocity distribution. This result suggests the absence of the enhancement of electron-electron collisions and/or incompleteness of electron fluid approach near the wall.