Comparison between experimental Langmuir probes and three species one-dimensional kinetic simulations

Abstract

The biasing of Langmuir probes to obtain the appropriate measurements in weakly ionized plasmas is addressed in this work. The experimental results show discrepancies when this measurement method is employed with sweep signals of different symmetries or frequencies. This means that the method provides signal dependent plasma parameters, which hinder its diagnosis. In this work, a three species one-dimensional kinetic model is applied with a sweep electric field to simulate the dynamics of a weakly ionized plasma close to a Langmuir probe. With the same symmetries and frequencies for the sweep signal in both the experimental and numerical problems, a qualitative similar behaviour is found. The response of the three species plasma due to changes of the electric field is analysed by means of a three species kinetic collisional model. The effect of constant averaged or energy dependent characteristic collision frequencies is studied. These numerical results are useful to provide a first explanation of the discrepancies found in the experimental data and improve the measurement process.

Propagator integral method for plasma physics kinetic equations with practical applications

“…Moreover, weakly ionized plasmas are usually cold and non-thermal, T e ∼ 1-10 eV while T i , T 0 ≪ T e , which means that characteristic time scales vary for each species in a wide range. These time scales are similar to those involved in experiments, which means that collisions are important and cannot be neglected in order to obtain a proper representation of the system dynamics [28]. The peculiarities of these complex systems lead to simplifications and fluid models that may not completely capture the microscopic dynamics of these systems failing to describe the physical behaviour of the problem In this chapter a three species one-dimensional kinetic description for weakly ionized plasmas is presented, including a set of numerical resolutions and test performed to analyse the response of the model.…”

“…The method is applied to solve arduous models, accounting for a variety of physical effects of disparate intensity, that are very difficult to implement with classical numerical methods, specially in Weakly Ionized plasmas (WIP), were collisions play a very relevant role [25,28]. Similar methods, based on propagators or other integral kernels, have been employed to solve a wide range of problems in physics and engineering [29][30][31][32][33].…”

“…Moreover, in plasmas is typical to analyse the electrical current density, defined as J γ = [q γ ] = q γ n γ u γ , which is a parameter related to the electrical current measured in laboratories by means of, for example, electrical probes [28,[83][84][85]. In addition, total heat flux can be computed by Q γ = [v 2 ]/2n γ .…”

“…In several practical applications the Dougherty operator [91,[109][110][111] has been used to describe non-lineal collisional terms, as we did in [25,28,112] for the one-dimensional case. This two-dimensional form can be extracted from the previous Hard Spheres potential.…”

“…New collision terms to account for the interaction of charged species with the large amount of neutrals must be included. These terms are usually neglected due to the short-distant interaction, but they become relevant in WIP due to the high density of neutrals acting as a viscous media [24,28]. Also, dynamics of the heavy species, as ions, is usually not accounted for, which may end up modifying the electron dynamics through the self-consistent electromagnetic field [101].…”