Langmuir probe diagnostic on magnetic plasma devices often encounters more challenges in data processing than in non-magnetized plasmas, the latest itself being far from simple. In this paper, a theory of particle collection by a probe at the plasma potential in collisionless weakly ionized plasmas is constructed, accounting for velocities distributed according to the Maxwell equation and different mechanisms of particle collection depending on their speed. Experimental validation of the presented theory has been done with 2 cylindrical probes (rpr = 75 μm and Lpr = 1 cm and rpr = 0.5 mm and Lpr = 1 cm) parallel to B→ on a linear plasma device Aline, with magnetic fields of 0.0024–0.1 T and plasma densities of 1015–1017 m−3 in helium. Cylindrical probe measurements are compared to data from a planar probe perpendicular to the magnetic field, and the results for electron density, temperature, and plasma potential are presented. The introduced theory is initially constructed for a cylindrical probe but is applicable to various probe sizes, shapes, and orientations. Alongside the main subject, a number of associated issues are addressed with different details: a probe design issue relative to the magnetized environment, the “intersection” method of plasma potential evaluation, and the robustness of the conventional “1st derivative” method, a current bump near the plasma potential, lower limit for electron temperature estimation, and self-consistent calculation of electron temperature and density.
In this paper we construct a numerical technique capable of solving Maxwell's equations in frequency domain, both in vacuum and in cold magnetized plasma, with a boundary condition that guarantees the existence of a potential associated with the radiofrequency electric fields tangential to certain surfaces. This potential is of interest to nonlinear sheath physics, since it enables the calculation of the time-dependent sheath current excited by a single-frequency electromagnetic wave, and thereby the associated DC sheath current and sheath potential.
Ion cyclotron range of frequencies (ICRF) wave propagation is calculated theoretically for tokamak conditions and for linear magnetized plasma device IShTAR which is dedicated to the RF sheath studies. Only the slow wave (SW) mode of ICRF waves can propagate and be studied in IShTAR. Therefore it is possible to decouple the role of the different ICRF modes in the RF sheath effects. Numerical simulations of the ICRF SW are done in COMSOL in the framework of the existing cold plasma modelling package RAPLICASOL and the SW is for the first time modelled in 3D. To date, RAPLICASOL existed as a 3D wave coupling modelling approach which targets the fast wave (FW). Plasma is implemented as a material with manually assigned physical properties and a perfectly matched layer (PML) is used to absorb the wave energy. Here it is demonstarted how to adjust the RAPLICASOL PML for models with propagating SW. Field structures in the resonance cone shape obtained for the SW differ significantly from the FW and exhibit strong dependence on the density profile in the close proximity of the antenna. The lower-hybrid (LH) resonance is a constant issue in the attempts to model the SW. In this work an approach to obtain correct numerical solutions in the LH resonance presence is demonstrated. Results of this work can be used to improve the complex tokamak ICRF simulations, where so far the SW propagation on the edge has been avoided.
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