New Applications of Langmuir Probes
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DISCHARGE AND PLASMA PHYSICS
New Applications of Langmuir Probes P. E. Masherova, A. F. Piskunkova, V. A. Riabya,*, V. P. Savinovb, and V. G. Yakuninb aResearch Institute of Applied Mechanics and Electrodynamics of the Moscow Aviation Institute (National Research University), b
Moscow, 125080 Russia Faculty of Physics, Moscow State University, Moscow, 119991 Russia *e-mail: [email protected] Received May 22, 2017
Abstract—In this work, two new possibilities for standard probe diagnostics are described. The first one can be used to study isotropic, collisionless low-pressure plasma in which the electron energy distribution function is close to a Maxwellian one. In such plasmas, the Boltzmann law, Bohm effect, and 3/2 power law are valid. Use of corresponding system of equations for cylindrical Langmuir probes allowed for measurements of probe sheath thicknesses and the mean ion mass. The solution of this task was provided by accurate probe diagnostics of inductive xenon plasma at pressure p = 2 mTorr that resulted in the determination of the Bohm coefficient CBCyl = 1.22. The second possibility of probe diagnostics includes a method and device for evaluation of ion current density to a wall under a floating potential using a radially movable plane wall Langmuir probe simulator. This measurement in the same xenon plasma served as the basis for development of an ion source in which the given wall was represented by an ion extracting electrode of the ion extraction grid system. Keywords: plasma, Boltzmann law, Bohm effect, 3/2 power law, Langmuir probe, RF inductive discharge, antenna coil, ferrite core DOI: 10.1134/S1063778817110114
INTRODUCTION Langmuir probes represent the most popular tool for local contact diagnostics of plasma. Standard probe measurements by the Druyvestein method allow registering the following parameters of plasma: function of distribution of electrons energies (FDEE) f(ε) [here ε is electron energy], electron temperature Те, electron concentration ne, floating potential of probe Vf, potential of space Vs, and its corresponding density of electron saturation current jes. These parameters quite sufficiently characterize the physical state of plasma. In the gas discharge space, they are usually measured by cylindrical probes, characterized by constructive simplicity, and they are measured by more complicated flat probes in the near-wall region. This work describes two new possibilities of probe diagnostics. The first of them complements the mentioned set of probe parameters by thicknesses δ of the probe layer of volume charge of the cylindrical probe and average ion mass Mi in Maxwell plasma. These parameters allow controlling the correctness of probe theory applied for interpretation of probe measurements and the purity level of plasma-forming substance: gas itself from the supply pipe and its possible mixture with air, which may flow into the vacuum chamber from the atmosphere. The second possibility of probe diagnostics provides a fast and relatively reasonable estimate of density of ion c
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