A cylindrical langmuir probe primary probe holder size effect on the results of local plasma diagnostics

Аuthors

Masherov P. E.

Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

e-mail: p.masherov@ya.ru

Abstract

The purpose of this work was to study the primary probe holder of a cylindrical Langmuir probe relative size effect on the results of local plasma diagnostics.

The primary probe holder radius, that should be far less than the electrons free path average length was the main subject of consideration in the presented paper, because it is the primary probe holder that is able to decrease the concentration of electrons and change other parameters of radiated plasma in the spots of its probing. It significantly affects the quality of local plasma diagnostics.

Three types of cylindrical probes made of tungsten thread of 0.15 mm in diameter were used. All three probes were provided with probe holders of the same diameter of 1.6 mm. The idea of the work consisted in obtaining measurements from the probes of various lengths under equal impact of the primary probe holder, which increased surface, allocated near the probed area contributes to recombination of charged plasma particles on its surface, and, thus reduces the level of ionization equilibrium in the radiated plasma. The cylindrical probe of a certain length averages the parameters of plasma in its scope. The invariable local distortion of plasma parameters near the probe holder affects differently the measured results for probes of various lengths. The work employs such probe lengths range that allows point out their bound, outside which the disturbances of the considered type become less than the total error of probe measurements.

The paper describes the experiment with HF induction (HFI) discharge in Xenon and probe measurements carried out using probe station Plasma Sensors VGPS-12. A number of technical features of VGPS-12 allows increase the accuracy of plasma diagnostics results, narrowing down, for example, electron density measuring error field to the value of about 10%. These features include: implementation of Dryuvesteyn method that does not require prior guesses on the shape of the electron energy distribution function in contrast to the other probe techniques; suppression of the most part of errors by protecting reference electrode with developed surface; probe surface cleaning by ion bombardment and HF current heating.

The work is concluded by the analysis of the obtained results, allowing formulate recommendations on selection of the main sizes cylindrical Langmuir probes to ensure the acceptable plasma diagnostics accuracy.

Keywords:

high frequency discharge, local plasma diagnostics, Langmuir probe, electron distribution function

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