On the reliability of probe diagnostics in RF plasma
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MA DIAGNOSTICS
On the Reliability of Probe Diagnostics in RF Plasma V. A. Ryabyi and V. A. Obukhov State Research Institute of Applied Mechanics and Electrodynamics, Moscow Aviation Institute, Volokolamskoe sh. 4, Moscow, 125810 Russia email: [email protected] Received March 25, 2012
Abstract—The errors of probe diagnostics of the resting or slowmoving plasma of Qmachines or dc dis charges are shown to lie typically within a range of ±(20–30)%. Problems of probe diagnostics of RF plasma and modern approaches to their solving are considered. The objectivity of probe diagnostics of RF plasma is established by the mutual agreement between the electron energy distribution functions measured in the same experiment using the Langmuir probes and the method of relative intensities of spectral lines. DOI: 10.1134/S1063780X13050164
INTRODUCTION The plasma parameter measurements providing the knowledge of the electron energy distribution function (EEDF) and the electron density ne and tem perature Te play an important role in experimental physics because they make it possible to verify that understanding of physics of the investigated plasma source is correct and that construction of the device (and its systems) is effective and developed with due competence. Of great importance is also the local plasma diagnostics that yields a detailed picture of the workingmatter ionization in the plasma source under study. The simplest tool for local plasma diagnostics is a Langmuir probe in the form of a small segment of thin wire that is inserted into the place of interest in the plasma generator. This is a cylindrical probe, whose volt–ampere characteristic with respect to the refer ence electrode (gasdischarge electrode or metallic surface coming in contact with plasma) provides data on the electron temperature Te and electron density ne, as well as on the plasma potential with respect to the same electrode. The mentioned information is obtained as a result of the probecharacteristic pro cessing based on the probe theory. Its simplest variant in respect to a dc glow discharge was proposed in 1924 by I. Langmuir [1], after whom an electrostatic probe was named (four years later, Langmuir introduced also the word “plasma” itself [2], which denoted a quasi neutral ionized medium). The Langmuir theory sug gests the Maxwellian electron energy distribution, which is most probable for nonisothermal plasma of dc glow discharge under a pressure below 10 Pa. This EEDF is even more probable for the currentfree iso thermal immovable or slowly moving surfaceionized plasma of Qmachines [3] that were applied, in partic ular, for studying the wave formation or diffusion pro
cesses in the plasma concerning thermonuclear research. Under these conditions corresponding to those of the Langmuir theory applicability, it seemed very interesting to reveal the degree of reliability of probe measuring results. For this purpose, researchers made measurements of the same plasma parameters using different methods in a single experiment. So, the v
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