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Diagnostics of VHF Argon Plasmas by Laser Thomson Scattering

Weiting CHEN, Kohei OGIWARA¹⁾, Koichiro KOGE, Kentaro TOMITA, Kiichiro UCHINO and Yoshinobu KAWAI

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

1)

Graduate School of Information Science and Electrical Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan

(Received 24 May 2013 / Accepted 5 June 2013 / Published 15 August 2013)

Abstract

The laser Thomson scattering (LTS) method has been applied to measure the electron density n_e and electron temperature T_e of very-high-frequency (VHF) argon plasmas. When the probing laser wavelength was 532 nm and the laser power density was ∼10¹⁵ W/m², the Thomson scattering spectrum was obviously deformed by the effect of the photo-ionization of metastable argon atoms. The threshold laser power density at which the scattered light intensity from electrons in the plasma and that from electrons produced by photo-ionization are equivalent was found to be unexpectedly low (4 × 10¹³ W/m²). To avoid the photo-ionization of metastable argon atoms, the laser power density was decreased to around 1×10¹³ W/m² by using a cylindrical lens as the focusing lens. Then, the n_e and T_e values measured by LTS and the probe method were compared for a VHF plasma using argon gas at a pressure of 100 mTorr. This comparison confirmed that the LTS method gave reasonable n_e and T_e values.

Copyright (c) 2013 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

Thomson scattering, VHF plasma, argon gas, photo-ionization, metastable atom

DOI: 10.1585/pfr.8.1306114

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This paper may be cited as follows:

Weiting CHEN, Kohei OGIWARA, Koichiro KOGE, Kentaro TOMITA, Kiichiro UCHINO and Yoshinobu KAWAI, Plasma Fusion Res. 8, 1306114 (2013).