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Application of Digital Phase Analysis to Far-Infrared Laser Interferometer for the Large Helical Device

Hikona SAKAI¹⁾, Kenji TANAKA^1,2), Yuki TAKEMURA^2,3), Yasuhiko ITO²⁾, Tokihiko TOKUZAWA^2,3), Ryo YASUHARA^2,3), Hiyori UEHARA^2,3), Toshiki KINOSHITA¹⁾, Tsuyoshi AKIYAMA²⁾, Kazuya NAKAYAMA⁴⁾ and Kazuo KAWAHATA²⁾

1)

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

2)

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

3)

The Graduate University for Advanced Studies, SOKENDAI, Toki, Gifu 509-5292, Japan

4)

College of Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan

(Received 31 August 2022 / Accepted 22 May 2023 / Published 2 August 2023)

Abstract

A digital phase analysis technique was applied to a 119-µm wavelength far-infrared (FIR) laser interferometer on the Large Helical Device (LHD). High-density plasma measurement without phase jumping was achieved, representing an improvement over measurements made using a conventional analog phase counter. Digital phase analysis became operational at a quarter of the threshold of the analog phase counter. The phase sensitivity was also improved using digital analysis because the noise level was reduced to approximately half that of the conventional analog phase counter. The improved phase sensitivity enabled measurement of small-amplitude fluctuations.

Copyright (c) 2023 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

plasma, interferometer, far-infrared laser, digital demodulation

DOI: 10.1585/pfr.18.1402062

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