Plasma and Fusion Research

Volume 17, 1402107 (2022)

Regular Articles

Development of Two-Color Laser Imaging Interferometer Using CO2 Laser and Quantum Cascade Laser in the Large Helical Device
Toshiki KINOSHITA, Kenji TANAKA1), Yuki TAKEMURA1,2), Shota TAKESHIDA and Hikona SAKAI
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
1)
National Institute for Fusion Science, National Institutes of Natural Sciences, Toki, Gifu 509-5292, Japan
2)
The Graduate University for Advanced Studies, SOKENDAI, Toki, Gifu 509-5292, Japan
(Received 6 September 2022 / Accepted 18 October 2022 / Published 9 December 2022)

Abstract

CO2 laser interferometers are a promising option for high-density plasma measurements. However, in low- and middle-density measurements, noise due to mechanical vibrations is a serious problem. To remove this noise, we developed a two-color laser imaging interferometer using a CO2 laser and quantum cascade (QC) laser, called the CO2/QC laser imaging interferometer, through benchtop experiments and installed it in the Large Helical Device (LHD). Benchtop experiments provided optical design guidelines for the CO2/QC laser imaging interferometer to minimize the influence of the unstable output wavelength of the QC laser. The optical system in LHD was designed according to this guideline, and the vibration noise was successfully reduced to 2.80 × 1018 m−3. We also demonstrate measurement examples of hollowed and peaked electron density profiles evaluated using Abel inversion and macro-scale instability. This is the first study to present the measurement results of high-temperature plasma using a CO2/QC two-color laser interferometer. The study outcomes provide important insights for the development of two-color laser interferometers in future fusion devices.

Keywords

plasma diagnostics, interferometer, CO2 laser, quantum cascade laser, LHD

DOI: 10.1585/pfr.17.1402107

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