CO2-laser Interferometer on the Large Helical Device

This study reports the development of CO₂ laser interferometers for electron density measurements on the large helical device (LHD). Two types of interferometers using approximately 10 μ m are developed. One is an imaging two-color laser interferometer (I-TCI) for measuring density profiles and macroscopic fluctuations of MHD instabilities, and the other is a single channel phase-modulated dispersion interferometer (PMDI) designed for reliable density monitoring. The diagnostic principles, systems, and analysis techniques are outlined, and representative measurement results are presented. The I-TCI system was also designed to operate in the deuterium experiments performed on LHD from 2017 until 2022. Shielding against neutron and γ -ray irradiation was necessary to prevent damage to the I-TCI detection system. Design strategies for neutron and γ -ray shielding are presented in the appendices.