Two-color CO₂ laser long-distance interference measurements with long-time stabilization in JT-60SA.

This article focuses on the development and performance of a two-color CO₂ laser interferometer system in JT-60SA, designed for long-distance transmission and long-term stabilization to measure line-integrated electron density in plasma. The system employs two CO₂ lasers at wavelengths of 10.59 µm and 9.25 µm, combined with a newly developed beam axis stabilizing system using an additional YAG laser, enabling stable beam transmission over a 214 m optical path for more than 11 hours. Experimental results demonstrate a density resolution of 2 × 10¹⁹ m⁻² and show that the two-color interferometer effectively reduces noise and oscillations caused by mechanical vibrations and plasma disruptions compared to one-color systems. However, residual phase drifts remain due to temperature-dependent refractive index changes in transmission media, which cannot be fully compensated by the two-color method. The article also discusses challenges related to optical line length differences, noise sources, and future improvements including wavelength selection and transmission media to enhance measurement accuracy in large fusion devices.