CO 2 laser imaging interferometer for high spatial resolution electron density profile measurements on LHD

Abstract

A CO2 laser (λ=10.6 μm) imaging interferometer was developed for detailed and precise electron density profile measurements and was installed on the large helical device in 2001. A CO2 laser is adopted to avoid fringe loss due to the short wavelength. An imaging scheme is employed that uses three slab-like beams and multichannel detector arrays to measure with fine spatial resolution. As an initial trial we used one slab-like beam that passes through the plasma edge. Phase distributions were measured without fringe loss even after pellet injection. Density profiles can be obtained by the slice and stack method with a spatial resolution of 16 mm (about 3% of the plasma minor radius). The electron density was found to rise to about 6×1019 m−3 just after pellet injection even near the last closed flux surface. Mechanical vibration effects would be negligible in such a case. A bench top experiment of a coaxial YAG laser (λ=1.06 μm) interferometer for vibration compensation was carried out.