Design and characterization of the polychromators for JT-60SA Thomson scattering systems

The Thomson scattering diagnostics are fundamental for a reliable determination of electron density and temperature profiles in Tokamak experiments. In JT-60SA, two systems are currently under development: one is dedicated to the core plasma region, the other to the edge region. They aim to measure the Te and ne profiles with a precision better than 10% and 5%, respectively, at an electron density of 1019 m-3 and with an electron temperature ranging from 10 eV up to 10 keV for the edge system and from 100 eV to 30 keV for the core one. The polychromators design, based on interference filters and APD detectors, is critical to achieve such performance. Each polychromator comprises 6 spectral channels measuring from 1064 nm down to 500 nm for the core system and to 660 nm for the edge one. The design aims at maximizing the efficiency and minimizing the noise under different plasma conditions. About 100 polychromators are manufactured by G.N.R.srl under an F4E contract. In this work, we present the polychromators design and performance. The key features of the optical design and of the mechanical structure are discussed. State of the art interference filters and APD detectors with custom electronics are described. Overall optical transmission measurements of the assembled polychromators and spectral channels calibration are then presented. More specific studies of the APD electrical noise and temperature dependence are discussed. The transmission functions are finally used to predict the error expected in the determination of the electron temperature and density under different levels of plasma light intensity.