Design of Vertical Thomson Scattering Diagnostics for the T-15MD Tokamak

Abstract

For measuring the parameters of the plasma electron component in the T-15MD tokamak, the complex of Thomson scattering diagnostics is being designed, which will make it possible to study different regions of the plasma. In this work, we discuss the design of the Thomson scattering system with vertical probing for T-15MD, which provides for information on the plasma parameters along the vertical diameter of the vacuum vessel. Plasma probing is performed using the Nd:YAG laser with multipass system for inject laser beam into the tokamak. The laser operates at the second harmonic λ = 532 nm. Scattered radiation is collected by one wide-angle lens. Light is transmitted to the registration system by the fiber-optic collector consisting of 159 fiber-optic assemblies with the size 2 × 1 mm. The registration system consists of three units; each unit includes relay optical system and polychromator with detector. The spectrum of scattered radiation is recorded using image intensifier with CMOS camera. For achieving high light transmittance of the diagnostics optical system, the parameters and design of each optical unit were calculated in detail: the collecting lens, the relay optical system, and the polychromator. As compared to the T-10 Thomson scattering diagnostics, the system transmittance was considerably improved due to selecting rigorously the optical materials, as well as applying new design of the relay optical system, consisting mainly of mirrors. Using synthetic diagnostics, the accuracy of measuring the electron temperature and density was estimated. The plasma spectra from the region of T-15MD limiter are used as the plasma background. The Thomson scattering diagnostics system with vertical probing will make it possible to measure the electron temperature of plasma of the T-15MD tokamak with an error of less than 10% in the range from 80 eV to 6 keV at an electron density of more than 6 × 10¹⁸ m⁻³ in the central plasma region. At the plasma edge, the error will be less than 10% for the T_e range from 100 eV to 2 keV at densities n_e > 1 × 10¹⁹ m⁻³. The spatial resolution of the diagnostics will be ~11 mm at the center of the plasma and ~22 mm in the edge plasma.