Infrared Laser Diagnostics for ITER

Abstract

Two infrared laser-based diagnostics are under development at the Oak Ridge National Laboratory (ORNL) for measurements on burning plasmas such as ITER. Our primary effort is the development of a CO₂ laser Thomson scattering diagnostic for the measurement of the velocity distribution of confined fusion-product alpha particles. This diagnostic utilizes small-angle collective scattering of infrared light from the electron cloud surrounding the alpha particles. Key components of the system include a high-power, single-mode CO₂ pulsed laser, an efficient optics system for beam transport and a multichannel low-noise infrared heterodyne receiver. A successful proof-of-principle experiment has been performed on the Advanced Toroidal Facility (ATF) stellerator at ORNL utilizing scattering from electron plasma frequency satellites. The diagnostic system is currently being installed on Alcator C-Mod at MIT for measurements of the fast ion tail produced by ICRH heating. A second diagnostic under development at ORNL is an infrared polarimeter for Faraday rotation measurements in future fusion experiments. A preliminary feasibility study of a CO₂ laser tangential viewing polarimeter for measuring electron density profiles in ITER has been completed. For ITER plasma parameters and a polarimeter wavelength of 10.6 µm, a Faraday rotation of up to 26° is predicted. An electro-optic polarization modulation technique has been developed at ORNL. Laboratory tests of this polarimeter demonstrated a sensitivity of ≤0.01 °. Because of the similarity in the expected Faraday rotation in ITER and Alcator C-Mod, a collaboration between ORNL and the MIT Plasma Fusion Center has been undertaken to test this polarimeter system on Alcator C-Mod. A 10.6 µm polarimeter for this measurement has been constructed and integrated into the existing C-Mod multichannel two-color interferometer. With present experimental parameters for C-Mod, the predicted Faraday rotation was on the order of 0.1°. Significant output signals were observed during preliminary tests. Further experiment and detailed analyses are under way.