Mechanical and optical design of the FIR laser Polarimeter/Interferometer on the HL-3 tokamak.

• A 13-channel far-infrared (FIR) laser Polarimeter/Interferometer has been designed for the HL-3 tokamak, including eight horizontally-viewing channels and five oblique channels. • The novel structure of support tower allows us to arrange multi-channel light-path on both sides. • Multichannel line-integral electron densities have been successfully measured by the interferometric measurement in 2023, with a time resolution of <1.0 microsecond. The interferometer system will be upgraded to the three-wave based Polarimeter/Interferometer. Aiming to achieve important parameters of the electron density and plasma current density profile, a multi-channel far-infrared (FIR) laser Polarimeter/Interferometer is being developed for HL-3 tokamak by using the formic-acid laser source (HCOOH, λ=432.5 μm). All the 13 probing channels were distributed in two columns, including eight horizontally-viewing channels and five oblique channels. The stainless steel support tower (7.0 m high) was specially designed, and itself novel structure allowed us to arrange the light-paths of horizontal channels and oblique channels on two sides, respectively. Copper corner-cube-reflector (CCR) was mounted in the vacuum vessel to retrace the incident wave. In 2023, multi-channel line-integral electron densities were successfully derived by the interferometric measurement, with a time resolution of 1.0μs, line-integral density resolution of ∼5.0 × 1016/cm2. Right now, the interferometer system is being upgraded to three-wave based Polarimeter/Interferometer, which is able to simultaneously measure the electron density and Faraday rotation angle. [ABSTRACT FROM AUTHOR]