Progress of the in-building J-TEXT FIR three-wave polarimeter

A Far-infrared (FIR) polarimeter has been designed and is under development on J-TEXT tokamak. J-TEXT polarimeter is proposed to measure internal plasma magnetic field and electron density simultaneously, with a temporal resolution of ∼ 1 μs and spatial resolution up to 1.5 cm. After earlier conceptual design, extended work has been done for the system set-up and optimization. The configuration of J-TEXT polarimeter has been improved for better performance, and is more compatible to adapt to limited port space. Three FIR lasers at 432 μm are already prepared and tested, which are capable to maintain more than 30 mW/cavity output for 8 hours without manual control. The output characteristics of beam expansion optics employed for the multichannel arrangement are measured and show a good agreement with expectation. A bench test is carried out to assure the feasibility and reliability of the system. Phase resolution of < 1 mrad is obtained, which meets the requirement of measurement. Vibration tests on the mechanical supporting structure are conducted to verify and improve the system environment. Vibration reduction is achieved by optimizing mechanical structures, and the preliminary results show a decrease on vibration amplitude of up to two orders. The first result of J-TEXT polarimeter is expected to be obtained during the next experimental campaign of J-TEXT tokamak.