Upgrade of the two-channel CO₂ laser dispersion interferometer on the HL-3 Tokamak

Abstract Electron density is one of the most important parameters in the fusion plasma. Aiming to obtain the information of electron density and its peaking factor after discharge, a two-channel CO 2 laser ( λ =10.6μ m ) dispersion interferometer (DI) has been commissioned on the HL-3 tokamak. The CO 2 laser DI is based on the second harmonic generation technology, and is insensitive to the surrounding vibration. Due to employment of the short wavelength laser source, it greatly reduces the possibility of fringe jump during the measurement. The optical design of the system is based on the Gaussian beam propagation and lens transformation, and two probing beams vertically and doubly pass through the HL-3 plasma, whose detection positions correspond to R = 1.78 m and 2.33 m. Based on the technology of Lock-in amplifier and FPGA, a high-speed data acquisition and processing system is exploited on HL-3. The two-channel CO 2 laser DI was put into operation in 2024, and successfully measured the electron density on HL-3, with time resolution of 0.1 ms and line-averaged electron density resolution of 1.5 × 10 17 m -3 . Furthermore, the CO 2 laser DI was firstly applied in China to real-time density feedback control, and plays a crucial role in the control and operation of plasma on HL-3.