Automatic Beam Alignment System for Thomson Scattering Diagnostic on Experimental Advanced Superconducting Tokamak

Thomson scattering (TS) diagnostic has been widely used as a standard diagnostic on tokomaks for more than 45 years, measuring the electron temperature and density profiles of fusion plasma. The pointing stability of the laser beam crossing the plasma could impact the measuring accuracy greatly. The laser beam path of the current TS diagnostic system on Experimental Advanced Superconducting Tokamak is about 40 m, equipped with seven mirrors and one focusing lens. The laser beam needs to be aligned regularly because of the instability of laser beam pointing caused by many factors. In this paper, an automatic beam alignment system used to calibrate the deviations of laser beam is introduced. This system employs two Charge Coupled Device (CCD) cameras and two two-dimension adjustable servo-motorized mirror mounts. CCD cameras capture images of the laser spots to calculate their positions by processing the images. Then compare the calculated centroids with the reference values to find the deviations. A fuzzy logic controller is designed with the deviations in pixels accepted as the inputs, and its outputs are the data required to adjust the mirror mounts automatically. This system has been tried and it is corroborated that the deviations could be corrected in several minutes by adjusting the two servo-motorized mirror mounts automatically in turn.