A precision control method for plasma electron density and Faraday rotation angle measurement on HL-2A

The precision of plasma electron density and Faraday rotation angle measurement is a key indicator for far-infrared laser interferometer/polarimeter plasma diagnosis.To improve the precision,a new multi-channel high signal-to-noise ratio HCOOH interferometer/polarimeter has been developed on the HL-2A tokamak.It has a higher level requirement for phase demodulation precision.This paper introduces an improved real-time fast Fourier transform algorithm based on the field programmable gate array,which significantly improves the precision.We also apply a real-time error monitoring module (REMM) and a stable error inhibiting module (SEIM) for precision control to deal with the weak signal.We test the interferometer/polarimeter system with this improved precision control method in plasma discharge experiments and simulation experiments.The experimental results confirm that the plasma electron density precision is better than 1/3600 fringe and the Faraday rotation angle measurement precision is better than 1/900 fringe,while the temporal resolution is 80 ns.This performance can fully meet the requirements of HL-2A.