FPGA-Based Digital Lock-In Technique for Real-Time Measurement of Polarization Angle in MSE Diagnostic on EAST

Real-time measurement of polarization angle of the Stark polarized light in motional Stark effect (MSE) diagnostic is essential for obtaining current density distribution, feedback control of which can be utilized to achieve advanced mode of tokamak device. In this article, a digital lock-in amplifier (DLIA) based on field programmable gate array (FPGA) is developed, which is used for real-time measurement of polarization angle in MSE diagnostic on the experimental advanced superconducting tokamak (EAST). By demodulating the intensity-modulated polarized light that contains multiple harmonic components, the DLIA-based system can efficiently extract the amplitudes of two second-harmonics and further obtain the polarization angle from the two extracted amplitudes. The operation period of the system is 260.84 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> s, which well satisfies the demands of MSE diagnostic for working cycle within 1 ms. Compared with the previous fast Fourier transform (FFT) method, the developed DLIA-based system has a lower latency. Compared to the commercial lock-in amplifiers (LIAs) currently used, the DLIA-based system in this work has the advantages of high integration and directly outputting the polarization angle of the Stark polarized light without extra signal processing. The performance of the DLIA-based system is demonstrated in laboratory test and field test of EAST. The maximum relative error in field test on EAST is less than 5%, meeting the requirement of MSE diagnostic.