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

Abstract

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.

High speed FPGA-based Phasemeter for the far-infrared laser interferometers on EAST

“…Compared with conventional digital phase detection technique, FPGA-based digital phasemeter has better performance on Signal-Noise Rate (SNR), time resolution and precision when dealing with the signals in the MHz range. Moreover, the phasemeter also has the ability of real-time data reduction in that real time output is possible [5,6]. Our Phasemeter is wherefore designed with dedicated plug-in front-end modules for specialized bandwidth.…”

“…Accordingly, Fast Fourier transform (FFT) method can be applied to obtain fundamental parameters, including phase information. (2.3) and (2.4) can be expressed in the form of (2.5) afterwards [5,6],…”

“…where θ is the phase of target wave, and Im[S(k)] is the imaginary part of the transformed signals while Re[S(k)] is the real part [5][6][7][8]. A finite impulse moisturizer response (FIR) filter with Hanning window is used to isolate target IF signals from three-wave signals.…”

An FFT overlap method for high bandwidth far-infrared laser Interferometer in HL-2A

“…Driven by the demand for high-bandwidth electron density diagnostics, the modulation 2021 JINST 16 P09010 signal frequency of the FIR interferometer is upgraded to 1-5 MHz with two methanoic acid lasers [12][13][14]. However, the phase difference measurement system is optimized for high accuracy, which limits high bandwidth measurement [15][16][17].…”

Research on the phase adjustment method for dispersion interferometer on HL-2A tokamak

“…Laser interference measurement, especially far-infrared laser interference measurement is an important diagnostic technique for plasma electron density measurement [2]. A heterodyne system, which consists of two individual HCOOH laser sources, has been installed on HL-2A for the density measurement [3][4][5][6]. It suffers from fringe jump problems due to the long wavelength of the HCOOH lasers, when the plasma density changes rapidly.…”