This paper introduces a real-time high precision measurement of phase difference based on Field Programmable Gate Array (FPGA) technology, which has been successfully applied to laser grating interference measurement and real-time feedback of plasma electron density in HL-2A tokamak. It can track the changes of electron density while setting the starting point of the density curve to zero. In a laboratory test, the measuring accuracy of phase difference is less than 0.1°, the time resolution is 80 ns, and the feedback delay is 180 μs.
In the real-time plasma electron density measurement using far infrared (FIR) laser interferometry, the plasma electron density can be calculated by measuring the real time phase difference between the reference signal and the probe signal. A novel Real-time Phase Jump Process (RPJP) method is applied to the HL-2A tokamak. With this method, the phase difference precision is up to 1 3600 fringe (1 fringe is equal to a phase shift of 2π), and the dynamic measurement range is extensible 65536 fringes. The time resolution of the phase difference is 80 ns, while the feedback delay is 180 µs.
Electron density and Faraday rotation angle are important physical parameters in nuclear fusion research. To measure them simultaneously, the three-wave polarimeter/interferometer diagnostic system is applied. Both the final probe output signal and the reference signal contain three frequency components. The time-varying phase difference curve of each frequency component can be measured by the Real-time Dynamic Spectrum Analysis (RDSA) method based on Field-Programmable Gate Array (FPGA). The phase difference precision is better than 0.1 o and the real-time feedback delay is less than 1 ms, which satisfy the requirements of 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.
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