High precision phase difference calculation based on adaptive mixed-radix All-phase FFT for Tokamak plasma electron density measurement

In many Tokamak devices, laser interferometers, such as HCN, DCN, CO2, etc, are employed to measure the plasma electron density by calculating the phase difference between the reference signal and the detector signal. This paper proposes an adaptive All-phase fast Fourier transform (Ap-FFT) method in the electronic density of the plasma. And analyze the reasons why the multi-frequency measurement phase is inaccurate. The proposed method can reduce the accuracy of phase calculation by adjusting the conversion length and mixed-radix to reduce the spectrum leakage and grid effect. At the same time, the experimental data verifies the correctness of the calculation results of this method, and it shows that the measurement uncertainty of the method is 10% higher than the 1024-point Ap-FFT, and 15% higher than the 1024-point FFT. This method can be effectively applied to the measurement of the electronic density of an ionic plasma for plasma and provides a valuable reference for other phase detection systems and equipment.