Study of MHD Activities in IR-T1 Tokamak Plasma Using Hilbert-Huang Transform

In the IR-T1 tokamak plasma, study of the MHD activities is presented by using the combination of singular value decomposition (SVD) and Hilbert-Huang transform (HHT). In this work, we have proposed the SVD+HHT method to study the MHD activity for mode number identification and time-frequency behavior. Also, we have used spatio-temporal structures of the Mirnov coil fluctuations in terms of correlation function to better identification of mode number and frequencies of dominant MHD modes. High correlation values of spatio-temporal wave-like structures in lifetime below 0.1 ms in the different poloidal separation were observed. It was shown that the main frequency rotation of the MHD activities is above 10 kHz. Spatio-temporal wave-like structures, with maximum value in poloidal separation 120 or 180 degrees, show that the wavelength of the MHD mode is related to m = 2 and m = 3. Also, there are small-scale structures, mostly contributed by high-frequency short wavelength fluctuating component in the bottom of the poloidal separation 90 degrees related to m = 4 or m = 5 mode numbers. These results have been compared with the SVD + HHT analysis results. In addition, amplitude modulation of Mirnov oscillations in IR-T1 tokamak plasma generates intra-wave frequency modulation. These amplitude variations of MHD oscillations cause a frequency fluctuation. Our results have indicated that in the Hilbert spectrum (HT) analysis, the intra-wave frequency modulation is small compared to the short-time Fourier transform (STFT) and wavelet transform (WT) spectrum.