Time–frequency analysis of nonstationary complex magneto-hydro-dynamics in fusion plasma signals using the Choi–Williams distribution

• Choi–Williams distribution yields excellent time–frequency resolution for discrete signal.
• CWD method provides clear time–frequency pictures of EAST and HT-7 fast MHD events.
• CWD method has advantages to wavelets transform scalogram and the short-time Fourier transform spectrogram.
• We discuss about how to choose the windows and free parameter of CWD method.

The Choi–Williams distribution is applied to the time–frequency analysis of signals describing rapid magneto-hydro-dynamic (MHD) modes and events in tokamak plasmas. A comparison is made with Soft X-ray (SXR) signals as well as Mirnov signal that shows the advantages of the Choi–Williams distribution over both continuous wavelets transform scalogram and the short-time Fourier transform spectrogram. Examples of MHD activities in HT-7 and EAST tokamak are shown, namely the onset of coupling tearing modes, high frequency precursors of sawtooth, and low frequency MHD instabilities in edge localized mode (ELM) free in H mode discharge.