Application of a bandpower correlation method to the statistical analysis of MHD bursts in quiescent Wendelstein-7 AS stellarator plasmas

This paper presents the analysis of Mirnov coil data from pure electron-cyclotron resonance heated Wendelstein-7 AS discharges with transient magnetohydrodynamical ( MHD) mode activity in otherwise MHD inactive plasmas. Amplitude modulations were found on the 100 mu s time scale, expected to originate from transient changes of the plasma pressure profile by anomalous transport events. The analysis was done using our custom-developed bandpower correlation method, which combines short-time Fourier transform for the decomposition of the signal with a correlation analysis to quantitatively characterize the statistical connection between the modulations in the power of different MHD modes. The effect of a confinement transition ( near a rational rotational transform) on the correlation between the power modulations of different frequency modes has been studied. As high as 50% correlation was found between different frequencies in 'bad confinement' shots with enhanced transient transport, while for 'good confinement' shots no significant correlation was observed.