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 µs 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.
Fourier decomposition of magnetic perturbations in toroidal plasmas using singular value decomposition
Mode structure of disruption precursors in TFTR enhanced reversed shear discharges