Mirnov oscillation analysis in IR-T1 tokamak

Summary form only given. Tokamak discharges are usually rather turbulent with a large spectrum of electric and magnetic fluctuations. Mirnov oscillations are caused by rotating magnetic island inside the plasma column originated by resistive instabilities. Depending on the size of the plasma column, the frequency spectrum of Mirnov oscillation is in the range from a few to tens of kHz. We employ the technique to analyze signals measured in the IR-T1 tokamak without and with resonant helical field (RHF). The time-resolved analysis just described applies to the time series of the signal produced by each Mirnov pick-up coil. In actual experimental condition, a large number of coils are used to discriminate the poloidal m toroidal n mode numbers of the oscillations. The characteristics of modes can then be further investigated by applying standard time correlation techniques. Since we suppose that the observed signals are produced by a rotation of magnetic islands, the signals of two neighbour Mirnov pick-up coils must be related by a delay. We see that the resonant helical field has almost no influence on the plasma current and loop voltage and there is a slight decrease of the floating potential at the plasma edge. The characteristic frequency of poloidal propagation of the magnetohydrodynamic (MHD) fluctuations can be determined by using the correlation techniques described. For this calculation we have used the signals from the coils located on the vacuum chamber with low magnetic field because they are better correlated than the one from the coils in the high field side. We have shown that time-resolved spectral analysis can be easily applied to analyse the (MHD) oscillations in tokamak.