Mirnov oscillation analysis in IR-T1 tokamak

摘要

Summary form only given. Tokamak discharges are usually ratherturbulent with a large spectrum of electric and magnetic fluctuations.Mirnov oscillations are caused by rotating magnetic island inside theplasma column originated by resistive instabilities. Depending on thesize of the plasma column, the frequency spectrum of Mirnov oscillationis in the range from a few to tens of kHz. We employ the technique toanalyze signals measured in the IR-T1 tokamak without and with resonanthelical field (RHF). The time-resolved analysis just described appliesto the time series of the signal produced by each Mirnov pick-up coil.In actual experimental condition, a large number of coils are used todiscriminate the poloidal m toroidal n mode numbers of the oscillations.The characteristics of modes can then be further investigated byapplying standard time correlation techniques. Since we suppose that theobserved signals are produced by a rotation of magnetic islands, thesignals of two neighbour Mirnov pick-up coils must be related by adelay. We see that the resonant helical field has almost no influence onthe plasma current and loop voltage and there is a slight decrease ofthe floating potential at the plasma edge. The characteristic frequencyof poloidal propagation of the magnetohydrodynamic (MHD) fluctuationscan be determined by using the correlation techniques described. Forthis calculation we have used the signals from the coils located on thevacuum chamber with low magnetic field because they are bettercorrelated than the one from the coils in the high field side. We haveshown that time-resolved spectral analysis can be easily applied toanalyse the (MHD) oscillations in tokamak