Magnetic fluctuation studies in extrap T1
1991
Magnetic field fluctuations in Extrap T1 plasma discharges are studied with internal and external arrays of pick-up coils. Statistical methods are employed to determine the spatial structure of the fluctuating modes. The discharge evolution is divided into two phases with different fluctuation characteristics. During start-up, dominatly m=0,1, low-n modes are seen. The poloidal harmonics are well correlated and correspond to horizontally-polarized perturbations. These fluctuations modes are attributed to variations of the toroidal equilibrium position. In the subsequent quasi-stationary stage mainly turbulent activity is observed. The experimental mode spectra are compared with a circular cylinder geometry, ideal MHD model. The stability calculation for the quasi-stationary phase gives comparable growth rates for many m-modes, which is qualitatively consistent with the measurements.
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