Study of the dynamic behaviour of the STP-3(M) RFP plasma using microwave reflectometry

Measurement of density perturbations in the STP-3(M) reversed-field-pinch (RFP) plasma has been carried out using a 35 GHz microwave reflectometer to study the dynamic behavior of the RFP plasma. Simultaneous magnetic fluctuation measurements using a toroidal flux loop with the microwave reflectometry measurement, where both were mounted on the same poloidal cross-section, show an apparent temporal correlation between magnetic fluctuations and density perturbations, which are consistently interpreted as the m = 0 mode. In microwave reflectometry, phase runaway phenomena were observed typically after the plasma current peak. This phenomena can be explained by considering periodic density perturbations such as the sawtooth oscillation, i.e. a peaking of density profile causes a movement of the cut-off from the outside to inside and a sudden flattening causes a jump towards the outermost cut-off position.