Characteristics of the magnetic field fluctuations in the ETA-BETA II reversed field pinch experiment

Magnetic field fluctuations have been studied in the ETA-BETA II experiment as a function of density, time and radius. The spectral analysis of the measured fluctuations has shown that the higher frequency portion of the spectrum (>50 kHz) is more sensitive to the density variation, whereas the lower frequency portion ( approximately 10 kHz) shows a radial profile which, especially at later times during the discharge, resembles the characteristic shape of MHD resistive modes. Radial correlation lengths of several centimeters are found. In general the amplitude of the fluctuations increases at lower densities and is larger toward the end of the discharge and in the outer plasma region. These trends may be correlated with the typical variation of the streaming parameter However, the lower frequency fluctuations, in particular, can be interpreted as MHD instabilities which can produce large distortion of the magnetic surfaces leading to island formation and overlapping resulting in stochastic behaviour of the magnetic field lines in a relevant radial extent ( approximately=1 cm) of the outer plasma regions.