Magnetic field fluctuations in the high aspect ratio Extrap T1 reversed field pinch

Magnetic field fluctuations have been investigated on the high aspect ratio (R/a = 8.8) Extrap T1 reversed-field pinch. A global mode structure is found, characterised by poloidal model numbers m = 0 and m = 1. The mode spectrum of m = 1 fluctuations is peaked around toroidal mode numbers n = -19,...,-14, corresponding to modes with resonances of the q-profile inside the reversal surface. The mode amplitudes are such that magnetic island overlapping should occur in the central part of the plasma causing stochastisation of the magnetic field. Nearly rigid body rotation of the internally resonant modes is observed, with poloidal and toroidal angular velocities of ωθ= 1.4x105 ± 0.2x105 rads/s and ωφ= 0.5x104±0.1x104 rad/s, respectively