GRADIENTS OF ELECTRON-TEMPERATURE AND DENSITY ACROSS M = 2 MAGNETIC ISLANDS IN RTP

Simultaneous measurements of oscillations of the electron temperature, the electron density and the poloidal magnetic field in the Rijnhuizen tokamak RTP are combined to obtain an insight into the structure of MHD mode perturbations. The diagnostics used are a 20 channel heterodyne ECE radiometer, a 19 channel FIR interferometer and a set of magnetic pick-up coils. An m/n = 2/1 island preceding a disruption is analysed in detail. Whereas the density and temperature are significantly lower in the O point than in the X point, no local minimum or even flattening of the profiles could be demonstrated. The gradients across the island may be understood in terms of a very long connection length inside the island, in combination with the relatively short mean free path along the field lines due to the high collisionality in the outer region of the plasma. The transport inside the island and the poloidal distribution of the radial heat flux over the X and O points are discussed.