MHD stability limits to the operation parameters of the FT tokamak

A systematic study of the macroscopic instabilities limiting the accessible operation parameters has been performed on the Ohmic discharges of the FT tokamak at B T = 40 and 60 kG. The MHD fluctuation behaviour and the modifications of the profiles associated with the precursor of the disruption are discussed in detail for the cases of breaking through q L = 3, low-q L operation, disruptions at the highdensity limit and disruptions following the disappearance of the sawtooth activity. In all these cases the power balance terms that appear associated with the development of the MHD instabilities are dominant either in the centre or at the edge of the discharge and so the transport in the intermediate confinement zone does not seem to be affected during the precursor of the disruption. The loop voltage negative spike of the disruption itself is found to be associated with the appearance of a burst of m = 3, n = 2 modes in the presence of m = 2, n = 1 precursor activity.