Study of slow n = 1, m = 1 reconnection in JET discharges with low central magnetic shear

Experiments designed for the attainment of the hybrid advanced mode of operation in JET revealed the existence of a new form of reconnecting modes with toroidal number n = 1 and poloidal number m = 1, which have the same radial structure as sawtooth precursors but are characterized by small growth rate and low saturation amplitude. These modes develop in repetitive cycles with typical growth times of 100 ms. Temperature perturbations are not in the form of sawteeth; in fact, there is slow and mild erosion of the central temperature without any crash. Comparison with theory of m = 1 modes indicates that the slow growth observed in the hybrid regime is caused by low magnetic shear (a measure of the radial variation of field line inclination) and possibly by diamagnetic effects in the plasma core