Sawtooth activity and current density profiles in JET

Extensive investigations of sawtooth activity in JET have led to a major revision of the understanding of sawteeth and to further theoretical developments. In addition, experiments with powerful auxiliary heating have revealed a new plasma regime in which sawteeth are stabilized and quiescent plasmas are achieved for periods of over 3 s. Measurements of current density profiles using a far infrared multichannel polarimeter show that the central safety factor q0 may be below unity in normal sawtoothing plasmas and may reach values as low as 0.6-0.8 during the stable periods. Analysis of pellet ablation and of density perturbations at rational q surfaces ('snakes') yields further information on the evolution of the q-profile during sawteeth. These results have significant implications for theories of the sawtooth instability and for explanations of the sawtooth stabilization. (author). 13 refs, 5 figs