Giant sawtooth stability and core-localized fluctuations in JET plasmas*

In ICRF heated plasmas giant sawteeth (ST) can develop with periods larger than one second. At low ICRH power ($<$3 MW) a well defined ST period that increases with power is observed. At higher powers a large variation in ST periods is observed with a long ST-free period followed by a phase of shorter ST periods. At higher ICRH powers Alfven eigenmodes (AEs) are also observed. ST are stabilized by the fast-ion pressure inside the q=1 surface but the pressure gradients drive AEs which can lead to fast ion losses and triggering of ST. Other ST trigger candidates are low-frequency MHD activity and broadband turbulence induced transport. Due to the improvement of core diagnostics, especially the X-mode reflectometer, details of the AE activity and of the turbulent fluctuations can be studied in great detail inside the q=1 surface. Experimental results will be shown and compared with modelling results. Evidence will be presented for possible causes of giant ST crashes.