Magnetic safety factor profile before and after sawtooth crashes investigated with toroidicity and ellipticity induced Alfvén eigenmodes

A study of toroidicity and ellipticity induced Alfvén eigenmodes (TAEs and EAEs) that are excited before and after sawtooth crashes during ion cyclotron range of frequency (ICRF) heating in JT-60U is presented. From the TAEs that are observed before sawteeth and that reside inside the q = 1 surface an upper limit has been set for q in the plasma centre at the time of the crash. After the sawtooth crash, EAEs that reside at the q = 1 surface are often observed. In a number of discharges the start of the EAE activity is delayed to up to 150 ms after the crash. In some cases TAE activity was observed between the sawtooth crash and the onset of the EAE activity. These TAEs could be modelled successfully only when it was assumed that the central safety factor (q₀) rises above unity after the giant sawtooth crash. The appearance of the TAE activity immediately after the giant sawtooth crash is a strong indication that the fast particle drive remains in the plasma centre. This is consistent with theoretical estimates for the confinement of deeply trapped ICRF ions. The delayed appearance of the EAEs is also consistent with the disappearance of the q = 1 surface from the plasma at the giant sawtooth crash. The only way to obtain agreement between the experimentally measured EAE frequencies and the NOVA-K simulations is to assume that the q = 1 surface reappears in the plasma at the start of the EAE activity. The delayed appearance of the EAE activity seems to be correlated with the electron temperature just before the crash.