Wave–particle resonances and redistribution/losses of fast ions in tokamaks

Abstract

Enhanced fast ion losses, mostly in the range of energies from around 1.2 to 2.4 MeV, were measured during the activity of tornado modes in the JET tokamak. Tornado modes are TAE localized inside the q = 1 surface, which do not extend to the outer regions of the plasma. Thus, it is necessary to find an explanation on how such modes can lead to the loss of fast ions. In this paper, a mechanism that allows explaining the loss of fast ions triggered by tornado modes is proposed. This mechanism is based on the combined effect of tornado modes and global TAEs over the fast ions (global TAEs were always observed along with the tornado modes in the experiments in which enhanced losses were measured). Tornado modes would trigger the process of loss by resonantly interacting with the fast ions near the centre of the plasma and transporting the ions to a more peripheral region where tornado modes and global TAEs coexist. The TAE would then transport convectively the fast ions, most efficiently through the first bounce resonances (p = 1), all the way to the plasma edge eventually leading to its loss. This mechanism of loss is supported by calculations carried out with the CASTOR-K code.