Fast ion loss in a 'sea-of-TAE'

Super-thermal fast ions provide a source of free energy to excite instabilities, which in turn can enhance the loss of fast ions. It has been proposed that when multiple modes with resonances closely spaced in phase space reach sufficient amplitude so that the fast ion trajectories overlap, very rapid non-linear growth can occur. The modification of the fast ion distribution by this loss may in turn excite additional, otherwise stable, modes, leading to an 'avalanche' effect greatly enhancing the transport of fast ions (Berk H.L., Brelzman B.N., Fitzpatrick J. and Wong H.V.. 1995 Nucl. Fusion 35 1661). It has been proposed that in ITER (ITER Physics Basis Editors et al 1999 Nucl. Fusion 39 2137), the transport of fast ions will be through a similar interaction of many modes. In NSTX (Ono M. et al 2000 Nucl. Fusion 40 557) bursts of multiple TAE-like instabilities are correlated with fast ion losses in a manner which qualitatively resembles avalanche behaviour.