Modelling frequency splitting of TAEs in JET

High frequency magnetohydrodynamic (MHD) activity in tokamak plasmas is driven by energetic ion populations. The time evolution of this MHD behaviour, notable frequency splitting, provides insights into the changing properties of both the energetic particle population and the ambient plasma. To extract this information, it is necessary to model the self-consistent nonlinear interaction of the energetic particles with the MHD modes, incorporating energetic particle resonance and background damping. Here we report new results on frequency splitting, obtained using a fully nonlinear numerical implementation (R. G. L. Vann et al., Phys. Plasmas 10, 623 (2003) of the Berk-Breizman augmentation of the Vlasov-Maxwell system (hereafter VM (BB)''); this model self-consistently evolves the fields with the energetic particle velocity distribution. First, the frequency splitting data for ICRH driven TAEs in JET pulse 40332 (A. Fasoli et al., Phys. Rev. Lett 81, 25 (1998)) are transformed at each time step to compensate for slow frequency sweeping. The Fourier transform of this data ten yields clear evidence for a sharply defined frequency splitting bifurcation. Such transitions are then shown to arise naturally from the fully nonlinear VM (BB) system, in response to slow monotonic evolution of the model parameter that characterises the energetic particle drive, with fixed wave damping. This is in contrast to frequency sweeping in the VM (BB) system, which has recently been shown to occur for constant parameter values (R. G. L. Vann et al., Phys. Plasmas, 12 accepted (2005). The key physics underlying the bifurcation sequence is determined by the extended to which energetic particles are captured by, trapped in, and released from the potential well of the excited mode. (Author)