Saturation phase of the Toroidal Alfven Eigenmodes in the Large Helical Device: MHD burst*

The aim of the study is reproducing the saturation phase of the Toroidal Alfven Eigenmodes (TAEs) in the Large Helical Device (LHD), particularly the destabilization of MHD bursts. A set of linear and nonlinear simulations are performed using the FAR3d code. The linear simulations indicate the overlapping of 1/2-1/1, 2/3-2/4 and 3/5-3/6 TAEs between the inner-middle plasma region in the frequency range of 45-75 kHz, destabilized by EP with an energy of 45 keV and EP β = 0.022. The nonlinear simulation reproduces the TAE saturation phase and the triggering of the MHD burst observed in the experiment. The nonlinear simulation indicates an energy transfer from the 1/1-1/2 TAE towards the 2/3-2/4 and 3/4-3/5 TAEs, leading to the destabilization of the n=2 and 3 TAEs. MHD burst is triggered by the broad radial overlapping of the unstable TAEs. The 1/1-1/2 TAE eigenfunction extends from the inner to the plasma periphery, thus the AE perturbation extends from the inner to the plasma periphery, leading to a collapse of the EP density profile at the middle-outer plasma region and a partial lost of the EP population by an enhancement of the outward EP transport. The nonliner simulations also show the formation of shear flows during the TAEs saturation phase, enhanced during the MHD burst.