Excitation of toroidal Alfven eigenmodes with counter-current NBI in the TCV tokamak

In Tokamak & aacute; Configuration Variable (TCV), unstable modes excited by resonant interaction between the shear Alfv & egrave;n waves in continuum gaps and energetic particles have been observed in scenarios with neutral beam injection (NBI). TCV is a middle-size device (R-0/a = 0.88/0.25) equipped with a 1 MW, 25 keV tangential neutral beam injector. In this paper the phenomenology of modes excited with on-axis NBI is presented. The Alfv & egrave;nic nature of the modes has been confirmed investigating their sensitivity against plasma parameters such as NBI energy, toroidal magnetic field, and cross-checking with the predictions from linear kinetic stability code. The mode radial profile is estimated using electron cyclotron emission measurement and agrees well with modelling results. In addition, the fast particle distribution function has been modelled using TRANSP/NUBEAM code. Even with counter-current NBI (leading to higher losses), the drive from the resonant particles is sufficient for the mode excitation. An ad-hoc additional diffusion model allows to estimate the fast particle transport, modifying the fast particle gradient at the mode location and matching the neutron rates.