Stability properties of toroidal Alfvén modes driven by fast particles*

Issues of Alfvén mode stability in advanced tokamak regimes are addressed on the basis of recent developments in theory, computational methods and progress in experiments. The instability of toroidal Alfvén eigenmodes (TAEs) is analysed for spherical tokamaks, such as the NSTX using the NOVA-K code. Modes in the Alfvén frequency range observed in JT-60U during negative ion based NBI heating at energies Eb0 = 360 keV, and in TFTR during ICRH experiments with chirping frequency, on various timescales, are analysed using the kinetic non-perturbative code HINST, which is able to resolve new resonant branches of the toroidal Alfvén modes called resonant TAEs (RTAEs). However, as is shown the mechanisms for frequency chirping may be different in different experiments. In TFTR, frequency chirping results from the slow variation of q profile between sawteeth. In JT-60U experiments some frequency chirping modes have very short timescales, which suggests the cause is the change in fast particle distribution.