Issues of Alfvén mode stability in advanced tokamak regimesare addressed on the basis of recent developments in theory, computational methodsand 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 bedifferent in different experiments. In TFTR, frequency chirping results fromthe slow variation of q profile between sawteeth. In JT-60U experiments some frequency chirpingmodes have very short timescales, which suggests the cause is the changein fast particle distribution.
Study of chirping toroidicity-induced Alfvén eigenmodes in the National Spherical Torus Experiment