Simulation of Alfvén frequency cascade modes in reversed shear discharges using a Landau-closure model

Abstract

The dynamics of energetic particle destabilized Alfve ́n frequency sweeping modes in tokamak reversed-shear safety factor discharges are modelled using a new Landau-closure model that includes coupling to geodesic acoustic wave dynamics and closure relations optimized for energetic particle–Alfve ́n mode resonances. Profiles and equilibria are based upon reconstructions of a DIII-D discharge (#142111) in which a long sequence of frequency sweeping modes were observed. This model (TAEFL) has recently been included in a verification and validation study of n = 3 frequency sweeping modes for this case along with two gyrokinetic codes, GTC and GYRO. This paper provides a more detailed documentation of the equations and methods used in the TAEFL model and extends the earlier calculation to a range of toroidal mode numbers: n = 2 to 6. By considering a range of toroidal mode numbers and scanning over a range of safety factor profiles with varying qmin, both up-sweeping frequency (reversed-shear Alfve ́n eigenmode) and down-sweeping frequency (toriodal Alfve ́n eigenmode) modes are present in the results and show qualitative similarity with the frequency variations observed in the experimental spectrograms.