HIGH-FREQUENCY MODE CASCADES IN THE ASDEX UPGRADE TOKAMAK

Using a novel diagnostic, cascades of high frequency modes have been o bserved in the core plasma of the ASDEX Upgrade tokamak. Their mode nu mbers obey the relation m = n + 1, with n up to 23. The appearance of cascades is always related to a region of low shear, which is ascribed to previous (m, n) = (1, 1) mode activity and leads to the instabilit y of tearing modes with high mode numbers. The destabilization and sta bilization of modes resulting in the cascade process can be reproduced by a simple quasilinear model.