High Frequency Mode Cascades in the ASDEX Upgrade Tokamak

Using a novel diagnostic, cascades of high frequency modes have been observed in the core plasma of the ASDEX Upgrade tokamak. Their mode numbers obey the relation $m=n+1\mathrm{}$, 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 instability of tearing modes with high mode numbers. The destabilization and stabilization of modes resulting in the cascade process can be reproduced by a simple quasilinear model.