Chapter 9: Performance-Limiting MHD Activity and Possibilities for Its Stabilization in ASDEX Upgrade

Sibylle Günter, Hartmut Zohm
2003 Fusion science and technology  
Performance-limiting magnetohydrodynamic (MHD) instabilities on ASDEX Upgrade are discussed. In the conventional H-mode scenario, the main MHD performance limitation is found to be the neoclassical tearing mode (NTM). The onset b of NTMs in ASDEX Upgrade scales with the poloidal ion gyroradius, in agreement with theoretical expectations. At higher b values, NTMs occur in a more benign form, the frequently-interruptedregime NTMs, which lead to a smaller confinement degradation than normal NTMs.
more » ... ctive control of NTMs by electron cyclotron current drive in the island has been demonstrated on ASDEX Upgrade. In advanced tokamak regimes with reversed shear, a variety of performance-limiting instabilities has been observed. The shear reversal zone can be unstable to double tearing modes or to infernal modes; both have been identified in ASDEX Upgrade. Due to the broad current profile in advanced tokamak discharges, the ideal external kink mode can be unstable at relatively low b N Յ 2; this is a main limitation to strongly reversed shear discharges with peaked pressure profiles. Finally, it is shown that fast-particle-driven modes such as fishbones can also have beneficial effects, such as providing stationary current profiles or triggering internal transport barriers.
doi:10.13182/fst03-a407 fatcat:tamhshpzzvdynkt7regwq6bvm4