The role of stochastization in fast MHD phenomena on ASDEX Upgrade

Studies of fast MHD events in ASDEX-U suggest that stochastization plays an important role in these processes. In spite of the short time duration and small region of stochastization, it can lead to very strong changes in plasma confinement. Main reason for such influence is a strong mixing of the magnetic field lines which destroy magnetic surfaces and strongly increase radial transport. In this paper three such phenomena are discussed: frequently interrupted regime of neoclassical tearing mode (FIR-NTM), minor disruption due to interaction of the (2,1) and (3,1) tearing modes and sawtooth crash. The role of stochastization of magnetic field lines is analyzed by applying the mapping technique to trace the field lines of toroidally confined plasma. The sawtooth crash dynamics is also studied by means of spectral analysis and reconstruction of phase trajectories using delay coordinates which are standard techniques for stochastic systems and have been employed for identification of the transition to chaos in different physical systems. (author)