Validation of a tearing mode locking model using a database of disruptive plasmas at ASDEX Upgrade

An exploratory study is presented that aims at validating a model for mode locking on the basis of a large set of ASDEX Upgrade (AUG) discharges. Not discriminating between plasma configurations, the model allows to estimate the duration of the deceleration phase, as well as the critical mode width for locking. Both quantities are important for the design of disruption avoidance algorithms that aim to affect the MHD mode rotation. It was found that the model successfully described locking of large modes in those cases where the deceleration started in a quasi-stationary phase of the discharge (i.e. with low variability of the global plasma angular momentum prior to mode seeding) and where deceleration took place over temporal intervals comparable to the momentum confinement time. Theoretical braking curves and locking durations predicted with the model were in good quantitative agreement with the experiment. On the other hand, the model failed to reproduce the braking curves of modes appearing towards the end of a transient phase, e.g. during an impurity influx or when approaching the disruptive density limit. It can be concluded that the modes were not the primary cause of the plasma momentum losses within the scope of the model. A modified mode equation of motion is proposed, which accounts for transient variation of the plasma density, e.g. during the development of a MARFE, and its impact on braking predictions is discussed. Furthermore, it was observed that a substantial fraction of modes was rotating at the onset of a major disruptive event. Consequences of this observation on disruption prediction schemes in AUG are examined.