Alternative Detection of <em>n</em> = 1 Modes Slowing Down on ASDEX Upgrade

• Published in: Applied Sciences
• Main Authors: Emmanuele Peluso, Riccardo Rossi, Andrea Murari, Pasqualino Gaudio, Michela Gelfusa, on behalf of the ASDEX Upgrade Team, on behalf of the EUROfusion MST1 Team
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Subjects: disruption avoidance; disruptions; MHD instabilities; Mode Locking; Magnetic Islands
• Online Access: https://www.mdpi.com/2076-3417/10/21/7891

Disruptions in tokamaks are very often associated with the slowing down of magneto-hydrodynamic (MHD) instabilities and their subsequent locking to the wall. To improve the understanding of the chain of events ending with a disruption, a statistically robust and physically based criterion has been devised to track the slowing down of modes with toroidal mode numbers <i>n</i> = 1 and mostly poloidal mode number m = 2, providing an alternative and earlier detection tool compared to simple threshold based indicators. A database of 370 discharges of axially symmetric divertor experiment—upgrade (AUG) has been studied and results compared with other indicators used in real time. The estimator is based on a weighted average value of the fast Fourier transform of the perturbed radial <i>n</i> = 1 magnetic field, caused by the rotation of the modes. The use of a carrier sinusoidal wave helps alleviating the spurious influence of non-sinusoidal magnetic perturbations induced by other instabilities like Edge localized modes (ELMs). The indicator constitutes a good candidate for further studies including machine learning approaches for mitigation and avoidance since, by deploying it systematically to evaluate the time instance for the expected locking, multi-machine databases can be populated. Furthermore, it can be thought as a contribution to a wider approach to dynamically tracking the chain of events leading to disruptions.