Simulation of feedback control system for NTM stabilisation in ASDEX Upgrade

• Published in: Fusion engineering and design Vol. 88; no. 6-8; pp. 1137 - 1140
• Main Authors: Rapson, Christopher, Monaco, Francesco, Reich, Matthias, Stober, Joerg, Treutterer, Wolfgang
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2013
• Subjects: ASDEX Upgrade; Deposition; Feedback control; Neoclassical tearing mode; NTM; Simulation; Neoclassical tearing mode; Feedback control; ASDEX Upgrade; Simulation; NTM
• ISSN: 0920-3796; 1873-7196
• DOI: 10.1016/j.fusengdes.2013.02.127

► Feedback loop to control the ECRH deposition location is modelled in Simulink. Controller optimised using simulation results. ► Apart from optimising the PID gain values, alternative architectures were trialed without risk to hardware. ► Off-normal events could be simulated, and the controller response improved. ► Optimised controller applied in experiment. Even for the low power used, partial stabilisation of NTM was observed. ► The simulation is useful outside its intended application, and for future developments of the NTM feedback control system. Neoclassical Tearing Modes (NTMs) are a class of MHD instability in high beta tokamak plasmas which significantly increase radial transport, thus capping the performance of fusion plasmas. More importantly, NTMs can lead to disruptions which compromise the lifetime of structural components. Several tokamaks have demonstrated that Electron Cyclotron Resonant Heating (ECRH) can stabilise NTMs if the power deposition is aligned with the mode location. The deposition location depends on the toroidal magnetic field, flux and density profiles, and can be controlled by tilting the mirror in the ECRH launcher. Until recently, the mirror angle was set by feedforward control at ASDEX Upgrade. In order to adapt automatically to different discharge scenarios, the system at ASDEX Upgrade has been extended to steer the mirror using feedback control. The mirror must react on the current diffusion time scale, on the order of 100ms. This is within the capabilities of the mechanical subsystem and real-time plasma diagnostics, but requires careful interfacing between these components. For example, asynchronous data transfer and non-linearities make it difficult to design an analytically optimal controller. Therefore a simulation has been used to test and tune different controller architectures. This simulation is the subject of the current contribution. Performing the optimisation process offline saves valuable experiment time and allows risk-free experimentation with novel designs. Settings which were optimised in the simulation led to considerable improvement of the system performance.