A Flexible System for the Control of External Magnetic Perturbations in the JET Tokamak
External magnetic perturbations are typically utilized in tokamak devices with two operational or experimental purposes: the correction of the intrinsic 3D error fields and the mitigation or suppression of Edge Localized Modes (ELMs). At JET, dedicated coils are used for the generation of these toroidally asymmetric perturbations. While error fields exist even in the absence of plasma, in ELM mitigation experiments the external fields are meant to ergodize slightly the magnetic topology in the plasma periphery hence reducing the drive for the destabilization of these instabilities. The control of the magnetic field produced by these coils is achieved by controlling the current flowing in them. The real-time system responsible for this control, recently underwent a number of functional improvements since its original implementation utilizing the present voltage controlled voltage sources. This work describes the overall system, built-in functionality, control algorithms and presents preliminary experimental results along with performance assessment studies. In particular, the main improvements are: the possibility of automatically reducing the current references in case the plasma amplifies the applied perturbation, real-time limitation of dI/dt to reduce the electromotive force in machine protection diagnostic systems, implementation of a model predictive controller as an alternative to the PID and the possibility of adapting the current references, in real-time, using an external system. The result is a flexible control system which is able to fulfil the operational and experimental requirements of an international and dynamic scientific environment.