The Concept of Causality Horizon and Its Application to Synchronization Experiment

Synchronization consists of the coordination of events to operate systems in harmony. Recently the importance of synchronization has been recognised in both the operation of artificial systems and in the investigation of natural events. In the last decade or so, synchronization of multiple interact ingdynamical systems has therefore become a lively field of study for control. In Magnetic Confinement Nuclear Fusion, various pacing concepts have been recently proposed to control various instabilities, from sawteeth to ELMs. Some of the main difficulties of these experiments reside in the quantification of the synchronization efficiency; poor accuracy in this respect affects the potential of the techniques for the understanding of the main physical mechanisms involved. In the paper, various independent statistical indicators are introduced to address these issues. In metallic Tokamaks, one of the most recent applications of ICRH heating on JET is sawtooth control by ICRH modulation, for avoiding triggering dangerous Neoclassical Tearing Modes and counteracting impurity accumulation. Various forms of Edge Localised Modes pacing have also been tried to influence their behaviour using external perturbations; the one studied in this paper, injection of pellets, seems the most promising in the perspective of future devices such as ITER. In the application to JET experiments with the ILW, the proposed indicators provide sound and coherent estimates of the efficiency of the synchronisation scheme investigated. They also confirm the interpretation that the fast ions play a fundamental role in the stabilization of the sawteeth, in both L and H mode.