Plasma-Control System Interactions in Magnetically Confined Fusion Experiments
The Joint European Torus (JET)  is the world’s largest magnetically confined fusion (MCF) experiment. A unique sequence of 120 almost identical JET plasmas recently provided two orders of magnitude more statistically equivalent data than ever previously available. The purpose was to study movement of eroded plasma-facing material from JET’s new Beryllium wall [2, 3], but it has allowed the statistical detection of otherwise unobservable phenomenon. This includes a sequence of resonant-like waiting times between edge-localised plasma instabilities (ELMs) [4, 5], instabilities that must be mitigated or avoided in large MCF devices such as ITER . Here we investigate the causes of this phenomenon, finding a clear link between plasma confinement, ELM occurrence, vertical plasma oscillations, and an oscillation in a control coil current not usually associated with ELM occurrence. The results suggest a strong and unanticipated edge-plasma dependence on control system behaviour, suggesting that plasma confinement, stability, and control, should not be studied independently.