  A unique experiment in the Joint European Torus (JET) consecutively produced
120 almost identical plasma pulses, providing two orders of magnitude more data
than is usually available. This allows the statistical detection of previously
unobservable phenomena such as a sequence of resonant-like waiting times
between edge-localised instabilities (ELMs). Here we investigate the causes of
this phenomenon. By synchronising data to the 1000s of ELM times and averaging
the results, random errors are reduced by a factor of 50, allowing
unprecedentedly detailed behaviour to be described. A clear link can then be
observed between plasma confinement, ELM occurrence, vertical plasma
oscillations, and an otherwise unobservable oscillation in a control coil
current that is not usually associated with ELM occurrence. The results suggest
a strong and unanticipated edge-plasma dependence on control system behaviour.
