  For a two week period during the Joint European Torus (JET) 2012 experimental
campaign, the same high confinement plasma was repeated 151 times. The dataset
was analysed to produce a probability density function (pdf) for the waiting
times between edge-localised plasma instabilities ("ELMS"). The result was
entirely unexpected. Instead of a smooth single peaked pdf, a succession of 4-5
sharp maxima and minima uniformly separated by 7-8 millisecond intervals was
found. Here we explore the causes of this newly observed phenomenon, and
conclude that it is either due to a self-organised plasma phenomenon or an
interaction between the plasma and a real-time control system. If the maxima
are a result of "resonant" frequencies at which ELMs can be triggered more
easily, then future ELM control techniques can, and probably will, use them.
Either way, these results demand a deeper understanding of the ELMing process.
