  A triggering mechanism responsible for the explosive onset of edge localised
modes (ELMs) in fusion plasmas is identified by performing, for the first time,
non-linear magnetohydrodynamic simulations of repetitive type-I ELMs. Briefly
prior to the ELM crash, destabilising and stabilising terms are affected at
different timescales by an increasingly ergodic magnetic field caused by
non-linear interactions between the axisymmetric background plasma and growing
non-axisymmetric perturbations. The separation of timescales prompts the
explosive, i.e. faster than exponential, growth of an ELM crash which lasts
${\sim}$ 500 ${\mu}$s. The duration and size of the simulated ELM crashes
compare qualitatively well with type-I ELMs in ASDEX Upgrade. As expected for
type-I ELMs, a direct proportionality between the heating power in the
simulations and the ELM repetition frequency is obtained. The simulations
presented here are a major step forward towards predictive modelling of ELMs
and of the assessment of mitigation techniques in ITER and other future
tokamaks.
