  Periodic edge localized modes (ELMs) are the non-linear consequences of
pressure-gradient-driven ballooning modes and current-driven peeling modes
becoming unstable in the pedestal region of high confinement fusion plasmas. In
future tokamaks like ITER, large ELMs are foreseen to severely affect the
lifetime of wall components as they transiently deposit large amounts of heat
onto a narrow region at the divertor targets. Several strategies exist for
avoidance, suppression, or mitigation of these instabilities, such as the
naturally ELM-free quiescent H-mode (QH-mode). In the present article, an ASDEX
Upgrade equilibrium that features a QH-mode is investigated through non-linear
extended MHD simulations covering the dynamics over tens of milliseconds. The
equilibrium is close to the ideal peeling limit and non-linearly develops
saturated modes at the edge of the plasma. A dominant toroidal mode number of
$n=1$ is found, for which the characteristic features of the edge harmonic
oscillation are recovered. The saturated modes contribute to heat and particle
transport preventing pedestal build-up to the ELM triggering threshold. The
non-linear dynamics of the mode, in particular its interaction with the
evolution of the edge safety factor is studied, which suggest a possible new
saturation mechanism for the QH-mode. The simulations show good qualitative and
quantitative agreement to experiments in AUG. In particular, the processes
leading to the termination of QH-mode above a density threshold is studied,
which results in the transition into an ELM regime. In the vicinity of this
threshold, limit cycle oscillations are observed.

Plasma Physics