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Nonlinear simulations of the early ELM phase based on a typical type-I ELMyASDEX Upgrade discharge have been carried out using the reduced MHD code JOREK.The analysis is focused on the evolution of the toroidal Fourier spectrum. Itis found that during the nonlinear evolution, linearly subdominant low-nFourier components, in particular the n = 1, grow to energies comparable withlinearly dominant harmonics. A simple model is developed, based on the ideathat energy is transferred among the toroidal harmonics via second ordernonlinear interaction. The simple model reproduces and explains very well theearly nonlinear evolution of the toroidal spectrum in the JOREK simulations.Furthermore, it is shown for the n = 1 harmonic, that its spatial structurechanges significantly during the transition from linear to nonlinearly drivengrowth. The rigidly growing structure of the linearly barely unstable n = 1reaches far into the plasma core. In contrast, the nonlinearly driven n = 1 hasa rigidly growing structure localized at the plasma edge, where the dominanttoroidal harmonics driving the n = 1 are maximal and in phase. The presentedquadratic coupling model might explain the recent experimental observation ofstrong low-n components in magnetic measurements [Wenninger et al., Non-linearmagnetic perturbations during edge localized modes in TCV dominated by low nmode components, submitted to Nuclear Fusion].

Nonlinear excitation of low-n harmonics in reduced magnetohydrodynamic simulations of edge-localized modes