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Access conditions for full suppression of Edge Localised Modes (ELMs) byMagnetic Perturbations (MP) in low density high confinement mode (H-mode)plasmas are studied in the ASDEX Upgrade tokamak. The main empiricalrequirements for full ELM suppression in our experiments are: 1. The poloidalspectrum of the MP must be aligned for best plasma response from weakly stablekink-modes, which amplify the perturbation, 2. The plasma edge density must bebelow a critical value, $3.3 \times 10^{19}$~m$^{-3}$. The edge collisionalityis in the range $\nu^*_i = 0.15-0.42$ (ions) and $\nu^*_e = 0.15-0.25$(electrons). However, our data does not show that the edge collisionality isthe critical parameter that governs access to ELM suppression. 3. The pedestalpressure must be kept sufficiently low to avoid destabilisation of small ELMs.This requirement implies a systematic reduction of pedestal pressure oftypically 30\% compared to unmitigated ELMy H-mode in otherwise similarplasmas. 4. The edge safety factor $q_{95}$ lies within a certain window.Within the range probed so far, $q_{95}=3.5-4.2$, one such window,$q_{95}=3.57-3.95$ has been identified. Within the range of plasma rotationencountered so far, no apparent threshold of plasma rotation for ELMsuppression is found. This includes cases with large cross field electron flowin the entire pedestal region, for which two-fluid MHD models predict that theresistive plasma response to the applied MP is shielded.

Experimental conditions to suppress edge localised modes by magnetic perturbations in the ASDEX Upgrade tokamak