The effect of normalized plasma pressure as characterized by normalized pressure parameter (βN) on the suppression of edge localized modes (ELMs) using resonant magnetic perturbations (RMPs) is studied in low-collisionality (ν* ⩽ 0.2) H-mode plasmas with low-triangularity (⟨δ⟩ = 0.25) and ITER similar shapes (⟨δ⟩ = 0.51). Experimental results have suggested that ELM suppression by RMPs requires a minimum threshold in plasma pressure as characterized by βN. The variations in the vacuum field topology with βN due to safety factor profile and island overlap changes caused by variation of the Shafranov shift and pedestal bootstrap current are examined numerically with the field line integration code TRIP3D. The results show very small differences in the vacuum field structure in terms of the Chirikov (magnetic island overlap) parameter, Poincaré sections and field line loss fractions. These differences do not appear to explain the observed threshold in βN for ELM suppression. Linear peeling–ballooning stability analysis with the ELITE code suggests that the ELMs which persist during the RMPs when βN is below the observed threshold are not type I ELMs, because the pedestal conditions are deep within the stable regime for peeling–ballooning modes. These ELMs have similarities to type III ELMs or low density ELMs.
Resonant magnetic perturbation experiments on MAST using external and internal coils for ELM control