Experimental Observations of the Plasma Shape Effect on the RMP-ELM Coupling for Optimization of the KSTAR ELM-Crash Control

Full text: Reliable and robust resonant magnetic perturbation (RMP) on the edge-localized-mode (ELM) crash control is crucial for the success of ITER and beyond. In recent KSTAR experiments, a critical dependence of RMP-ELM coupling on the plasma shape was found to be as important as much as q₉₅. In application of the low-n RMP fields, small variations in the lower triangularity by controlled R_X,lower (radial position of lower X-point) made significant changes on RMP coupling, suggesting a narrow window for the ELM-crash suppression. For the shape effects found in 2016, such RX window for the RMP induced ELM-crash suppression was surprisingly narrow (δ_lower = 0.74 ± 0.04 and R_X,lower = 144 ± 2 cm), while the other shape parameters, such as Z_X, seem to have weak effects. Also it was found that the same shape valid for the n = 1 RMP was not effective for the n = 2 RMP induced ELM-crash suppression. In 2017, further study revealed that such a strict condition of triangularity (R_X,lower) can be relaxed by allowing for an additional small up-down asymmetry on the plasma shape. Applying this new optimized plasma shape led us to make substantial improvements on reliability and robustness of the RMP induced ELM-crash control. As a result, the n = 1 RMP induced ELM-crash suppression were successfully demonstrated in a wide range of q₉₅ even with a fixed RMP spectra, achieving a record-long sustainment of ELM suppression more than ∼ 30 s. Similarly, the n = 2 RMP-induced ELM-crash suppression was achieved with the same shape (universality) at ITER-relevant low q₉₅ = 3.3-3.5. Furthermore, we found a singular response to the shape change for the RMP-plasma coupling, with a support of ideal MHD modelling. (author)