Solitary perturbations prior to the partial collapse of the edge pedestal in KSTAR H-mode plasmas

Solitary perturbations (SPs), localized both poloidally and radially, are frequently observed on the KSTAR tokamak within $\sim$100 $\mu$s before the partial collapse of the edge pedestal. The perturbation structure measured by 2D imaging diagnostic [1] is clearly distinguished from the quasi-steady filamentary modes that are routinely observed during the inter-crash period [2, 3]. In particular, the SPs have a low toroidal mode number (typically unity) and smaller pitch angle. The SPs are similar to the irregular mode structure with large amplitude [3] appearing near complete crash. The resemblance suggests that the generation of SPs is strongly connected to the crash mechanism. [1] G.S. Yun \textit{et al}, \textit{Rev. Sci. Instrum.} \textbf{81} 10D930 (2010) [2] J.E. Lee \textit{et al}, \textit{Nucl. Fusion} \textbf{55} 113035 (2015) [3] G.S. Yun \textit{et al}, \textit{Phys. Plasmas} \textbf{19} 056114 (2012)