Microtearing mode in electron temperature pedestal evolution and collapse of KSTAR H-mode plasmas

While the electron transport barrier remains in its final form before an edge-localized mode crash, edge turbulence manifests as fluctuations in electron temperature. Because edge turbulence is closely related to the evolution and collapse of pedestal, the microscopic spatial structure and dynamics of electron temperature fluctuations during the electron temperature pedestal evolution phase are studied using broadband electron cyclotron emission measurements. The cross phase between the electron temperature and potential fluctuations is evaluated using a velocimetry technique to identify the nature of turbulence. A comprehensive comparison of the properties of various instabilities confirms that the micro-tearing mode is a leading candidate associated with the electron temperature pedestal evolution and collapse. The quadratic transfer function reveals that the energy within the pedestal is nonlinearly transferred to the interior of the electron temperature pedestal before the pedestal collapse, resulting in radial change in the mode structure and dynamics.