Observation of wide and high pedestal formation triggered by coherent edge mode activities in KSTAR hybrid scenario discharges*

Hybrid scenarios, one of the candidate scenarios for the International Thermonuclear Experimental Reactor (ITER), exhibit distinctive features such as a central  above unity and an extended region of low or zero magnetic shear in the core. These characteristics reduce neoclassical tearing modes induced by sawtooth activities and enable higher beta operation. Extensive efforts have been made to comprehend the underlying mechanisms responsible for the improved performance observed in hybrid scenarios on present-day tokamaks.

In recent experiments in the Korea Superconducting Tokamak Advanced Research (KSTAR), a transition-like performance enhancement was observed in hybrid scenario discharges. This enhancement was associated with the formation of a wide and high pedestal. A comparative analysis discovered that coherent edge mode (CEM) activities were responsible for changing the pedestal. CEM, a coherent edge-localized mode, occurs during the pedestal recovery phase and is stabilized after the subsequent edge-localized mode (ELM) crash. Measurements showed that particle and heat transport tended to increase during CEM activities, providing a consistent explanation for the observed pedestal broadening. Furthermore, dedicated experiments changing the amount of external gas fueling revealed a correlation between increased external fueling and intensified CEM activities, indicating an available control knob for accessing high performance.