The Investigation of Fast Ion Effects in the Internal Transport Barrier Operation in KSTAR

Fast ions generated from the fusion reaction will be used as a self-heating source for sustaining the future burning plasmas. Therefore, it is essential to understand the fast ion effects on fusion plasmas. In recent, it has been known that fast ions can suppress turbulence through various mechanisms including an increase in the pressure gradient [1], dilution effects [2], and changes in the zonal shearing rate [3]. Various physical mechanisms of fast ion effects for turbulence suppression during the internal transport barrier (ITB) operation were investigated in KSTAR plasmas. We observed that a gyrokinetic simulation predicted a significant reduction of thermal energy fluxes when fast ions were included in the KSTAR ITB plasma [4] heated by neutral beam injection with a high fast ion fraction and a peaked fast ion density profile. It turned out that the dominant physical mechanism for turbulence suppression by fast ions in this KSTAR ITB plasma was dilution effects including both effects of reduced main ion density fraction and change of main ion density gradient. Furthermore, it was found that the sole effect of the inverted main ion density gradient was sufficient to suppress the turbulence enough for ITB sustainment. The significance of this finding lies in its potential to explore novel operational modes with improved confinement through effective turbulence control in the future. By manipulating the main ion density gradient through various methods such as pellet injection, impurity puffing, or the addition of fast ions, the ability to regulate turbulence can be achieved. Additionally, fast ion effects on KSTAR ITB plasmas with the different experimental conditions [5,6] will be discussed.