Impact of carbon impurities on the confinement of high-ion-temperature discharges in the Large Helical Device

Effects of carbon impurities on the thermal confinement properties were discussed for carbon-pellet-injected high-Ti discharges in the Large Helical Device (LHD). To clarify their role, the amounts of carbon impurities introduced into plasmas were scanned by varying the number of injections in a discharge or the size of the pellet, and the changes of thermal confinement properties with these variations were examined. In all cases, strong correlations between the densities of carbon impurities and the thermal diffusivities of plasmas were found. Thermal diffusivities were small when the carbon densities were greater than a certain value, e.g. ~1.5 × 1017 m3 at r/a ≈ 0.72, and the thermal diffusivity degraded drastically below this density. This indicates the existence of a threshold density of carbon impurity for an improved confinement in plasmas. Combined with the formation of the impurity hole, the variation of confinement property with carbon density can explain the degradation of ion temperature in high-Ti discharges with carbon pellet injection in the LHD.