LAUSR

Recent experiments in the Large Helical Device (LHD) clearly show the ability of electron cyclotron heating (ECH) to control accumulation of impurities, as demonstrated by external injection of a tracer-encapsulated solid pellet (TESPEL). The impurity introduced locally into the core region of the LHD plasma by the TESPEL has strongly accumulated in a high-density LHD plasma, as well as an intrinsic impurity, such as a carbon. When a high-power (1.5 MW) ECH is applied just after the TESPEL injection for such a plasma, the accumulation of the impurity injected by the TESPEL was almost completely suppressed. This result indicates that applying ECH enhances the outward radial impurity flux. Although a neoclassical ambipolar radial electric field in stellarators has a stronger impact on the transport, particularly on the impurity transport, than in tokamaks, the sign of the radial electric field measured and calculated is directed to a negative even in the 1.5 MW ECH case, resulting in the inward impurity flux. By revisiting the neoclassical formulation, an increment of ion temperature and its gradient could be one of the candidates for the explanation, since the ion temperature is also increased globally by ECH.Recent experiments in the Large Helical Device (LHD) clearly show the ability of electron cyclotron heating (ECH) to control accumulation of impurities, as demonstrated by external injection of a tracer-encapsulated solid pellet (TESPEL). The impurity introduced locally into the core region of the LHD plasma by the TESPEL has strongly accumulated in a high-density LHD plasma, as well as an intrinsic impurity, such as a carbon. When a high-power (1.5 MW) ECH is applied just after the TESPEL injection for such a plasma, the accumulation of the impurity injected by the TESPEL was almost completely suppressed. This result indicates that applying ECH enhances the outward radial impurity flux. Although a neoclassical ambipolar radial electric field in stellarators has a stronger impact on the transport, particularly on the impurity transport, than in tokamaks, the...