Observation of an impurity hole in the Large Helical Device

An extremely hollow profile of carbon impurity (denoted as an ‘impurity hole’) is observed associated with an increase in the ion temperature gradient after an impurity injection in the Large Helical Device. The central carbon density drops to 0.3% of the plasma density due to a strong outward convection driven by the ion temperature gradient, while an inward convection is predicted by neoclassical theory. Transport analysis gives a low diffusion coefficient of 0.4 m 2 s −1 and an outward convection velocity of 3 m s −1 at half of the minor radius.