Formation of impurity transport barrier in LHD plasmas with hollow density profile

The iron impurity density in the central column of Large Helical Device (LHD) plasmas with a hollow electron density profile is found to be much smaller than those with a peaked density profile based on the radial density profile measurement of iron ions in several ionization stages. The iron impurity transport has been studied with the help of a 1D simulation code. The radial structure of the convective velocity suggests that an impurity transport barrier is formed by a strong density gradient effect on the impurity convection near the edge region of LHD plasmas with a hollow electron density profile.