Observation of carbon impurity flow in the edge stochastic magnetic field layer of Large Helical Device and its impact on the edge impurity control

The parallel flow of carbon impurity in a thick stochastic magnetic field layer called the 'ergodic layer' located at the edge plasma of the Large Helical Device (LHD) is studied by space-resolved vacuum ultraviolet (VUV) spectroscopy, using a 3 m normal incidence spectrometer. A full vertical profile of C3+ impurity flow is evaluated from the Doppler shift of the second order of CIV line emission (2 × 1548.20 Å) at a horizontally-elongated plasma position of LHD. The carbon flow at the top and bottom edges in the ergodic layer has the same direction toward the outboard side along the major radius direction. The observed flow quantitatively agrees with the simulation results calculated with a 3D simulation code, EMC3-EIRENE. It experimentally verifies the validity of edge parallel flow driving the impurity screening.