24th IAEA Fusion Energy Conference - IAEA CN-197

EX/P5-18: Low Concentration of Iron as First Wall Material in LHD Plasmas with Edge Ergodic Layer

11 Oct 2012, 08:30

4h

Poster Room (Area F-B)

Poster EXD - Magnetic Confinement Experiments: Plasma–material interactions; divertors; limiters; scrape-off layer (SOL) Poster: P5

Speaker

Mr Shigeru Morita (Japan)

Description

Experiments of Large Helical Device (LHD) have been successfully conducted during past 14 years, based on the first wall fully covered with rectangular protection plates made of stainless steel, basically having no serious impurity problem. The impurity behavior of iron has been quantitatively studied with a spectroscopic system newly developed for measuring two-dimensional distribution of extreme-ultraviolet (EUV) impurity lines. The iron density in core plasmas is found to be fairly low, at least five orders of magnitude smaller than the electron density. The highest density of iron is observed during impurity accumulation triggered by pellet injection. The central density of Fe^23+ ions analyzed from FeXXIV profile reaches 2.6x10^9cm^-3 during the impurity accumulation phase with appearance of a large inward convective velocity of V=-6m/s, whereas it is only 3x10^8cm^-3 before the accumulation occurs. No accumulation is observed for light impurities. The accumulation is mainly caused by the density gradient. These results are very similar to tokamak ones, but only the value of the iron density is different between tokamaks and LHD. The stochastic magnetic filed layer of LHD can effectively fulfill its function as the impurity screening in collisional regime of 10

Country or International Organization of Primary Author

Japan