Simultaneous Observation of Silicon and Boron Impurity Behaviors in the Core Region of a Mid-Density LHD Plasma

Line emissions from both silicon (Si) and boron (B) impurity ions introduced by a single tracer-encapsulated solid pellet (TESPEL) containing silicon hexaboride (SiB6) powders were successfully observed using the extreme ultraviolet (EUV) spectrometer and charge-exchange spectroscopy (CXS) technique in the Large Helical Device. The CXS diagnostic shows clearly that a hollow radial profile of fully ionized B impurities was created immediately after the TESPEL injection, and such a hollow profile was relaxed with time. At the same time, Li-like emissions from the highly ionized Si impurities were also observed with the EUV spectrometer, SOX-MOS. Therefore, the decay times of these impurities could be estimated under the same plasma conditions. The estimated decay time of the Si impurities, tau(Si) = 0.12 +/- 0.01 s, was found to be slightly longer than that of the B impurities, tau(B) = 0.09 +/- 0.01 s. (C) 2021 The Japan Society of Plasma Science and Nuclear Fusion Research