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, SOXMOS. Therefore, the decay times of these impurities could be estimated under the same plasma conditions. The estimated decay time of the Si impurities, τ_Si = 0.12 ± 0.01 s, was found to be slightly longer than that of the B impurities, τ_B = 0.09 ± 0.01 s. (author)