Transport characteristics of tracer and intrinsic impurities depending on the density of LHD plasmas

By injecting a tracer-encapsulated solid pellet (TESPEL) with triple tracers, V (Z = 23), Mn (Z = 25) and Co (Z = 27), into a plasma in the Large Helical Device (LHD), it was found in the high-density case (typically ne = (5–7) × 1019 m−3) that the Kα emissions from the intrinsic impurities were strongly suppressed, while those from all the three tracers were retained for a long time. In the medium-density case (typically ne = (3–4) × 1019 m−3), Kα emissions from the intrinsic impurities were observed clearly, while those of the tracer impurities were observed to decay much faster than the high-density case. When the intrinsic impurities penetrate into the plasma core through the plasma periphery in the plasma build-up phase under relatively low-density conditions, then such impurities are found to be kept for a long time in the later phase under high-density conditions. By implementing supersonic Ar gas puffing in addition to the TESPEL injection, Ar Kα emission was clearly observed together with Kα emissions from the tracers in the medium-density case. In contrast to this, Ar Kα emission was completely suppressed in the high-density case. This result shows that the suppression of the intrinsic impurity coming from outside the plasma is indeed working in the high-density case, while the impurities deposited inside the plasma are kept for a long time.