Description
Recently, enhanced impurity transport was observed in the core region of high-density NBI-heated discharges in the Large Helical Device (LHD) during the continuous dropping of lithium (Li) granules into its plasma edge [1]. It was observed in these experiments that continuous Li-granule dropping induced also an enhancement of plasma confinement, this being attributed to a reduction of turbulence levels. Similar observations were reported previously for boron powder dropping in the LHD [2].
In this work, we present results from dedicated experiments aimed at broadening initial studies of the impact of low-Z powder injections on impurity confinement and transport [1]. For this, impurity confinement times are estimated from the temporal behaviors of Ti and Mo impurities injected by using the Tracer-Encapsulated Solid Pellet (TESPEL) [3]. In the first instance, a series of experiments were performed in which the Li-granule dropping rate was varied. Analysis of spectral lines emitted by highly-ionized states of the injected impurities revealed that outward transport of such mid- and high-Z impurities increases with Li-granule injection rate while such transport decreases with electron density. In parallel, plasma energy confinement is found to improve for moderate Li injection levels compared with reference discharges without Li, confirming the beneficial impact of Li on overall plasma performance. In the second instance, the impacts of boron (B) and silicon (Si) powders were investigated. In this case, it is found that both powders can enhance impurity transport, this being more pronounced for B than for Si. These results demonstrate that the enhancement of mid-/high-Z impurity transport in high-density NBI-heated LHD plasmas by low-Z powder dropping is dependent on powder type and drop rate.
A comparative analysis of impurity transport in NBI-heated LHD plasmas during low-Z powder dropping is presented for a range of injection rates and low-Z powders (Li, B, and Si). This is done to establish a broad experimental database. Possible physical mechanisms responsible for the observed trends are discussed.