The first attempt to characterize the divertor footprint profile in the heliotron device LHD was done, by using a number of Langmuir probes and the multivariable analysis technique.In order to clarify the generation mechanism of the private-side peak on the footprint profile, which has not been reproduced in the modeling study, over 6000 time points were extracted by excluding time points with profile modifications due to already-known reasons. A characterization index ⁄ was newly defined from the multivariable analysis result, and its dependences on upstream parameters were investigated. As a result, it was found that the footprint profile correlates with the pressure gradient at the edge inside the core region with a fixed beta, suggesting that change of the plasma pressure profile could modify the edge magnetic field structure even if the volume integral of the plasma pressure was constant.
Superior control of particle recycling and hence full governance of plasma density has been established in the Large Helical Device (LHD) using largely enhanced active pumping of the closed helical divertor (CHD). In-vessel cryo-sorption pumping systems inside the CHD in five out of ten inner toroidal divertor sections have been developed and installed step by step in the LHD. The total effective pumping speed obtained was 67 ± 5 m 3 s −1 in hydrogen, which is approximately seven times larger than previously obtained. As a result, a low recycling state was observed with CHD pumping for the first time in LHD featuring excellent density control even under intense pellet fueling conditions. A global particle confinement time (τ * p ) is used for comparison of operation with and without the CHD pumping. The τ * p was evaluated from the density decay after the fueling of hydrogen pellet injection or gas puffing in NBI plasmas. A reliably low base density before the fueling and short τ * p after the fueling were obtained during the CHD pumping, demonstrating for the first time full control of the particle balance with active pumping in the CHD.
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