Optimized close divertor configurations in the large helical device (LHD) are investigated using fully-three dimensional simulation code for neutral particle transport [1],[2]. The mitigation of the heat load on divertor plates from the main plasma is an essential issue for nuclear fusion power plant. In Tokamak plasma confinement systems, it is proved that the reduction of the heat load on divertor plates and the neutral particles in the plasma periphery is important factors for achieving and sustaining H-mode plasmas. For these purposes, a closed divertor concept has been proposed; the neutral particles are locally confined in the divertor region by installing the baffle plates in order to minimize the particle influx to the main plasma from the divertor plates. In an LHD plasma vacuum vessel, helically-twisted shaped plasmas are produced, leading to three-dimensionally complicated particle deposition pattern on the divertor plates. This is because ions and electrons basically move along the magnetic field lines from the main plasma to divertor plates via an ergodic magnetic layer where the magnetic field line structure is stochastic and complicated. Thus, a fully-three dimensional plasma model is essentially needed for neutral particle transport analyses [3]. In the plasma model, it is assumed that the distribution of neutral particle fueling from the divertor plates corresponds to that of the striking points of particle trajectories traced from uniformly distributed positions just inside of the last closed magnetic surface with a random walk process. A CCD camera with an interference filter for monitoring H_α intensity images has been installed to measure the density profiles of neutral hydrogen around lower divertor plates. Three-dimensional distribution of the neutral particles in various magnetic configurations will be analyzed by comparing the calculated H_α intensity images with the measurements by the CCD camera, which can contribute the investigation of the availability of the three dimensional model and the assumption about the gas fueling distribution from the divertor plates. LHD has a future plan for closed divertor configurations in Phase II. The neutral particle density profiles in the divertor region are calculated by using the neutral transport simulation code including the baffle plates, which will propose the most optimized closed divertor configuration for LHD.

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