(EX/P4-15) Particle Balance Study in the Large Helical Device

S.M. Masuzaki1), M. Kobayashi1), R. Sakamoto1), J. Miyazawa1), M. Shoji1), M. Goto1), S. Morita1), N. Ohyabu1), A. Komori1), O. Motojima1), LHD Experimental Group1)
1) National Institute for Fusion Science, Toki, Japan

Abstract.  Achieving an effective particle control using a divertor system is a crucial issue in fusion experiment. Particle balance studies are necessary for understanding neutral particle behavior, and are also important from the viewpoint of tritium inventory in vacuum vessel for next step fusion devices. In the Large Helical Device (LHD), plasma experiments with an intrinsic helical divertor (HD) and a local island divertor (LID) have been performed, respectively. The HD is an open divertor at this stage, and the LID is a closed divertor equipping baffle structure and pump-system. In the LID configuration, most particles from the core are well guided by the outer-separatrix of m/n=1/1 island to the closed divertor module located locally in a toroidal section, and the substantial part of fuelled particles are evacuated. On the other hand, in the HD configuration, the most part of fuelled particles are retained in the vacuum vessel. In a typical ice-pellet fueled discharge, over 80% of fuelled particles are evacuated in the LID configuration, and about 75% of fuelled particles remain in the vacuum vessel in the HD configuration, respectively. Therefore the discharge history strongly affects the density control in the HD configuration. The difference in the neutral particle behavior between the HD and the LID configurations is considered to be explained by following conditions in the LID configuration: (1) The low charge exchange particle flux to the first wall due to the relatively low edge neutral density reduces the amount of implantation of the neutral particles in the first wall. (2) The small carbon amount released by physical and chemical sputtering from the helical divertor plates due to the small ion flux to the plates in the LID configuration leads the co-deposited particles to be small. (3) For the high operational temperature (over 1000K) of the LID divertor plates in the LID configuration, the amount of desorbed particle from the plates is large. On the contrary, in the HD configuration, the operational temperature of the helical divertor plates is typically less than 700K, and it is too low for the particle desroption from the carbon divertor plates.

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