N. "Ohno

N. Ashikawa

S. Masuzaki

T. Yamada

Y." Tomita

National Institute for Fusion Science (NIFS-Repository)

Study of Dust Transport in LHD PlasmasOhno, N., Yamada, T.(Nagoya Univ.) Ashikawa,N., Masuzaki, S., Tomita, Y.Dust will be a serious problem in fusion devices be-cause dust formation in next-step devices is expected tobe enhanced due to a long pulse operation. Dust accu-mulation inside the vacuum vessel contributes to hugetritium inventory. In order to understand accumulationprocess of dust particles, we need to study dust trans-port. In the previous experiments of the LHD, therefore,we injected carbon dust particles by using material probesystem into a plasma and the motion of dust particleswas measured with a high-speed camera. In this study,we have developed 3 D dust transport code for aim atanalyzing dust motion observed in LHD.In the simulation code, we take into account gravi-tational force, electrostatic force, Lorentz force, ion-dragforce and friction force between a dust particle and neu-tral gases as forces acting on a dust particle in a plasma.In order to evaluate the validity of the developed sim-ulation code, we compared with the calculated resultsand experimental ones of the dust transport obtained inthe NAGDIS-T. We used spherical carbon dust particleswhose radius is about 8 micron. We dropped dust parti-cles from the top of the vacuum vessel into a deuteriumplasma. We set containers on the bottom in toroidal andradial directions to collect dust particles transported ina deuterium plasma.Figure 1 shows coordinates used in the simulation.The direction of major radius and hight are x- and y-axes,respectively and the troidal direction is z-axis. x =0 cm and y = 0 cm correspond to surface of inner walland that of top wall. The dust feeder is located at aposition of (9, 0, 0 ). Troidal magnetic ﬁeld strength is4.0×10−2T, and vertical one is 1.8×10−3T, and neutraltemperature is 400K. Reﬂection coeﬃcient of a carbondust particle on the surface of stainless steal wall is as-sumed to be 0.78, estimated based on a shore hardness ofa sphere carbon particle. Plasma parameters in the simu-lation, such as electron density, temperature and plasmaﬂow, were obtained in the experiment. The experimentalresult indicates that plasma rotates poloidally around aplasma column due to E × B drift, where the plasmacenter is x = 10 cm and y = -7 cm.Figure 2 shows calculated dust trajectories . In Fig.2(a) ( the dust radius is 0.5 micron ), in the upper regionof the vacuum vessel from -7 to 0 cm, a dust particle, in-troduced from the top, receive strong outward ion dragforce due to ExB plasma ﬂow, so a dust particles is trans-ported outward, and is reﬂected at the outer wall. Thereﬂected dust particle moves inward, but the directionof the dust motion is changed by the ion drag force andthe dust particle hit the outer wall again. Finally, thedust particle reaches on the bottom. On the other hand,Fig. 1: Coordinate in the simulation code.Fig. 2: Calculated dust trajectories of a dust particleswith a radius of (a): 0.5 micron and (b) 6.0 micron.a large dust particle of 6 micron can not reach the outerwall, and fall down on the bottom of the vacuum vesseldirectly. Figure 3 shows radial distributions of mass ofFig. 3: Comparison between radial distributions of massof dust particles in the experiment and the simulation.dust particles collected by containers. The simulation re-sult is in a good agreement with that in the experiment,except for data points around x = 7 cm and x = 13 cm.The discrepancy is found to be caused by an inﬂuence ofthe viewing ports located in the bottom of the vacuumvessel.We will analyze the dust transport date in the LHDby using the developed 3D dust transport code in nearfuture.54§30. tud  of  Transport in LHD PlasmasOhno, N., Yamada, T. (Nagoya Univ.), Ashikawa, N., Masuzaki, S., Tomita, Y.

Study of Dust Transport in LHD Plasmas

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