Performance characteristics of pebble flow for pebble divertor

The pebble divertor concept is proposed for the high-power density fusion reactor. In this system, a large number of multi-layer pebbles are dropped in the divertor region to form the pebble curtain as the divertor plate. The multi-layer pebble consists of three layers, i.e. kernel, a plasma facing layer and an immediate tritium permission barrier. The performance characteristics of the pebble flow are related to flow rate and flow pattern for shielding the incident particle flux. In this study, we have studied fundamental designs of the dropping system. The dropping system consists of a container with the single slit at the bottom. The pebbles are stored in the cuboid container. In the experiment, the dependence of the flow rate of the pebble on the slit width is observed. Al2O3 pebbles and active carbon pebbles were used at the point of view for mechanical performance instead of using the multi-layer pebbles because the multi-layer pebble is very expensive now so that it is difficult to prepare a large number of pebble enough to perform the experiment. In the calculation, we estimated the dependence of the flow rate on the slit width. The calculation results have been coincident with the experimental results. It is possible to estimate the relation between slit width and flow rate of the multi-layer pebble with SiC kernel. From this study, the pebble curtain can shield more than 90% of heat flux from core plasma when the slit width is more than 3 cm.