Abstract.A marked feature of the pebble divertor is an effect by use of functional multi-layer coated pebble, which consists of a surface plasma facing layer, an intermediate tritium permeation barrier layer, and a kernel for heat removal. The dimensions, structure and the irradiation conditions of pebbles are the important issues for the development of the pebble divertor. From the view point of resistance of the induced thermal stress, the pebble is taken as small as possible in size. On the other hand, from the view point of the pumping performance, the suitable irradiation temperature range of the surface layer of pebble was estimated from the experiments and the numerical analysis. The pumping process enhanced by dynamic retention is available to extend the higher allowable irradiation temperature range from 900K to 1100K. As taking the temperature rise limitation due to pumping effect and the fractural strength due to the induced thermal stress limitation, it was found that the diameter of the pebble is possible to be 1-2 mm in about 20 MW/m 2 for the SiC kernel and 2-3 mm in less than 30 MW/m 2 for the graphite kernel.

A `pebble drop' divertor concept is proposed for future commercial fusion reactors. The marked feature of this system is the use of multilayer pebbles that consist of a central kernel and some coating layers. By using multilayer pebbles the pebble drop divertor has advantages such as steady state wall pumping with low bulk tritium retention. The performance of the whole divertor system depends on the characteristics of the multilayer pebble. In particular, the maximum heat load of the system is determined by the dimensions, the layer structure and the material of the kernel. The kernel also has an important role in determining surface temperature, which affects the wall pumping efficiency. Numerical results for the maximum allowable heat load and the surface temperature of the divertor pebble are presented. From the numerical estimation of thermal stress and surface temperature, it is found that the radius of a divertor pebble with a ceramic kernel should be 0.5-1 mm.

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