Pebble divertor concept for high power density fusion reactors

The pebble divertor is a new concept applicable to the strike zone with very high heat (>20 MW/m2) and particle load (>1024 atoms/m2s). In this concept, a large number of small pebbles made from refractory materials with multifunctions are used as a moving surface. A marked feature of our pebble divertor concept is the use of multi-layer pebbles. The multi-layer pebble consists of a kernel, a plasma facing layer (PFL) and an intermediate tritium permeation barrier. In the case of the pebble with a diameter of 1–2 mm, it is found that the pebble can resist high heat loads of about 30 MW/m2. When the PFL of the pebble is made from carbon material, pebbles also perform fuel gas pumping due to the hydrogen gettering characteristic of carbon. From the estimation of the energy conversion in the pebble divertor system, the maximum operation temperature should be larger than about 800 K to obtain a energy gain, which is the ratio of recovery energy in Calnot cycle to the additional heat energy, larger than one in the divertor system.