Operating windows of the pebble divertor

Abstract

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.

Moving Divertor Plates in a Tokamak

“…4. Similar processes steps were described previously for the other moving divertor ideas [4][5][6][7][8][9].…”

“…It is not yet clear whether a moving plate system would be more or less feasible than other similar ideas for movable divertors, e.g. moving belts [4,5], pebble divertors [6,7], or liquid walls [8,9]. Perhaps the main advantage of the moving plate system is that the plasma facing surface will be more robust and more similar to conventional divertor plates than these other systems.…”

“…The most closely related is a moving belt limiter [4,5], which cools and recoats the divertor surface using a flexible in-vessel moving belt. The pebble divertor [6,7] would perform similar functions, but with a gravity-driven stream of small pebbles falling through the divertor. The liquid wall divertor would perform the same functions with a moving liquid stream or gravity-driven droplets [8,9].…”

Rapidly Moving Divertor Plates in a Tokamak

“…Substituting τ from Eq. [3] and then d from Eq. [4] gives T surf = 2T ave , independent of Q or material properties.…”

“…The most closely related is a moving belt limiter 1,2 , which cools and recoats the divertor surface using a flexible invessel moving belt. The pebble divertor 3,4 would perform similar functions, but with a gravity-driven stream of small pebbles falling through the divertor.…”