Investigation of extruded pebble rods for plasma-facing components in magnetic fusion reactor divertors

Design of divertor plasma-facing components which can survive extreme heat loads in future magnetic fusion reactors is extremely challenging. This work presents first manufacture and testing of extruded pebble rods for use as plasma-facing components. The pebble rods consist of 1 mm diameter spheres bound together by a porous carbon matrix. Reactor-relevant (up to 50 MW/m²) steady-state front-surface heat loads are applied at normal incidence by laser heating. Pebble release at rod recession rates of order 0.5 cm/s and intact pebble recovery are demonstrated. A wide range of pebble materials have been tested, including C, B, BN, SiN, SiC, and W. Recession rate tunability has been demonstrated by varying pebble material, matrix fraction, and pebble size. Comparisons with finite-element simulations suggest that the pebble rod recession can be understood in terms of pebble expansion followed by matrix cracking. Tests of the extrusion of the pebble-based rods demonstrate that friction between the rods and the stainless-steel extrusion channel is sufficiently low (<50 N for the expected channel length) for reliable use in a reactor. Front surface outgassing rates below 1000 Torr-L/m²/s are achieved, believed to be sufficiently low for use in typical magnetic fusion reactor divertors.