Implications of net erosion and redeposition of solid-surface plasma facing material in long-pulse fusion devices

It is estimated that long-pulse fusion devices may experience rates of net erosion and deposition of solid PFC (Plasma Facing Component) material of 10³ – 10⁵ kg/year, whatever the material used. Even if the net erosion (wear) problem can be solved, the redeposition of so much material has the potential for major interference with operation. The potential implications appear to be no less serious than for plasma contact with the divertor target, i.e., a dust explosion or major UFO-disruption could be as damaging as target failure for an actively-cooled device. It will therefore be necessary to manage material deposits to prevent fouling operation. This situation appears to require a fundamental paradigm shift regarding meeting the challenge of taming the plasma-material interface; in that any acceptable solid PFC material will in effect be flow-through, like liquid-metal PFCs, although at far lower mass flow rates. Therefore, the solid PFC material will have to be treated as a consumable like brake pads in cars. A critical open issue is the formation of large/thick redeposited material in the divertor region, colloquially called slag. Unfortunately, understanding and predictive capability for the formation and stability of these thick layers is almost completely lacking, but new developments, like powder dropping into discharges, provide a unique opportunity to carry out controlled studies on the management of low-Z slag in all current magnetic confinement devices. The implications for such a paradigm shift and near-term research needs will be discussed.