Dust generation from plasma-surface interactions

Formation of dust and debris in burning plasma experiments and fusion reactors could ultimately impact device operation, with possible adverse effects on investment protection and public safety. Dust is oftentimes observed with various diagnostics during discharges in present plasma devices, and collection during machine venting has produced quantities of dust suitable for characterization. Present fusion plasma devices, however, operate in regimes substantially different from those required of burning plasma experiments. Power exhaust, energetic particles, and localized cross-field transport will be of greater magnitude, challenging materials located at the plasma periphery. The quantity, characteristics, and behavior of dust generated from plasma-surface interactions with these materials can be markedly different from dust collected in existing devices. Strategies ensuring the safety and performance of future burning plasma experiments and fusion reactors must address the detection and possible removal of dust. Development of dust detection and removal techniques relies on understanding sources of dust and predicting quantities and locations of dust accumulation. Additionally, a defensible safety argument to regulatory authorities for experiment or reactor licensing must demonstrate the dust's impact on safety analyses of postulated accidents and present limitations on the dust quantity at various locations in the machine. Several mechanisms have been identified as possible sources of dust in burning plasma experiments. Significant surface heat loads associated with plasma disruption, vertical displacement, large ELM's, arcing, runaway electrons, and other phenomena readily erode material surfaces by mechanisms such as evaporation, melt layer ejection, and explosive degeneration. Erosion products may condense and deposit nearby or be transported to distant locations, possibly hidden in grooves or behind structures, no longer in sight of plasma. Moderate heat loads associated with continuous power exhaust, interaction of energetic particles, sputtering, and chemical erosion may also produce detritus yielding dust. Mixing of eroded material from varying base materials complicates the chemical behavior of the dust, modifying properties of dust growth, transport, and deposition. The purpose of this paper is to describe various mechanisms of dust generation resulting from plasma exposure of materials in burning plasma experiments and fusion reactors. Dust production rates for ITER will be estimated and characteristics will be evaluated, including particle size distribution, surface mass distribution, and tritium and/or activation product content. The analysis results would be useful in the evaluation of dust detection and removal strategies, and provide a point of comparison for dust limits derived from radiological release criteria. (author)