Carbon deposition in the gaps between tiles: Data from the Tokamak Fusion Test Reactor and laboratory simulation studies

Abstract

The bumper limiter in the Tokamak Fusion Test Reactor (TFTR) is lined with graphite tiles. Between the tiles are gaps of width ∼3 mm. Interaction of the hydrogen plasma with the graphite tiles is known to lead to the deposition of hydrogen-rich carbon layers on low-flux regions of the tiles. When these tiles were disassembled for examination, thick carbon deposits were found to penetrate ∼2 cm down the side faces of the gaps. These deposits were several microns thick, at a depth ∼5 mm below the plasma-facing surface. Experiments were performed in the plasma simulator Laser-Material-Plasma Experiment (LAMPE) to study the mechanism of carbon deposition in narrow gaps. In conjunction with calculations using the neutral transport code DEGAS, the results show that sputtered carbon can be transported deep down the gap by multiple reflections off the faces of the gap. This understanding should be taken into account in the design of tiles for future fusion reactors to minimize codeposition buildup in these nearly inaccessible regions.