EROSION AND SURFACE MODIFICATIONS OF DIFFERENT FT LIMITERS

Various processes (sputtering, evaporation, arcing, blistering, chunk emission, etc.) acting in the scrape-off region have proven to be the main responsible mechanism for surface erosion. This paper analyzes the behaviour of three different limiters used in an FT machine for several discharges by means of the SEM-EDXA technique. The first limiter analyzed has a circular cross-section of the surface facing the plasma. The second-one has an elliptic cross-section to obtain a uniform distribution of the heat flux. The third one is slightly different from the others: movable mushrooms are placed on a stem as a mounting support. All limiters are manufactured in AISI 316 L stainless steel. Thermal energy dumped during a disruption results in the vaporization and production of molten layers. The instability of this molten layer depends on the strength of the magnetic forces acting on the limiter surface and may give rise to the so-called "chunk-emission". Microstructural changes (fissures, dendrites growth, etc.) on the damaged areas and sputtering and arcing on the ion side of the limiters were observed. © 1984.