Structural and chemical modifications of AISI 316 SS FT limiters compared with the disruption simulation damage

The effects of both tokamak nondisruptive and disruptive discharges and repetitive pulse heating on the molten and evaporated layers of the AISI 316 SS have been simulated by means of an electron beam gun with pulse times ranging from 5 to 20 ms and power peak density ranging between 8x107 and 6x108 W/m2. Physical and chemical changes induced by thermal shock on the laboratory samples have been compared with modifications observed after 134 discharges on the FT (Frascati Tokamak) mushroom-limiter samples. Analyses were performed by means of SEM-EDXA and Auger spectroscopy. An appropriate facility allows the collection of the metallic vapor produced during simulation tests. Results indicate a Mn and Cr depletion in both laboratory and tokamak samples. The results are also in accordance with the increase of the Mn-line intensity in the plasma column, found by means of VUV-spectroscopic measurements during the transient phase of all 134-FT discharges. All phenomena occuring on both FT limiter pieces and simulative-samples are very similar. Melting by thermal shock, displacement of the molton material, microstructural changes, fissures, preferential evaporation and chemical composition change are the main features of this analogy. (orig.)