Properties of thick layers deposited on plasma facing wall components of TEXTOR-94.

Redeposition of eroded carbon can form carbon-rich layers on the plasma fac ing components of experimental fusion devices. The trapping of hydrogen iso topes in these deposits may represent a potential safety hazard in T-D oper ated devices. Understanding the properties of these deposits, like composit ion, structure, chemical activity etc, is an important prerequisite to pred ict the hydrogen retention, and to develop in-situ removal techniques. Thick layers (similar to 150 mu m), which tend to flake off, were found on different wall components of TEXTOR-94 after long term operation (6110 disc harges). Flakes with a size of a few mm(2) had been removed from the main p oloidal limiter. They have rough, bubbly appearance, and are porous but mec hanically hard. The deposits were examined by SEM, NRA, EPMA, EDX, and OES. They consist of about 80%-90% carbon and incorporate non-uniformly distrib uted deuterium, Si, B and metals (mainly Fe, Cr, Ni, also as clusters). The D/C-ratio found in these flakes measured by TDS is 4x10(-4). Baking the fl akes in the air at 250 degrees C reduces their thickness at a rate of about 1.2 mu m/h, baking at 450 degrees C gives a rate of about 2.6 mu m/h. Afte r baking the flakes turn to ash. The main residues are metals, B and Si, el ements that do not form volatile oxidation products.