Properties of Thick Layers Deposited on Plasma Facing Wall Components of TEXTOR-94

Abstract

Redeposition of eroded carbon can form carbon-rich layers on the plasma facing components of experimental fusion devices. The trapping of hydrogen isotopes in these deposits may represent a potential safety hazard in T–D operated devices. Understanding the properties of these deposits, like composition, structure, chemical activity etc, is an important prerequisite to predict the hydrogen retention, and to develop in-situ removal techniques.Thick layers (~150 µm), which tend to flake off, were found on different wall components of TEXTOR-94 after long term operation (6110 discharges). Flakes with a size of a few mm2 had been removed from the main poloidal limiter. They have rough, bubbly appearance, and are porous but mechanically hard. The deposits were examined by SEM, NRA, EPMA, EDX, and OES. They consist of about 80%–90% carbon and incorporate non-uniformly distributed deuterium, Si, B and metals (mainly Fe, Cr, Ni, also as clusters). The D/C-ratio found in these flakes measured by TDS is 4 × 10-4. Baking the flakes in the air at 250°C reduces their thickness at a rate of about 1.2 µm/h, baking at 450°C gives a rate of about 2.6 µm/h. After baking the flakes turn to ash. The main residues are metals, B and Si, elements that do not form volatile oxidation products.