Protection of limiter surfaces by films locally deposited during TEXTOR discharges

Abstract

The erosion of first wall material is a serious problem in future fusion devices. A concept is suggested to compensate erosion by the local injection of reactive gases through divertor plates or limiters forming a suitable deposit on their surfaces. The technique can be implemented during plasma operation with feed back control. Initial experiments were performed in TEXTOR in which silane gas (SiD 4) was blown through a hole in a graphite test limiter during ohmic discharges. The gas was ionized and partly deposited in the vicinity of the interaction hole, at areas subject to the highest heat load. Without gas puffing these are erosion dominated zones. The deposition of a silicon layer over an area of about 700 mm 2, peaked at the injection hole with a maximum thickness of about 900 nm and with a poloidal extend of about 20 mm was observed. It contained a fraction of about 4–5% of the total number of injected Si atoms. In addition, about the same quantity of carbon was co-deposited, significantly exceeding the C fluence arriving from the plasma during normal discharges. An additional local carbon source during silane injection must be invoked, caused e.g. by carbon erosion due to Frank-Condon or charge exchange atoms impinging on the limiter itself on nearby wall areas.