PROTECTION OF LIMITER SURFACES BY FILMS LOCALLY DEPOSITED DURING TEXTOR DISCHARGES

The erosion of first wall material is a serious problem in future fusi on devices. A concept is suggested to compensate erosion by the local injection of reactive gases through divertor plates or limiters formin g a suitable deposit on their surfaces. The technique can be implement ed during plasma operation with feed back control. Initial experiments were performed in TEXTOR in which silane gas (SiD4) was blown through a hole in a graphite test limiter during ohmic discharges. The gas wa s ionized and partly deposited in the vicinity of the injection hole, at areas subject to the highest heat load. Without gas puffing these a re 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 t hickness 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 o f injected Si atoms. In addition, about the same quantity of carbon wa s co-deposited, significantly exceeding the C fluence arriving from th e plasma during normal discharges. An additional local carbon source d uring 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.