BEHAVIOR OF BORON-DOPED AND TITANIUM-DOPED GRAPHITE LIMITERS UNDER HIGH HEAT LOADS IN TEXTOR

Boron-doped graphite (3.5% B in C (S2508, Carbon Lorraine), 20% B in C (GB120, Toyo Tanso)) and titanium-doped graphite (2.1% Ti in C (RGTi, supplied by Efremov Institute, St. Petersburg)) have been tested in T EXTOR as a single main limiter for typically 20-40 discharges. Convect ive power loads up to about 30 MW/m(2) led to maximum surface temperat ures between 2400-2800 degrees C. Severe cracking has occurred on the 20% boron-doped graphite material showing that it is not applicable as plasma facing material under high heat loads. Some large cracks have also been observed on the RGTi limiter, probably caused by the strong anisotropy of the material. The 3.5% boron-doped material shows simila r thermomechanical properties as a usual fine-grain graphite (EK98). L arge thermal sublimation of boron and titanium has been observed start ing at surface temperatures of about 2000 degrees C and 1800 degrees C , respectively. Very thick layers of redeposited boron or titanium (> 1 mu m) were formed near the areas of high sublimation. Besides the do ping species (boron, titanium) these materials sublime various (metall ic) impurities at elevated temperatures. No significant increase of th e emitted carbon flu has been observed below surface temperatures of 2 000 degrees C showing that radiation enhanced sublimation (RES) is sig nificantly reduced under these conditions compared with expected value s extrapolated from beam experiments.