Influence of boronization on the plasma performance in TEXTOR

The plasma-facing surfaces of TEXTOR have recently been coated (plasma chemical deposition) by a thin layer of hydrogen-rich carbon and boron (ratio 1:1). The paper describes the improved plasma performance which resulted when this coating replaced the amorphous carbon (a-C:H) layer used before. The main differences were a pronounced reduction of the radiation losses, a decrease of Z eff from 1.4 to 1.2 and a significant change in the hydrogen recycling: the recycling coefficient was smaller than unity during current flat top at a wall temperature T w =150 degrees C whereas this could only be achieved at T w >or=300 degrees C with a-C:H. Density excursions during ICRH pulses were avoided even at a power level of more than 2 MW. Possible explanations for the reduction of the oxygen concentration and for the change in hydrogen recycling are suggested and discussed.