HIGH-TEMPERATURE EROSION OF GRAPHITE DURING EXTREME LIMITER LOADS IN TEXTOR

Neutral beam heated discharges at the tokamak TEXTOR were used to heat up preferentially parts of the poloidal limiter system representing 15% of the effective area. Convective loads of about 2 kW/cm2 led to maximum temperatures of 2000-degrees-C. During the heat-up phase the effective carbon erosion yield increased from 1100 to 2000-degrees-C by more than one order of magnitude. This behavior is in accordance with laboratory measurements on radiation enhanced sublimation processes (RES). For the TEXTOR limiters the derived activation energy is 1.4 eV. The strong increase of the carbon flux leads to an increase of the carbon density by a factor of 4, and to a doubling of the electron density, the radiated power and Z(eff). The fuel dilution is about 50%. However, even under strongest heating scenarios applied, the plasma does not disrupt.