Comparison of chemical sputtering yields for different graphites at high ion flux densities

Graphite is widely used inside the vacuum vessel of magnetic fusion devices and is proposed as target material for future machines like ITER. There are, however, uncertainties concerning the erosion of the material by chemical sputtering via hydrocarbon formation at high ion flux densities. We report on experiments at the plasma generator PSI-1 using a stationary quasi-neutral plasma beam. The ion flux densities used cover the range from 4.10(20) to 1.2.10(22) m(-2) s(-1). They are thus filling the gap between the upper limit of ion beam experiments (10(20) m(-2) s(-1)) and tokamak relevant values (> 10(23) m(-2) s(-1)). To suppress impurity-induced erosion the hydrogen discharges were carefully conditioned and checked for possible impurities, especially oxygen, Samples of different advanced carbon fiber composites (CFC)-including a silicon-doped one - were exposed to various plasma conditions. A calibrated mass spectrometer monitored the CxHy-formation in situ and the axial dependence of the CH-band intensities at 431 and 432.4 nm in front of the target was detected. We have studied the temperature dependence (250-700 degrees C) of the erosion yields at ion flux densities up to more than 10(22) m(-2) s(-1) in hydrogen discharges. Weight loss measurements and scans with an optical profilometer were used to determine the mass loss. For Si-doped CFC an erosion yield of 1% was found, which is a factor of two less than for pure CFC.