VELOCITY DISTRIBUTION OF CARBON-ATOMS RELEASED FROM A TOKAMAK LIMITER BY CHEMICAL AND PHYSICAL SPUTTERING

The Doppler broadened profile of a C I line in the infrared (lambda = 909.5 nm, P-3(2) --> P-3(2)0) has been measured with high spectral resolution (lambda/DELTA-lambda > 10(5)) in front of a graphite limiter in the TEXTOR tokamak. The line shows a strong Zeeman effect, but by selecting the pi component with a polarizer, the influence of the magnetic field of TEXTOR on the line shape can be practically eliminated, and the profile is determined only by the velocity distribution of the carbon atoms. By a comparison of shots with a 'detached plasma' and shots with injected gas (CO, CH4) it could be shown that the carbon flux from the limiter is mainly determined by chemical sputtering in a 'detached plasma'. In a plasma attached to the limiter, the energy of the released carbon atoms increases with the electron temperature (T(e)) at the limiter, the typical width of the line gradually increases from about 16 pm to 50 pm (corresponding to velocities of 5 x 10(5) to 1.5 x 10(6) cm/s or energies of 1.5 to 13.5 eV), indicating that physical sputtering dominates at high T(e) and that chemical sputtering then gives only a small contribution (< 15%) to the impurity production at the limiter.