FLUXES OF BORON IN THE SCRAPE-OFF PLASMA OF TEXTOR FOLLOWING BORONIZATION

TEXTOR was boronized in March 1988. During the first three weeks following boronization, collector probes were exposed to ohmic and RF heated discharges in a wide range of plasma densities. With a fresh boronization layer on all surfaces, the dominating relation to main plasma parameters was an increase in boron flux with decreasing plasma density. This behavior is the same as that earlier observed for metal impurities and reflects the energy dependence of physical sputtering yield at the limiters. In January 1989 the boron flux in the scrape-off layer was reduced by two orders of magnitude compared to the situation immediately after boronization. During a gradual change-over from deuterium to helium filling gas there was a drop in boron flux with increasing helium content in the plasma, suggesting that in the situation with only traces of boron left in the machine, the release was dominated by chemical erosion.