EVOLUTION OF BE-CONTAINING LAYERS IN THE JET BOUNDARY REGION

Collector probes were exposed to the first evaporation of beryllium in the JET vessel and to the first few plasma shots thereafter. The probes were removed under ultrahigh vacuum to an on-line analysis facility before, during and after the sequence of pulses. The amount of Be deposited during evaporation, and the amount of impurity species collected during the plasma pulses, were thus determined. Carbon was observed from the first pulse, despite the coverage of the limiters with Be, and the co-deposition with Be led to carbide formation. Oxygen was gettered very effectively by the evaporated Be layers well away from the plasma, but less efficiently where plasma deposition occurred.