IMPURITY RELEASE FROM THE ICRF ANTENNA SCREENS IN JET

During ICRF (Ion Cyclotron Range of Frequencies) heating in JET (Joint European Torus), highly-localized impurity influxes from the Faraday screens (FSs) of r.f. antennas are observed. The influxes result from the acceleration of deuterium and impurity ions in the r.f.-enhanced Bohm-Debye sheaths. The r.f.-enhanced sheaths form when the equilibrium magnetic field line, linking a large r.f. magnetic flux, connects two surfaces and closes a loop where the r.f. voltage is induced. The FS influxes depend on two intimately-linked parameters, the antenna voltage and the density at the FS, as well as on the magnetic field angle, antenna phasing, FS geometry and material. The influx of Beryllium (Be) from the front face of the Be-gettered FS is phi-Be(FS) = (3 +/- 1) x 10(19) atoms MW-1 s-1 in monopole (k parallel-to = 0 m-1) phasing when the torus has carbon (C) belt limiters. The contribution of the Be front-face influx and the remaining Ni influx from the FS gaps to the ion dilution is delta-Z(eff)/P(r.f.) < 0.015 MW-1. When the C-belt limiters were replaced by Be-belt limiters, the influxes were reduced by more than a factor of 2. In dipole phasing (k parallel-to = 7 m-1) the front-face influx is negligible.