Impurity release from the ICRF antenna screens in JET

During ICRF heating in JET highly-localized impurity influxes from the Faraday screens (FS) of RF antennas are observed. The influxes result from the acceleration of deuterium and impurity ions in the RF-enhanced Bohm-Debye sheaths. The RF-enhanced sheaths form when the equilibrium magnetic field line, linking a large RF magnetic flux, connects two surfaces and closes a loop where the RF 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.