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The installation of beryllium Faraday screens (FS) at the ICRF antennae in JET has resulted in the elimination of the RF specific effects on the plasma boundary by the impurity influx originating at the screens. In dipole phasing, the influx is for all purposes negligible. In monopole, the beryllium influx does not exceed 1.0 E19 atoms/MW/s and the corresponding deltaZ(eff)/P(RF) is less than 0.005/MW. The observed dependencies (in monopole phasing) on plasma density, antenna voltage, antenna phasing and the angle between FS elements and the magnetic field in the boundary confirm that the release mechanism is the sputtering by the ions accelerated in the RF-enhanced Bohm-Debye sheaths forming at the front face of the FS. At an angle of about 22 degrees, the fraction of the RF power radiated by the antenna, dissipated at the screen, can reach 40%. At high antenna voltage, arcing across the FS can occur. With dipole phasing the heating efficiency is not degraded, even with the large angle, and all the power coupled by the antenna is absorbed at the resonance position near the plasma centre. The open screen design does not introduce any disadvantages. The experience from the JET operation at powers up to 22 MW shows that, if the necessary conditions are met, i.e. if RF rectification is minimised and material with the low sputtering coefficients at energies of 0.5-1 keV is used, the influx from the FS is eliminated.

Additional information

Authors: BURES M ET AL., JET Joint Undertaking, Abingdon, Oxon. (GB);D'IPPOLITO D, Lodestar Research Corporation, Boulder, Colorado (US);MYRA J, Lodestar Research Corporation, Boulder, Colorado (US)
Bibliographic Reference: Report: JET-P(91)37 EN (1991) 16 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
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