ASSESSMENT OF BERYLLIUM FARADAY SCREENS OF THE JET ICRF ANTENNAS

The JET ICRF antennas, equipped with beryllium (Be) Faraday screens (FS), can be operated in such a way that the RF specific effects on the plasma boundary, by the impurity influx originating at the screens, are reduced to a negligible level. In dipole phasing, k(parallel-to) = 7 m-1, the influx is for all purposes negligible. In monopole phasing (k(parallel-to) = 0 m-1) the beryllium influx does not exceed phi(Be)FS = 1 x 10(19) atoms.MW-1.s-1 and the corresponding delta-Z(eff)/P(RF) is <0.005 MW-1. The observed dependences of phi(Be)FS (in monopole phasing) on plasma density, antenna voltage, antenna phasing, and the angle between FS elements and the magnetic field in the boundary, B over arrow pointing right (a) = B(theta) over arrow pointing right (a) + B(T) over arrow pointing right (a), confirm that the release mechanism is sputtering by ions accelerated in the RF enhanced Bohm-Debye sheaths forming at the front face of the FS. When the angle between FS and B over arrow pointing right (a) is almost-equal-to 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 did not introduce any disadvantages. The experience from JET operation at powers of up to 22 MW shows that, if the necessary conditions are met, i.e. if RF rectification is minimized, antennas are phased as dipoles and material with low sputtering coefficients at energies of 0.5-1 keV is used, then the influx from the FS is, for all practical purposes. eliminated.