Role of the antenna screen angle during ICRF heating in JET

Two comparative heating experiments were carried out to test the role of the antenna screen angle during high power ICRF heating in JET. In the first experiment the magnetic field was practically aligned with the screen elements, whereas in the second experiment the toroidal field was reversed and the angle between the elements and the field was 25 deg. There were clear differences in the impurity influx, the heating efficiency and the coupling resistance between discharges with normal field and discharges with reversed field. The results indicate the impact of the Faraday screen design on the levels of RF enhanced metal influxes originating at the screen. This should be of particular relevance for the ICRF antennas in a reactor where, in contrast to small devices, most of the interaction between the RF fields and the plasma edge is expected to take place around the antenna structures because of the highly localized RF field patterns. A numerical code based on three-dimensional single-pass and full wave calculations of antenna-plasma coupling, including an arbitrary angle between the screen elements and the magnetic field, can qualitatively reproduce the observed difference in the coupling resistance. The experimental results are discussed with regard to possible mechanisms that can account for the observed differences in the two cases studied. (author). 36 refs, 16 figs