ICRF impurity and antenna studies

The paper summarizes recent work on three related topics: (i) Recent experiments and theory of metal impurity influx from the Faraday screens (FS) in JET are compared and show good agreement in both magnitude and scalings with key parameters. The theoretical model describes the acceleration of plasma ions by induced RF sheaths on the screen and the resulting sputtering of the screen material. It is shown that metal impurities can be reduced to negligible levels by proper antenna design and the use of a low atomic mass screen coating. (ii) Experiments on RF sheath formation at the FS of a fast wave ICRF antenna in Phaedrus are described. It is shown that the radial feeders of a typical two-strap fast wave antenna induce a large voltage on the FS with respect to an adjacent limiter. The magnitude of the voltage varies with strap phasing. Experiments with a low power single strap antenna mounted in the Phaedrus-B central cell demonstrate RF self-biases at the FS. (iii) A computational investigation of the vacuum properties of ICRF antennas is reported on. The numerical simulation code, ARGUS, is described briefly and results from a study of the PPPL TFTR ICRF antenna are given.