Theoretical and experimental investigation of the impact of surface waves and bulk absorption on ICRH fields measured at the plasma edge in tokamaks

A planar model which takes into account the periodicity in poloidal and toroidal direction, as well as the radial variation of the toroidal magnetic field, was used to calculate the RF fields in the edge. It is shown that surface waves can exist in a wide plasma parameter range and are excited in TEXTOR in particular. The situation in JET was also examined. Measurements were performed on TEXTOR with small RF pick-up antennas mounted on a radially movable manipulator, in order to determine the electric and magnetic RF field components in two perpendicular directions. The measurements show a wave polarization in agreement with the theoretically predicted existence of surface waves. The measured radial profile of the RF magnetic field shows qualitative agreement with theory. Harmonic generation and a clear anticorrelation between bulk absorption and amplitude of RF fields in the edge were observed, on TEXTOR as well as on ASDEX. Strong asymmetries in the RF fields corresponding to top- and bottom antenna excitation were observed on TEXTOR and compared with ray tracing calculations. (orig.)