Coupling of ion Bernstein waves to tokamak plasmas by waveguide antennas

A computational model has been developed that allows calculation of the radiofrequency (RF) power reflected by grill waveguide antennas, employed for additional heating of tokamak plasmas. The plasma surface impedance is evaluated using a full wave analysis with the SEMAL code (Sauter, O., Vaclavik, J., Comput, Phys. Commun. 84 (1994) 226), which solves the integrodifferential equations for the electric field of the wave in configuration space, in an inhomogeneous plasma, for any cyclotron harmonic and without any approximation with respect to the Larmor radius. These features are necessary for solving the problem of the antenna coupling of the ion Bernstein wave (IBW) plasma heating experiment, operating at an ion cyclotron harmonic higher than the second. In this experiment, the antenna coupling has to be carefully considered, in order to find suitable antenna-plasma conditions for avoiding parametric instabilities, a phenomenon that inhibits wave propagation in the plasma interior. As a result, for an IBW experiment on the FTU tokamak, a good antenna coupling is expected by assuming a low plasma density near the antenna-plasma interface. Nevertheless: by considering higher plasma. densities, so that a low activity of the parametric instabilities is expected, a marginal antenna coupling has been found, with about 40% of RF power reflected at the antenna.