FAST WAVE ANTENNA COUPLING TO SLOW WAVES AND ION BERNSTEIN WAVES DURING ICRF HEATING OF TOKAMAK PLASMAS

The coupling of the fast wave antenna to short wavelength waves is analysed by taking into account the imperfect alignment of the antenna screens along the magnetic field lines of a Tokamak plasma. Analytical and numerical estimates for the fraction of the power going into the short wavelength modes are given for various edge plasma parameters and their profiles. The functional dependence of this fraction on the screen angle and the excited spectrum of the short wavelength waves are computed. According to the results a nonnegligible amount of power could be radiated from the present ICRF antennas in the form of ion Bernstein waves or slow waves for realistic edge density profiles. For edge plasmas extending to the screen, the excited electrostatic wave spectrum is peaked for large parallel refractive indices nz and the electrostatic wave coupling is due to the parallel electric field along the magnetic field lines which therefore requires some misalignment of the screens. When the density near the screens is sufficiently low to allow the presence of the slow mode resonance, the excited electrostatic wave spectrum is peaked for mod n z mod <1 and the coupling is mainly due to the confluence between the fast wave and slow wave and does not necessarily require any misalignment. Possible connections of these phenomena to the deleterious edge modification and enhanced impurity influx from the antennae, observed in the heating experiments of JET, are discussed.