FLR mode conversion effects in the coupling analysis of the ICRF strap antenna in the IGNITOR machine

The effect of mode conversion on antenna-plasma coupling in the ICRH range is studied. A collisionless, weakly inhomogeneous plasma in the 'slab' approximation, in which mode conversion is described by the tunnelling equation with first harmonic majority (omega = 2 Omega(cM)) and fundamental minority (omega = Omega(cm)) ion cyclotron absorption, is considered. Green's function, connecting the electric field on an ICRH antenna with the current flowing in the same antenna, has been evaluated, essentially in closed form, and has been employed to evaluate the antenna radiation resistance. Numerical results have been obtained, for the parameters of the IGNITOR machine and a deuterium-hydrogen plasma; in the cold plasma limit, in which mode conversion effects are disregarded, our results are in agreement with those of an established code. Mode conversion effects produce a decrease of the radiation resistance for the monopole antenna configuration, and much weaker effects on the dipole configuration. This is related to the behaviour of the real part of the Green's function, which tends to vanish rapidly for toroidal wavenumbers approaching zero as a result of reflection from the mode conversion laver.