Introduction. In this report the development of a numerical model for studying interaction between LH waves and thermonuclear -particles in tokamak geometry is described in view of the DT experiments on JET. Motivation for this work is the obvious fact that a considerable part of -particles are born in a tokamak plasma as trapped particles. Their interaction with the LH waves can not be described accurately enough using the standard quasilinear (Fokker-Planck) approach. In fact, the RF effect on the a-particles is adequately described as the quasilinear diffusion in the space of variables which are constants of drift motion. In an axisymmetrical magnetic configuration, particle drift orbits are determined by three independent integrals of motion which can be, for example, the particle energy, the transverse adiabatic invariant and the canonical toroidal momentum. Adding to that the fast ion spatial diffusion makes the problem at least four-dimensional. This seems much too difficult for any comprehensive analysis. To keep the computer run time and required memory amount for numerical calculations within acceptable limits one is forced to sacrifice a part of the full description. In the model adopted in this work, no spatial diffusion is assumed and the "thin banana" approximation is used. This reduces calculation of the fast ion distribution function to solving a two-dimensional Fokker-Planck equation for each radial grid point. The computational basis of this work is an improved version of the fast ray tracing code (FRTC) described in Ref.[1]. The most important modifications of the code are the possibility to use arbitrary equilibrium configurations, including those with X-points and a module permitting self-consistent treatment of a-particles with the use of the 1D model of Ref.[2]. The report is organized as follows. Calculation of the magnetic toroidal coordinates is described in Sec.2. For the reader's convenience, the ray tracing procedure is outlined in Sec.3, following Ref.[1]. Section 4 is devoted to the alpha particle treatment. It includes discussion of the model, formulation of the Fokker-Planck equation, analytical investigation of the limiting cases and outline of the used numerical approach. Finally, numerical calculations for JET conditions and conclusions are given in Sec. 5.