In the present thesis the role of large orbits of high energy minority ions during Ion Cyclotron Resonance Heating (ICRF) experiments on a two-component plasma in the Joint European Torus (JET) is considered. The model presented here includes guiding centre orbits in the calculation of the minority ion distribution function and of the radial profiles of the collisional power with electrons and background ions. The results substantially differ from previous models in the estimate of the number of high energy particles and in the electron power deposition profile, which deviates considerably from the usual Gaussian shape. The orbit model thus introduced is then applied to a number of problems. Firstly, it is shown how to calculate correctly the fusion yield in ICRF heated (3He)D plasmas, and the results are compared with high fusion yield experiments carried out on JET and extrapolated to (3He)D experiments on ITER. Then the problem of the saturation of the central electron temperature at high values of the RF power per particle is addressed. It is firstly shown that the orbit-induced modification of the electron power deposition profile has important consequences for the evaluation of the transport coefficients in the plasma centre. Then the temperature saturation is shown to be caused by a combination of orbit effects and degradation of plasma confinement with increasing electron temperature. Tomographic reconstructions of γ-ray emission due to nuclear reactions of 3He with 9Be impurities present in the plasma show clear evidence of trapped ion orbits. These γ-ray emission profiles are simulated for both on-axis and off-axis ICRF heated (3He)D plasmas by taking into account orbits of minority ions with energies above the threshold for nuclear reactions to occur and a distribution which is function of both parallel and perpendicular velocity and pitch-angle.