[en] The excitation of ion cyclotron modes by the perpendicular neutral beam injection has been investigated by means of linear stability analysis. Strong instability was found for the switching on phase of the NI. The stationary slowing down distribution by Coulomb collisions, however, is expected to be mainly stable. The nonlinear Fokker-Planck equation for Coulomb interaction has been solved by Monte Carlo technique for Hdeg injection in a H⁺/D⁺ plasma mixture. By means of Monte Carlo simulation, the ion confinement time is estimated as a function of the ambipolar potential. For the thermal ion component in the plateau regime, the transport properties are strongly reduced by the ambipolar electric field. Finally, the bounce averaged drift kinetic equation and the basic principles of the corresponding Monte Carlo simulation are discussed shortly