The authors discuss the problem of the distribution function of injected ions in a plasma, which is reduced to a linear two-dimensional parabolic differential equation. By means of numerical integration of the problem it is verified that the presence of impurities in the plasma increases the diffusion of fast ions into the loss cone. The authors have elucidated the conditions under which the aggregate influence of impurities and the loss cone leads to a substantial change in the fast-ion energy balance. The magnitude of the additional energy transferred from the electric field to the hot ions has been determined together with the conditions of transition to the "whistle" regime.