Necessary and sufficient conditions are obtained for the description of particle motion by a "quasi-potential" in an inhomogeneous static magnetic field and an r.f. electric field. This analysis leads to a new concept of "critical energy", the height of the adiabatic barrier, which is proportional to , E0 being the strength of the r.f. field in the plasma. The theoretical results are confirmed by numerical calculations. Although this concept holds in a general magnetic field, analysis is restricted to the cusped magnetic field.