Alfvén spectra in a reversed-shear tokamak plasma with a population of energetic ions exhibit a quasiperiodic pattern of upward frequency sweeping (Alfvén Cascade).This paper presents an interpretation of Alfvén Cascades in terms of a new energetic particle mode localized around the point of zero magnetic shear.Existence of this new mode requires a sufficient density gradient of energetic ions that are characterized by rapid drift through the radial mode structure.A negative density gradient determines the observed direction of frequency sweeping.