A mechanism of island self-excitation is discussed in detail, by studying the dynamical interaction, in a tokamak plasma, of a suitably defined ensemble of ions with the electrostatic field of densely packed island chains, assuming that the ion Larmor radius is larger than the island width. It is shown that the loss of particle kinetic energy, consequent to a transverse adiabatically slow drift produced by the interaction, drives an instability through which this energy is pumped directly into the islands. In this way the islands feed themselves through the ion-chain interaction