The dynamics of electric field generation and radial acceleration of ions by a laser pulse of relativistic intensity propagating in an underdense plasma has been investigated using a one-dimensional electrostatic, ponderomotive model developed to interpret experimental measurements of electric fields (Kar S et al 2007 New J. Phys.9 402). Ions are spatially focused at the edge of the charge-displacement channel, leading to hydrodynamical breaking, which in turn causes the heating of electrons and an 'echo' effect in the electric field. The onset of complete electron depletion in the central region of the channel leads to a smooth transition to a 'Coulomb explosion' regime and a saturation of ion acceleration.