Perturbations of the spherical symmetric flow of a plasma generated by an intense light ion beam impinging on a pellet are studied. Scaling laws for the ablation pressure and mass flow rate relating the main parameters of the target and ion beam are given. The model also includes an analysis of the Rayleigh-Taylor instability of a thin compressed shell for perturbation wavelengths comparable to the shell thickness. As in the laser fusion context (TAKABE et al., 1985, Physics Fluids 28, 3676; KULL, 1989, Physics Fluids B1, 170), the results show, by means of a similar formula, that the instability growth rate is reduced to below the classical value due to ablative effects. This reduction is given in terms of the parameters of the target and ion beam.