  We determine conditions allowing to simplify the description of the impact of
a short and arbitrarily intense laser pulse onto a cold plasma at rest. If both
the initial plasma density and pulse profile have plane simmetry, then suitable
matched upper bounds on the maximum and the relative variations of the initial
density, as well as the intensity and duration of the pulse, ensure a strictly
hydrodynamic evolution of the electron fluid (without wave-breaking or
vacuum-heating) during its whole interaction with the pulse, while ions can be
regarded as immobile. We use a recently developed fully relativistic plane
model whereby the system of the (Lorentz-Maxwell and continuity) PDEs is
reduced into a family of highly nonlinear but decoupled systems of
non-autonomous Hamilton equations with one degree of freedom, with the
light-like coordinate $\xi=ct\!-\!z$ instead of time $t$ as an independent
variable, and new apriori estimates (eased by use of a Liapunov function) of
the solutions in terms of the input data (initial density and pulse profile).
If the laser spot radius $R$ is finite but not too small the same conclusions
hold for the part of the plasma close to the axis $\vec{z}$ of cylindrical
symmetry. These results may help in drastically simplifying the study of
extreme acceleration mechanisms of electrons.
