We report on the experimental demonstration of a technique to generate steep density
gradients in gas jet targets of interest to laser plasma ion acceleration. By using an
intentional low energy prepulse, we generated a hydrodynamic blast wave in the gas to
shape the target prior to the arrival of an intense CO2 (λ ≈ 10 µm) drive pulse. This
technique has been recently shown to facilitate the generation of ion beams by shockwave
acceleration (Tresca et al. 2015). Here, we discuss and introduce a model to understand
the generation of these blast waves and discuss in depth the experimental realisation
of the technique, supported by hydrodynamics simulations. With appropriate prepulse
energy and timing, this blast wave can generate steepened density gradients as short as
l ≈ 20 µm (1/e), opening up new possibilities for laser-plasma studies with near-critical
gaseous targets.