  In the past two decades, laser-accelerated ion sources and their applications
have been intensely researched. Recently, it has been shown through experiments
that proton beams with characteristics comparable to those obtained with solid
targets can be obtained from gaseous targets. By means of Particle-In-Cell
simulations, this paper studies in detail the effects of a near-critical
density gradient on ion and electron acceleration after the interaction with
ultra high intensity lasers. We can observe that the peak density of the gas
jet has a significant influence on the spectrum features. As the gas jet
density increases, so does the peak energy of the central quasi-monoenergetic
ion bunch due to the increase in laser absorption while at the same time having
a broadening effect on the electron angular distribution.
