  The emergence of a density bump at the front of a collisionless electrostatic
shock wave have been observed experimentally during the ablation of an
aluminium foil by a femtosecond laser pulse. We have performed numerical
simulations of the dynamics of this phenomena developing alongside the
generation of a package of ion-acoustic waves, exposed to a continual flow of
energetic electrons, in a collisionless plasma. We present the physical
interpretation of the observed effects and show that the bump consists of
transit particles, namely, the accelerated ions from the dense plasma layer,
and the ions from the diluted background plasma, formed by a nanosecond laser
prepulse during the ablation.
