Enhanced backward-acceleration of ions is experimentally observed when ultra-short, high-intensity and ultra-high-contrast laser pulses interact with thin foils having thicknesses in the order of the penetration depth of the laser light. Below the experimentally observed optimum foil thickness for the maximum ion energy versus thickness, there arises a second peak. 1D simulations on foils with an initial plasma density gradient show a similar trend as the experiment. It appears that in this regime of extremely thin foils it is important to take into account the limited expansion of the plasma that is formed by ultra-high-contrast pulses.
Loch, R. A., Levy, A., Ceccotti, T., Quere, F., Thaury, C., George, H., ... Martin, P. (2009). Enhanced ion acceleration with extremely thin foils. European physical journal. Special topics, 175(1), 133-138. https://doi.org/10.1140/epjst/e2009-01130-7