An ion acceleration mechanism in laser illuminated targets with internal electron density structure

1D and 2D PIC simulations of light ion acceleration by irradiation of structured thin foils by short pulse, high intensity lasers have been performed. When the back coating material is thicker than the typical scale length for TNSA fields the ion energy spectra contain peaks. These peaks are found to be due to the acceleration of ions by an electric field which builds up at internal interfaces between materials with different electron number densities. The electric field is generated by incomplete neutralization of the charge built up at the head of the hot electron beam as it crosses the interface between the two materials. In these simulations the back material layer is sufficiently thick that the sheath field drops to zero before reaching the vacuum boundary. The acceleration responsible for the peaks is therefore from within the material and is superimposed on the normal TNSA acceleration from the back material–vacuum surface, i.e. there are two distinct sites of particle acceleration. It is possible that this mechanism may explain features previously observed in simulations. A comparison is also made with experiments with thick rear coatings.