Quasi Mono-Energetic Heavy Ion Acceleration From Layered Targets

In the present work, we demonstrate acceleration of quasi monoenergetic heavy ions during the interaction of a high-intensity short-pulse laser with multi-layer targets. The targets, consisting of layers of high-Z (gold) and low-Z (carbon) species a few nm thick, have been used to tailor the energy spectra of the high-Z ion species. Au-ion bunches of energy around 500keV with an energy spread of less than 20% are observed. Particle-in-cell simulations provide explanation for a number of features of the experimental observations. Several behaviors, in addition to the expected sheath-field acceleration, were found to be involved. It is found that the Au layer is pistoned outward by the underlying Si substrate whilst simultaneously being tamped at its leading edge by the carbon overlay. The simulations show best agreement with the experiments when the carbon layer is first rarefied by the laser prepulse. In these cases, the simulations reproduce the double-humped spectra found in the experiment. Ion-electrostatic instabilities rapidly lead to the formation of a single trapping-like structure in phase space of relatively long wavelength. This long-lived structure dominates the ion acceleration and produces a double-peaked energy spectrum. It is suggested that the instability responsible may be of the Pierce-type.