Issues in Radiation Pressure Acceleration

Radiation Pressure Acceleration by circularly polarized laser pulses has been proposed and widely investigated theoretically as a promising scheme for efficient generation of ions. In particular, the use of ultrathin targets may allow to reach energies per nucleon beyond the GeV range. The first, recently reported experiments appear to confirm the basic mechanism and instantiate open challenges such as achieving a monoenergetic spectrum. In the present work we extend the "accelerating mirror" or "Light Sail" textbook model by including the effect of nonlinear "relativistic" transparency. The comparison with particle­in­cell simulations shows that the simple model gives meaningful predictions for the foil velocity and its "optimal" thickness for acceleration. At the same time, simulations show that the underlying dynamics is complex and involves a high degree of self­organization of the system. In particular, electron heating in the final stage of acceleration appears to be responsible for late broadening of the ion spectrum. We also report results on ion acceleration by extremely short pulses in liquid hydrogen jets to achieve high repetition rate in the "hole boring" regime. Finally, preliminary results on radiation friction effects are also reported.