Description
Overcoming current limitations on laser-driven ion energies requires the development of advanced acceleration mechanisms with enhanced efficiency. Here, we present particle-in-cell simulations of a hybrid ion acceleration scheme based on optically tailored plasma density profiles. In this approach, ions are initially pre-accelerated in a near-critical density plasma slab via the hole-boring regime of Radiation Pressure Acceleration, before being self-injected into an electrostatic field, co-propagating with the driving laser pulse, formed in a relativistically underdense rear-surface density gradient. This phase matched configuration enables sustained ion acceleration over an extended plasma length. Using Bayesian Optimisation, we tailor the density profile for a fixed laser power, achieving significantly enhanced maximum ion energies. Compared with circularly polarised laser acceleration from ultrathin foils (the most efficient acceleration mechanism investigated so far), the maximum proton energy exhibits a markedly steeper scaling with laser intensity, highlighting the potential of target density tailoring for next-generation laser-driven ion sources.