A hybrid laser-RF compact proton accelerator to 250 MeV

High-charge proton beams with 200+ MeV and small energy spread are important for applications such as therapy of deep-seated tumors. We use large 2D and 3D particle-in-cell (PIC) simulations to explore the development of a hybrid accelerator that combines the advantages of laser-driven (compact, high-charge, 10s MeV) proton beams with high-gradient RF acceleration (controllable beam energy and energy spread) in a meter-scale compact system, eliminating the need for large and expensive RFQ to perform bunching. We use an adaptive mesh technique to do fully-kinetic modeling of the system self-consistently, from the laser-solid interaction, transport, to the meter-scale acceleration in the RF. We find that space-charge field is minimized in transport due to the screening of accelerated electrons, but must be controlled in the RF stage for effective acceleration. We show that by tuning the distance of laser-plasma foil to the first RF cavity entrance, the space-charge field can be controlled such that it actually increases the beam capture and makes the accelerator more robust to the injection phase. We present end-to-end 3D simulations showcasing the possibility to develop a hybrid accelerator that captures a laser-driven proton beam at 40 MeV and produces a high-quality, high-charge 250 MeV beam in a few meters.