Acceleration and focusing of multispecies ion beams using over-dense targets with curved front surfaces*

High-flux, low-emittance ion beams are of interest to a wide range of applications. In particular, Fast Ignition (FI) of Inertial Confinement Fusion (ICF) requires a sub-ps ignitor beam with high energy flux to ignite a compressed DT fuel. We propose a scheme to generate high-flux ion beams via hole-boring radiation pressure acceleration using over-dense plasma targets with curved front surfaces. These low-emittance (on the order of 0.01 mm mrad) beams can focus to a few-micron spot size at a predefined focal length. The focal length and ion mean energy can be independently controlled: the former by changing the front-surface curvature and the latter by tuning the laser- plasma parameters. We interpret the results using simple models and validate them using first-principles PIC simulations. We demonstrate the applicability of the scheme for different laser transverse profiles and multi-ion species targets, with laser powers in the 100TW to 100 PW range, and the focal length in the 20-60 micron range. We identify factors limiting the ion beam spot size at focus and discuss methods to extend beam focal length to mm range – in accordance with ion beam stand-off distance required for igniting a FI target.