Collisionless shock acceleration of carbon ions in 1μm-laser-driven near-critical plasma*

Collisionless shock acceleration of charged particles is ubiquitous in the cosmos and its successful adaptation in the laboratory using laser-driven plasmas has the potential for compact particle accelerators suitable for several applications. We report collisionless shock acceleration of narrow-energy-spread carbon ions to 30 MeV with 4% conversion efficiency. This is achieved using a 100 TW linearly polarized laser interacting with a carbon nanofoam target of near-critical density for the 1μm-wavelength laser. The use of nanofoam near-critical target improves upon previous experiments with gas jets leading to low conversion efficiency or with exploding solid foils for which target pre-expansion needs to be optimized empirically. The variations in the accelerated ion spectra among different carbon ion species and proton radiography of the laser-driven near-critical plasma, together with kinetic simulations, provide detailed insight into the dynamics of the laboratory laser-driven collisionless shocks.