Laser Acceleration of Ultra-thin Multi-Ion Foil: Accelerating Quasi-Monoenergetic Proton Beam by Combination of Radiation Pressure and Heavy-Ion Coulomb Repulsion

Using ultra-thin proton-carbon foils, we found a critical ratio of proton/carbon concentration, below which there is a self-organized triple layer of electrons, protons and carbons. Because of its smaller charge-to-mass ratio, the protons layer is accelerated by the RPA in the front of the carbon layer. When the electron layer becomes transparent due to the Rayleigh-Taylor instability, laser energy leaks out and the radiation pressure can no longer accelerate the electron layer, thus stopping the acceleration of proton layer. However, the Coulomb repulsion by the carbon layer continues to accelerate the proton layer, sustaining a quasi-mono energetic prtoton spectrum. Using a normalized peak laser amplitude of a = 5 and a carbon-proton target with 10{\%} protons, our PIC simulation shows that the combined acceleration of the proton layer by radiation pressure and Coulomb repulsion can last as long as 60 laser periods, and the resulting quasi-monoenergetic (less than 10{\%} energy spread) proton energy is $\sim $ 70 MeV.