Intense Laser Acceleration of Quasi-Monoenergetic Protons

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By irradiating an ultra–thin overdense foil with an intense circularly polarized laser beam, the laser radiation pressure can push the foil forward. This scheme, laser radiation pressure acceleration, is one of the most actively studied laser–plasma acceleration scheme to generate quasi–monoenergetic proton beams. However, during the acceleration process, the Rayleigh–Taylor instability may destruct the foil into a bubble–like structure with interleaving high and low density regions. The laser will then penetrate through the underdense transparent regions and cease to push the electrons effectively.

To overcome the short acceleration duration problem, a multi–species foil instead of a pure hydrogen foil is applied. The proton layer can continue to be accelerated by the Coulomb repulsion force from the partially shielded heavy ions even after electrons becoming underdense. The scheme combining shielded Coulomb repulsion and radiation pressure acceleration can significantly extend the acceleration time and obtainable proton energy with quasi–monoenergetic properties.

In this work, we examine by numerical simulation the whole process of the laser proton acceleration scheme, including the energy evolution of radiation pressure acceleration, the development of the Rayleigh–Taylor instability, the effect of shielded Coulomb repulsion using a multi–species foil and further improvement in the scheme itself to pursue a high energy quasi–monoenergetic proton beam accelerated by an intense laser beam.