High-quality monoenergetic proton generation by sequential radiation pressure and bubble acceleration

Two-dimensional particle-in cell simulation shows that protons in a small target located in an under-dense high-mass plasma can be accelerated by the radiation pressure of a short circularly polarized laser pulse as well as by the wake bubble field of the laser in the background plasma. The radiation-pressure preaccelerated protons are easily trapped and accelerated stably in front of the bubble for a relatively long distance. It is found that a quasimonoenergetic proton beam of 38 GeV peak energy and 12.6% energy spread as well as small divergence angle can be obtained with a 10(23) W/cm(2) 18.26 kJ laser pulse in a tritium plasma of density 5.2 x 10(20) cm(-3).