Two-dimensional particle-in cell simulation shows that protons in a small target located in an underdense 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 ${10}^{23}\mathrm{W}/{\mathrm{cm}}^2$ 18.26 kJ laser pulse in a tritium plasma of density $5.2\times {10}^{20}{\mathrm{cm}}^{-3}$.

• Received 25 June 2009

DOI:https://doi.org/10.1103/PhysRevSTAB.12.121301

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