Influence of a large oblique incident angle on energetic protons accelerated from solid-density plasmas by ultraintense laser pulses

The acceleration of energetic electron, proton, and heavy ion beams produced by ultrahigh-intensity laser pulses through thin plastic targets is studied using two-dimensional hybrid particle-in-cell simulation. When the laser is incident at a large angle, the proton beams accelerated from the front and rear surfaces of the target deviate from the normal direction because of the formation of non-Gaussian asymmetric sheath field at the target surfaces. In particular, the simulations clearly show that the proton beam in the backward direction can have higher Bragg peak energy than that of the forward direction if the incident angle is sufficiently large.