Energetic proton acceleration and bunch generation by ultraintense laser pulses on the surface of thin plasma targets

Energetic proton acceleration from concave targets, the front of which were irradiated with 40 fs laser pulses with an intensity of 10²⁰ W∕cm², has been studied as a function of the depth of the concave shape. Three kinds of targets, a triangular concave target, a circular concave target and a parabolic concave target are considered. When the depth of the concave shape was varied, the peak proton energy showed a maximum. The underlying mechanism for the existence of a maximum peak proton energy is presented by tracing the proton trajectory. It is concluded that a parabolic concave target is the best, among the targets considered, for accelerating a proton beam, since a proton beam from a parabolic concave target goes through the strongest electric field.