Proton acceleration from overdense plasma target interacting with shaped laser pulses in the presence of preplasmas

We report two-dimensional particle-in-cell simulations to study the interaction of shaped laser pulses with overdense plasma targets. Preplasma is introduced as a linearly increasing plasma density ramp in front of the overdense plasma target and the shaping of laser pulse is introduced by considering different durations of leading and trailing pulse edge. The preplasma electrons are pushed towards the target due to the direct action of the laser ponderomotive forces on these electrons. The ponderomotive force associated with the sharp edge asymmetric pulses is significantly enhanced due to change in velocity gradient, hence, the electrons get stronger push inside the target. Thus, it is expected to reach more energetic electrons at the rear side of the target to accelerate the proton by the charge separation field. The combined effect of pulse asymmetry in the presence of preplasma enhances the peak energy of the protons for the case of fast leading edge pulse. We demonstrate the significant role played by the asymmetric pulse interaction with an overdense target to accelerate the proton more effectively from the rear side of the target. The asymmetric pulse with fast leading edge together with preplasma generates a proton beam of energy 12.5 MeV in comparison with the slow leading edge pulse (which generates proton beam of 6.0 MeV energy). This study suggests that the pulse asymmetry can be used as a tuning parameter in the presence of preplasma to improve the proton beam quality in TNSA scheme.