Manipulation of laser-accelerated proton beam spatial distribution by laser machined microstructure targets

Experimental study of laser proton acceleration was carried out using laser machined line targets and cross targets with tens of micrometers scale. We have found that both the shape and material of the microstructure targets have significant influences on the distribution of the proton beam. For the aluminum line target, the proton beam spot expands in a direction perpendicular to the boundary of the target; while for the plastic line target, it expands parallel to the boundary of the target. Detailed PIC simulations of the aluminum target have been carried out, which show that due to the lateral transport of hot electrons and the sheath fields accumulated at the edge of the microstructure target, the divergence angle of the accelerated proton beam is changed accordingly, thereby modulating the spatial distribution: the elliptical beam spot can be obtained from the line target, and the quasi-square beam spot can be obtained from the cross target. Simulations of the plastic target indicate that the difference in the electron transport properties for the two types of targets may be the reason for the completely different beam spot shapes. This work shows that the microstructure targets can be a potential method to manipulate the spatial distribution and uniformity of the proton beam.