Hollow target for efficient generation of fast ions by ultrashort laser pulses

The efficiency of ion acceleration driven by high-power femtosecond laser pulses strongly depends on the target thickness and on the absorption of laser pulse energy into the ionized solid target. Enhanced absorption has been demonstrated for targets with submicrometer structures deposited on their front surface. However, increasing the overall thickness of the target by adding the layer with structures is undesirable. Here, microstructured hollow targets are proposed to enhance the absorption of the laser pulse energy while keeping the target thickness to minimum. It is demonstrated by full 3D particle-in-cell simulations that the efficiency of proton acceleration from hollow targets substantially exceeds the efficiency of the acceleration from flat foils of the same thickness. The fabrication of an ultrathin hollow target (prototype) by focused ion beam milling is also described.