| A systematic experimental studies on laser ion acceleration based on both micron-thick Aluminum foils and nanometer-thick diamondlike carbon (DLC) foils irradiated by a linear polarized laser pulse with 30TW power and 10^-6 contrast has been presented. For the aluminum foils, proton energy up to 8MeV was observed at the target normal direction. For the DLC targets, distinct proton and C3+ was observed at both laser direction and target normal direction with maximum energy around 1MeV. The scaling of proton beam parameters with target thickness and laser power has also been studied. Two-dimensional particle-in-cell simulations reveal that the low laser contrast caused the deformation of the target, which lead to the shift of proton emission direction and suppress the acceleration efficiency . In order to further improve the laser energy transmission efficiency, a high intensity, high contrast laser pulse with steep front is required. By both 3D particle-in-cell(PIC) simulation and analysis, a plasma lens with near critical density is proposed. When the laser passes through the plasma lens, the transverse self-focusing, longitudinal self-modulation and prepulse absorption can be synchronously happened. If the plasma skin length is properly chosen and kept fixed, the plasma lens can be used for varied laser intensity above 10^19W/cm^2. The plasma lens can be implemented by a micron-scale glass cone irradiated by the prepluse with precisely controlled timing before the main pulse. Simulation shows by combining the cone target and the DLC target, both acceleration efficiency and proton energy can be about 3 times higher than in RPA regime and 6 times higher than in TNSA regime. It shows 180 MeV proton beam can be generated at laser intensity of 10^20W/cm^2. |
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