Above 100 MeV proton beam generation from near-critical-density plasmas irradiated by moderate Laguerre-Gaussian laser pulses

Abstract High-quality protons have many potential applications in tumor therapy, proton radiography, fast ignition and nuclear physics. Over the past decades, significant progress has been made in generating such proton beams with lasers. However, it is still challenging to achieve a compact proton beam with energy of hundreds of MeV under currently available laser intensities. Here, we propose a novel scheme to produce above 100 MeV protons in the nonuniform near-critical-density (NCD) plasmas driven by a Laguerre-Gaussian (LG) laser pulse. When a linearly-polarized LG laser pulse with the intensity of ∼ 1020 W/cm2 enters the NCD plasmas with a trapezoidal density profile, a donut-like double-channel structure with an electron column on the axis is produced behind the laser pulse, together with a unique, strong magnetic field. At the end of the density plateau, the magnetic field begins to expand in both longitudinal and transverse directions. The magnetic pressure force displaces the electrons with regard to the protons, resulting in a quasistatic electric field Ex S. This electrostatic field can keep robust and sustain for a long time since the magnetic field decreases very slowly, i.e., Ex S ∝ ∇Bz 2. Thus, the protons can be accelerated by the longitudinal electrostatic field and confined by the generated transverse focusing field. Three-dimensional particle-in-cell simulations confirmed that a well-defined 100s MeV proton beam with cut-off energy at least 3.5 times larger than that of the normal Gaussian laser could be obtained, making the proton beams a potential source for tumor therapy in the future.