High-quality protons have many potential applications in tumor therapy, proton radiography, fast ignition and nuclear physics. Over recent 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 an energy of hundreds of MeV under the laser intensities currently available. Here, we propose a novel scheme to produce above-100 MeV protons in 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 cm−2 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 quasi-static electric field . This electrostatic field can remain robust and be sustaintained for a long time since the magnetic field decreases very slowly, i.e. . 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 above-100 MeV proton beam with a 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.
Observation of proton modulations in laser–solid interaction