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Laser-driven high-quality ion beams hold immense potential for applications in diverse fields such as tumor therapy, fast ignition, and so on. However, current experimental ion beams are often constrained by either a large energy spread or relatively low energy. In this paper, we proposed a novel scheme for generating quasi-monoenergetic proton beams by irradiating near-critical-density plasmas, which have a density gradient with a picosecond laser pulse. This approach leverages two key aspects: first, the sustained interaction between the laser pulse and the plasma enhances the duration of magnetic vortex acceleration, thereby promoting extended ion acceleration. Second, the utilization of a multi-species target facilitates the formation of a dual-peaked electric field, which leads to the accumulation of protons in the negative gradient of the accelerating phase, resulting in a quasi-monoenergetic proton beam. The two-dimensional particle-in-cell simulation reveals that by employing a laser intensity of 1.37 × 1020 W/cm2 with a pulse duration of 0.5 ps, we can achieve a carbon ion beam with an energy of 50 MeV/u, and a quasi-monoenergetic proton beam exhibiting a cutoff energy of 160 MeV/u, a peak energy of 75 MeV/u, an energy spread of 3.1 % , and an angle divergence of ∼ 3.2°. Furthermore, the quasi-monoenergetic property is corroborated in three-dimensional simulation results, underscoring the robustness and effectiveness of our proposed scheme. [ABSTRACT FROM AUTHOR]

The mode conversion efficiency (CE) of the relativistic Laguerre–Gaussian (LG) laser is researched in detail within the context of current petawatt laser facilities. The topological charge, radial integer, laser central wavelength, laser bandwidth, and the design of reflective phase plate are integrated into a unified equation in theory. It is found that the vortex laser mode can be expanded as a series of LG modes, with calculations indicating that the LG10 mode predominates, constituting ∼78% of the total mode distribution. Our analysis reveals that mode CE tends toward a saturation value as the number of steps of the reflective phase plate increases. The 32-step phase plate utilized in relativistic systems is fine enough to obtain a higher CE for LG10 mode lasers, which is also verified in three-dimensional particle-in-cell simulations. This research holds promise for optimizing the design of reflective phase plates to enhance the conversion efficiency of intense LG lasers, thereby facilitating broader applications in intense vortex laser technologies. [ABSTRACT FROM AUTHOR]

We present an investigation of high harmonic generation (HHG) from relativistic plasmas driven by two-color co-rotating circularly polarized (CP) laser fields at various frequency ratios ω 2 / ω 1 . The theoretical analysis and the particle-in-cell simulation results reveal that when ω 2 / ω 1 ≥ 4 , CP harmonics with opposite helicities can appear simultaneously due to the relativistic mirror effect and the related conservation laws. Meanwhile, the harmonic spectra features (including the HHG efficiency, the allowed harmonic orders, and their polarization states) can be tuned through adjusting the frequency and intensity ratio of the two driving field components. This finding might be helpful for a consummate understanding of the bi-circular HHG technique. [ABSTRACT FROM AUTHOR]

We report experimental observation of periodic modulations in the energy distribution of C1+ ions dominantly accelerated in the interaction of a 15 μm thick tape target with intense laser pulses of intensities ∼1018 W/cm2 in a defocused configuration. Moreover, the influence of laser intensity on the acceleration of low- and high-charge-state species of carbon ions is observed. Two-dimensional (2D) particle-in-cell simulations elucidate the dynamics of ionization-dependent acceleration of different species in different laser focusing conditions. By comparison, 1D simulations suggest that the modulations of C1+ ions are due to the longitudinal recirculation dynamics of hot electrons in the target, which modulates the sheath field for acceleration of C1+ ions. [ABSTRACT FROM AUTHOR]

The interactions of ultraintense circularly polarized laser pulses with a mixed solid target and a double-layer target are studied by two-dimensional particle-in-cell simulations. Different carbon and proton compositions in the targets are used in the simulations. It is shown that the proton acceleration mechanisms in both targets are very sensitive to the ion density ratios between protons and carbon ions. For a mixed solid target, a relatively low proton density gives rise to monoenergetic peaks in the proton energy spectrum while a high proton density leads to a large cut-off energy and wide energy spread. With the increase of the ratio, the so-called directed-Coulomb-explosion becomes dominated over the radiation pressure. Surprisingly, for a double-layer target with a front proton layer and an ultrathin rear carbon layer, a highly monoenergetic proton beam with a peak energy of 1.7 GeV/u, an energy spread of ∼4%, and a divergency angle of 2° can be obtained, which might have diverse applications in medical therepy and proton imaging in future. [ABSTRACT FROM AUTHOR]