Shaped Targets Boost Laser-Driven Particle Beams by 49 Per Cent

Scientists are continually seeking methods to improve the efficiency and energy of laser-accelerated ion beams for applications ranging from hadron therapy to high-energy density physics. Mohammad Rezaei-Pandari, Mahdi Shayganmanesh, and Mohammad Hossein Mahdieh, all from Iran University of Science and Technology, present a detailed Particle-In-Cell simulation investigating Target Normal Sheath Acceleration from both conventional flat foils and innovative annular sector targets. Their research demonstrates a significant enhancement in ion acceleration using the annular geometry, achieved through the combined effects of electromagnetic cavity confinement and geometric plasma focusing. This novel approach increases laser energy absorption to 49%, more than triple that of flat targets, and elevates proton cut-off energies to 22 MeV, establishing a promising route towards the development of compact and highly efficient laser-ion sources.

This work details a comprehensive two-dimensional Particle-In-Cell (PIC) simulation study comparing conventional flat foils with a newly designed annular sector, or C-shaped, target.

Under identical ultra-intense laser irradiation with a normalised vector potential of a0=10 and a pulse duration of τ=25 fs, the annular sector geometry demonstrates markedly enhanced acceleration performance. This improvement stems from the synergistic interplay of electromagnetic cavity confinement and geometric plasma focusing, mechanisms that dramatically alter how laser energy is absorbed and converted into accelerated ions.
Analysis reveals the target void functions as an optical trap, sustaining oscillating electromagnetic fields for over 300fs through multiple internal reflections. This prolonged confinement increases total laser energy absorption to 49%, a significant increase compared to the 16% absorption observed with standard flat targets.

Consequently, peak electron temperatures reach 5.1 MeV, more than double the 2.2 MeV attained in flat target experiments. Phase space diagnostics further confirm that ion bunches accelerated from the converging cavity walls converge at the geometric centre, creating a highly localised, high-density focal spot.

This innovative target design substantially increases the proton cut-off energy to 22 MeV, a dramatic improvement over the 0.12 MeV achieved with flat targets, and accelerates Carbon ions to energies exceeding 60 MeV. These findings establish that precisely tailoring target curvature to exploit both optical trapping and geometric focusing provides a robust and efficient pathway for developing compact, high-performance laser-ion sources with applications in hadron therapy and high-energy density physics. The study utilized ultra-intense laser irradiation with a normalised amplitude of a₀ = 10 and a pulse duration of τ = 25 fs, applied identically to both target geometries.

This simulation work focused on comparing the acceleration performance achieved with each target type, revealing significant differences in energy absorption and ion beam characteristics. The annular sector targets, specifically, were designed to exploit electromagnetic cavity confinement and geometric plasma focusing.

Analysis of the simulation data demonstrated that the void within the annular target functioned as an optical cavity, sustaining oscillating electromagnetic fields for a period exceeding 300 fs through multiple internal reflections. This prolonged confinement resulted in a total laser energy absorption of 49% for the annular target, a substantial increase compared to the 16% absorption observed with the flat foil targets.

Consequently, peak electron temperatures reached 5.1 MeV with the annular target, more than double the 2.2 MeV attained with the flat targets. Phase space diagnostics were implemented to track the evolution of accelerated bunches, confirming that bunches originating from the converging cavity walls superimposed at the geometric center, forming a localized high-density focal spot.

This focal spot contributed to a marked increase in proton cut-off energy, reaching 22 MeV for the annular target compared to 0.12 MeV for the flat target. Furthermore, the annular target boosted Carbon ion energies beyond 60 MeV, demonstrating a significant enhancement in heavy ion acceleration capabilities. These findings establish that tailoring target curvature to harness optical and geometric focusing provides a viable route towards developing compact, high-efficiency laser-driven sources.

Enhanced ion acceleration via electromagnetic cavity confinement in annular targets

Laser-driven ion acceleration research demonstrates a peak electron temperature of 5.1 MeV using an annular sector target, more than double the 2.2 MeV achieved with standard flat targets. This enhancement stems from synergistic electromagnetic cavity confinement and geometric plasma focusing within the novel target geometry.

Analysis reveals the target void functions as an optical trap, sustaining oscillating electromagnetic fields for over 300fs through multiple internal reflections. This confinement directly results in a total laser energy absorption of 49%, a substantial increase compared to the 16% absorption observed with flat targets.

Phase space diagnostics confirm that ion bunches accelerated from the converging cavity walls superimpose at the geometric center, creating a localized high-density focal spot. Consequently, the annular target elevates the proton cut-off energy to 22 MeV, a significant improvement over the 0.12 MeV observed from flat targets.

Furthermore, the research shows Carbon ion energies boosted beyond 60 MeV using the annular sector target design. The study utilized two-dimensional Particle-In-Cell simulations with a simulation domain of 50μm x 30μm and a spatial resolution of 25nm to resolve the plasma skin depth. A linearly polarized Gaussian laser pulse with a central wavelength of 800nm, a full width at half maximum duration of 25fs, and a normalized vector potential of 10 was employed. Initial ion densities were set at nC = nH = 26.44 ncr, representing fully ionized hydrogen and triply ionized carbon.

Enhanced Ion Acceleration via Electromagnetic Cavity and Plasma Focusing

Researchers have demonstrated substantial enhancements in laser-driven ion acceleration using annular sector targets compared to conventional flat foils. Particle-In-Cell simulations reveal that the novel target geometry facilitates improved performance through electromagnetic cavity confinement and geometric plasma focusing.

These mechanisms sustain oscillating electromagnetic fields within the target void for extended periods, leading to significantly increased laser energy absorption, reaching 49% for the annular target versus 16% for flat foils. Consequently, the annular target achieves a peak electron temperature of 5.1 MeV, more than double the 2.2 MeV observed with flat targets, and elevates proton cut-off energies to 22 MeV, a considerable increase from 0.12 MeV in flat targets.

Carbon ion energies are also boosted beyond 60 MeV, indicating a marked improvement in acceleration capabilities. Analysis of ion temperature evolution reveals a multi-stage acceleration process, where reflected laser pulses contribute a secondary heating phase, particularly evident in the step-like heating profile of the annular target.

Phase space diagnostics further confirm that accelerated ion bunches converge at the geometric center of the cavity, creating a localized region of high momentum density. The findings establish that manipulating target curvature to harness optical and geometric focusing provides a viable route towards developing compact, high-efficiency laser-based ion sources.

The authors acknowledge that the simulations are limited to two dimensions and that further research is needed to validate these results in three-dimensional scenarios and with more complex laser pulse profiles. Future work should also investigate the impact of target material and density on the observed acceleration mechanisms, potentially optimising designs for specific applications in hadron therapy and high-energy density physics.