Enhanced TNSA Ion Acceleration via Optical Confinement and Geometric Plasma Focusing in Annular Sector Targets

TL;DR

This study investigates how an annular sector (C-shaped) target geometry can boost laser-driven ion acceleration via Target Normal Sheath Acceleration (TNSA). Using 2D PIC simulations at $a_0=10$ and $ au=25~\mathrm{fs}$, the authors compare flat foils with annular cavities, revealing that optical confinement within the cavity and geometric focusing of expanding plasma markedly increase energy absorption, electron temperature, and ion cut-off energies. Specifically, the annular target achieves $k_B T_e \approx 5.1~\mathrm{MeV}$ (vs $2.2~\mathrm{MeV}$) and proton cut-off energies up to $\approx 22~\mathrm{MeV}$ (vs $12~\mathrm{MeV}$) with carbon ions exceeding $60~\mathrm{MeV}$, aided by sustained EM fields for $>300~\mathrm{fs}$. The results demonstrate a robust, geometry-driven mechanism to enhance laser-ion source performance, offering a pathway to compact, high-energy beams for applications in medicine and high-energy-density physics, while highlighting the need for targeted fabrication and optimization of cavity dimensions.

Abstract

Enhancing the conversion efficiency and maximum energy of laser-driven ion beams is a critical challenge for applications in hadron therapy and high-energy density physics. In this work, we present a comprehensive two-dimensional Particle-In-Cell (PIC) simulation study comparing Target Normal Sheath Acceleration (TNSA) from standard flat foils and novel annular sector (C-shaped) targets. Under identical ultra-intense laser irradiation (a0=10, tau=25 fs), the annular sector geometry demonstrates a substantial enhancement in acceleration performance driven by two synergistic mechanisms: electromagnetic cavity confinement and geometric plasma focusing. Our analysis reveals that the target void acts as an optical trap, sustaining oscillating electromagnetic fields for over 300fs via multiple internal reflections. This confinement results in a total laser energy absorption of 49% (compared to 16% for flat targets), which yields a peak electron temperature of 5.1 MeV more than double the 2.2MeV observed in flat targets. Furthermore, 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 increases the proton cut-off energy to 22MeV (vs. 12MeV for flat targets) and boosts Carbon ion energies beyond 60MeV. These findings establish that tailoring target curvature to exploit optical trapping and geometric focusing offers a robust pathway for developing compact, high-efficiency laser-ion sources.

Figures (7)

• Figure 1: Schematic of laser-solid target interaction. (a-1) and (b-1) depict a laser pulse propagating in the x-direction and interacting with a annular sector and a flat target, respectively. (a-2) and (b-2) show simulation snapshots of the electric field intensity and electron distribution during these interactions.
• Figure 2: Comparison of ion and electron energy densities in laser-solid target interactions for flat and annular sector targets. The left column displays results for the flat target, while the right column shows the annular sector target. In each subfigure, the top panel (labeled ‘ion’) presents ion energy density (red color bar) and laser intensity (green-blue color bar). The bottom panel (labeled ‘electron’) shows electron energy density (red color bar) and laser intensity (green-blue color bar). A lineout through the center of the simulation box displays the electric field component along the y-direction for the ion plots and the x-direction for the electron plots. Snapshots (a, b), (c, d), and (e, f) correspond to simulation times $t = 100\,\mathrm{fs}$, $130\,\mathrm{fs}$, and $140\,\mathrm{fs}$, respectively.
• Figure 3: Comparison of ion and electron energy densities in laser-solid target interactions for flat and annular sector targets. The details are the same as in Fig. 2; snapshots (a, b), (c, d), and (e, f) correspond to simulation times $t = 180\,\mathrm{fs}$, $260\,\mathrm{fs}$, and $420\,\mathrm{fs}$, respectively.
• Figure 4: Temporal evolution of particle energy spectra. The columns correspond to electron (left), proton (middle), and Carbon ion (right) spectra for the flat (red curves) and annular sector (blue curves) targets. The rows represent simulation snapshots at $t = 130, 150, 180, 260, \text{and } 420\,\mathrm{fs}$. Solid lines depict the simulation data, while dashed lines indicate the Maxwellian fits used to estimate effective temperatures.
• Figure 5: Temporal evolution of the effective particle temperatures for (a) electrons, (b) protons, and (c) Carbon ions. The blue curves correspond to the flat target, and the red curves correspond to the annular sector target. The annular geometry exhibits an earlier onset of heating and consistently higher peak temperatures. Notably, the Carbon temperature evolution (c) for the annular target reveals a step-like increase (at $t \approx 130\,\mathrm{fs}$ and $t \approx 160\,\mathrm{fs}$), correlating with the optical transit time across the cavity void.