Enhancement of Proton Acceleration via Geometric Confinement in Near Critical Density-filled Targets

Abstract

High-quality proton beams generated by laser-plasma interactions are of significant interest for applications ranging from tumor therapy to fast ignition in inertial confinement fusion. However, simultaneously achieving high energy coupling efficiency and beam collimation remains a challenge. In this work, we investigate the enhancement of proton acceleration via geometric confinement in Near-Critical Density (NCD) plasma-filled micro-structured targets using two-dimensional particle-in-cell (PIC) simulations. To optimize laser-to-particle energy transfer, we systematically compared various target configurations, such as rectangular tubes, hybrid funnels, and straight cones. Our results reveals that increasing geometric complexity does not necessarily translate to superior acceleration performance. Instead, the relatively simple NCD-filled straight-cone target outperforms more complex hybrid geometries, achieving a maximum proton cutoff energy of 181.7 MeV and a reduced divergence of approximately $12^{\circ}$ at a laser intensity of $5.5 \times 10^{20}$ W/cm$^2$. This enhancement is attributed to the synergistic effect of relativistic laser self-focusing within the NCD channel and the strong spatial confinement of hot electrons by the conical walls. Furthermore, we identify a unique double-peak structure in the temporal evolution of the electron energy, which serves as a signature of sustained electron refluxing. This refluxing mechanism maintains a robust sheath field over an extended duration, driving the superior acceleration. The proposed target design offers a robust pathway for generating high-flux, high-energy proton beams suitable for next-generation high-repetition-rate laser facilities.

Figures (11)

• Figure 1: Initial electron density distribution for representative PIC simulation cases: (a) rectangular-grooved target, (b) straight-cone target, (c) hybrid funnel target with a cone-front geometry, and (d) hybrid funnel target with a cone-back geometry. The laser pulse is incident from the left boundary along the $x$-axis. The primary 60 nm hydrogen target is positioned at $x = 20\,\mu\text{m}$.
• Figure 2: (a) Proton energy spectra at $t = 680\,\text{fs}$, (b)Angular distribution ($dN/d\theta$) of protons with energy $\epsilon_p > 10\,\text{MeV}$ at t = 680fs, (c) Temporal evolution of proton cutoff energy in the straight cone target.
• Figure 3: Spatiotemporal evolution of the laser transverse electric field and electron density distribution in the NCD-filled straight cone target at (a) $t=100\,\text{fs}$, (b) $t=150\,\text{fs}$, and (c) $t=200\,\text{fs}$.
• Figure 4: Comparison of the on-axis transverse electric field distribution $E_y(x)$ for the Straight Cone target with (orange line) and without (blue line) NCD plasma filling at $t=150\,\text{fs}$.
• Figure 5: Smoothed electron energy spectra detected behind the target for different geometric configurations at $t=200\,\text{fs}$.