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

TL;DR

This study demonstrates that simple NCD-filled straight-cone targets optimize proton acceleration by leveraging relativistic self-focusing and electron confinement, achieving high-energy, well-collimated proton beams in laser-plasma interactions.

Contribution

It reveals that simple geometric designs outperform complex ones in proton acceleration efficiency, highlighting the importance of relativistic self-focusing and electron refluxing mechanisms.

Findings

Maximum proton energy of 181.7 MeV achieved

Reduced beam divergence to approximately 12 degrees

Double-peak electron energy structure indicating sustained refluxing

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.