Description
Near-critical-density gas jets irradiated by ultra-intense (∼10²⁰–10²² W/cm²), ultrashort (<100 fs) laser pulses offer a promising route toward compact, high-repetition-rate (HRR) ion accelerators. Compared with solid targets, gas jets provide intrinsic self-replenishment, negligible debris, and flexibility in ion species. However, unlike solids where target normal sheath acceleration (TNSA) typically dominates, laser–gas-jet interactions can involve multiple competing mechanisms—such as plasma-channel dynamics, collisionless shocks, and magnetic vortices—often leading to broad ion angular distributions. Since most applications require forward-directed ion beams, achieving efficient forward acceleration demands carefully tailored interaction conditions, as suggested by particle-in-cell (PIC) simulations.
Our initial experiments demonstrated efficient coupling of dense, near-critical gas jets with ∼10²⁰ W/cm², 70 fs laser pulses, resulting in the forward emission of energetic (∼1 MeV/amu) alpha particles with fluxes of order 10¹⁰ particles/sr. PIC simulations reproducing the experimental conditions indicate that these fast ions most likely originate from a collisionless-shock-acceleration (CSA)–like mechanism developing at the rapidly expanding walls of the laser-driven plasma channel [V. Ospina-Bohórquez et al., Phys. Rev. Res. 6, 023268 (2024)].
Building on this work, we present new PIC simulations exploring ion acceleration in dense gas jets at higher laser intensities, up to 10²² W/cm², with the goal of optimizing forward acceleration. The simulations show that around 5 × 10²¹ W/cm² the interaction generates a collisionless shock capable of accelerating protons to tens of MeV within a well-defined forward cone. The laser pulse duration is found to be a critical parameter, with pulse lengths of ∼40–50 fs producing substantially higher forward energies than either shorter or longer pulses. These results identify a clear and experimentally accessible parameter window, which will be directly tested at the ELI-NP E5 (Romania) and Apollon (France) laser facilities.
This work is particularly timely in light of the recent approval of the EIC Pathfinder EUROPA project, which aims to develop a laser-plasma-based facility for artificial radioisotope (ARI) production using gas jets. A central requirement of this project is the realization of a stable, high-repetition-rate ion source delivering ions in the ∼10 MeV energy range.
