High-flux ion acceleration in structured foam targets

Laser-driven ion sources are critical for fast ignition schemes in inertial confinement fusion. Short-pulse, kilo-Joule class lasers are excellent tools for generating relativistic ion beams, but achieving the ion flux demanded by fast ignition remains an open challenge. Custom 3D-printed targets, enabled by two-photon polymerization, can tune average plasma densities to the near-critical limit and greatly increase laser-to-ion conversion efficiency. Particle-in-cell simulations reveal that tuning targets to the relativistically transparent regime enables an extended period of electron heating by direct laser acceleration (DLA). The DLA heating drives a long-lived sheath allowing high-flux and high-energy ion acceleration. 2D simulations estimate nearly a 3-fold improvement in the J/J proton dose (> 10 MeV) using structured targets over foils, with a total efficiency of 15% J/J. This enhanced yield, combined with the scalability of two-photon polymerization, makes 3D-printed microstructures a prime candidate for ion-based fast ignition schemes.