Systematic study of proton beam focusing with hemispherical foils*

In the Proton Fast Ignition scheme, detailed point design studies have demonstrated the need to accelerate and focus a laser-driven proton beam to a diameter of 40 μm. The most robust divergence mitigation technique leverages the physics of target normal sheath acceleration where protons are accelerated along the normal of the target’s rear surface and are ballistically focused using a curved foil. While previous experiments have shown valuable results on proton focusing, the increasing interest in inertial fusion energy requires a comprehensive exploration of the design space in this area of research. This can be achieved by leveraging the enhanced shot rate capabilities of current and future high-power laser systems but poses significant challenges in target and diagnostic development.

We will present first experimental results obtained at the ALEPH laser system (20 J, 40 fs), where we performed a systematic study of proton focusing with spherically shaped foils. The campaign designed was informed by 2D particle-in-cell simulations, and its execution was enabled by substantial advancements in target fabrication and alignment capabilities. Target frames containing up to 200 hemispherical foils were manufactured and successfully tested.