Ultrasmall divergence of laser-driven ion beams from nanometer thick foils

TL;DR

This study demonstrates that nanometer-thick diamond-like carbon foils produce proton beams with extremely low divergence and high collimation when irradiated by intense lasers, promising for advanced nuclear physics applications.

Contribution

It provides experimental evidence and theoretical modeling of ultra-low divergence proton beams from nanometer-thick foils, a significant advancement over micrometer targets.

Findings

Proton beams with 2-degree divergence observed

Reduction of divergence by an order of magnitude compared to thicker targets

Agreement between experimental results, analytical model, and simulations

Abstract

We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2 degree are observed in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from micrometer thick targets. We demonstrate that this reduction arises from a steep longitudinal electron density gradient and an exponentially decaying transverse profile at the rear side of the ultrathin foils. Agreements are found both in an analytical model and in particle-in-cell simulations. Those novel features make nm foils an attractive alternative for high flux experiments relevant for fundamental research in nuclear and warm dense matter physics.