Strong-field focusing of high-energy particles in beam-multifoil collisions

TL;DR

This paper reports the first experimental demonstration of a new self-focusing mechanism for high-energy particle beams using metallic foils, enabling ultrahigh density beams for advanced physics research.

Contribution

It introduces and experimentally validates a novel multifoil focusing technique that uses reflected magnetic fields for high-energy beam concentration.

Findings

Observed strong, cumulative focusing across various beam configurations.

Measurements agree with analytical models and simulations.

Demonstrated a simple, self-aligned method for ultrahigh density beam generation.

Abstract

Extreme beams of charged particles and photons, reaching ultrahigh densities or producing intense gamma-ray bursts, are central to accelerator physics, laboratory astrophysics, and strong-field quantum electrodynamics research. Yet their generation is hindered by conventional focusing methods at multi-GeV energies that rely on massive magnetic assemblies, limiting compactness and attainable density. Here we report the first experimental observation of a fundamentally new focusing mechanism, in which a high-energy charged-particle beam is focused by its own magnetic field reflected from a stack of thin metallic foils via near-field coherent-transition-radiation. The experiment, performed at SLAC's FACET-II facility, reveals strong, cumulative focusing across a broad range of beam configurations, enabled by the delivered 10 GeV, 1 nC, 10 Hz electron beam. The measurements closely agree with predictions from an analytical model and particle-in-cell simulations. These results demonstrate that multifoil focusing is a remarkably straightforward, self-aligned approach to the generation of ultrahigh density beams, opening a path to explore unprecedented regimes of beam-matter interaction and high-energy radiation.