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

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.

Figures (5)

• Figure 1: Scheme for multifoil-induced beam focusing. (a) Experimental setup at SLAC's FACET-II accelerator facility. (b) Beam colliding with a multifoil and undergoing strong-field focusing. (c) 3D view of the multifoil target. (d-e) Evolution of the beam’s transverse size while traversing the multifoil target, as predicted by analytical theory, for the cases where the vacuum beam waist is located (d) at the target position and (e) upstream of the target. The beam size $\sigma_r$ is expressed in units of the beam size at the vacuum waist $\sigma_{r,0}$, while the propagation coordinate $z$ is normalized to the vacuum beta function at the waist $\beta_0$. The dashed line indicates the beam envelope for a vacuum drift.
• Figure 2: Waist scan of the long-bunch regime around the multifoil position. Horizontal divergence (rms) versus the waist position relative to the multifoil for $\beta=50cm$ (a) and $10cm$ (b). Red: 40-foil target. Blue: no-foil reference including the expected multiple-scattering contribution for 40 foils.
• Figure 3: Multifoil focusing of the compressed beam for varying foil numbers. (a-f) Waterfall plots of the angular charge distribution for (a) 0 foil, (b) 1 thick (100µm) aluminum foil, (c) 20, (d) 40, (e) 60, and (f) 111 thin aluminum foils. All foils are 0.9µm thick and are separated by 100µm. (g) Shot-averaged angular distributions corresponding to (a)-(f). (h) Beam divergence as a function of foil number. Red data points are the experimental measurements (with the error bar representing the standard deviation), pink data points indicate lower-bound experimental divergence values, the blue curve represents the analytical model and the black curve represents the PIC simulation.
• Figure 4: Multifoil focusing with beam waist upstream. (a)-(e) Waterfall plots of the angular charge distribution of the 10.06GeV slice for (a) 0 foil, (b) 20, (c) 40, (d) 60, and (e) 111 thin aluminum foils. (f) Shot-averaged angular distributions corresponding to (a)-(e). (g) Divergence as a function of electron energy (increasing from the front to the back of the beam).
• Figure 5: Beam chirp at different compressions and longitudinal phase space. (a) Variation of energy chirp with the uncalibrated value $B$ of the sector 14 bunch length monitor. Longitudinal phase space (b) and current profile (c) reconstructed from X-TCAV measurements for $B = 4500$ (using all shots with $B$ in the range 4400 to 4600). Positive $z$ is the front of the beam. The black solid line in (b) is a linear fit with a slope of -0.39µ.