The beam squeezes itself express
An electron beam in an accelerator is like a flock of birds: it flies in a tight group, but over time it spreads out. To keep particles together, heavy magnets are usually used. Here they did without them — the beam squeezed itself.
In the experiment at the FACET-II facility, a beam of 10 billion electrons (charge 1 nC) with an energy of 10 GeV was sent through a stack of thin metal foils. As the beam passes through a foil, its magnetic field reflects off the foil and returns to the beam. This reflected field acts on the beam itself and compresses it, as if an invisible hand gathers the scattering birds back into the flock.
Each subsequent foil enhances the focusing. Measurements showed that the beam becomes noticeably denser after passing through the stack. The results matched calculations and computer simulations.
This focusing method is simple and requires no precise tuning: the beam aligns itself with the foils. It could be useful for studying ultra-dense beams and the intense radiation they produce.
A charge of 1 nC is about 10 billion electrons packed into a beam thinner than a human hair.
In science fiction, dense particle beams are often used as weapons or tools for cutting materials — here, a similar focusing principle works without bulky magnets.