In a ring accelerator, particles race around in circles like cars on a roundabout. Normally, when new particles are injected, they take random spots on that roundabout — the beam becomes 'fluffy.' In a new experiment at the SNS accelerator, the eigenpainting method was applied: particles were injected so that they only entered one 'lane'—a single oscillation mode associated with the ring.

It's like letting cars onto the roundabout along one perfect trajectory instead of all lanes at once. As a result, the beam in four-dimensional space (two transverse dimensions and two trajectory angles) occupied nearly zero volume — all particles gathered into a thin 'thread' instead of a smeared blob. Even with the mutual repulsion of particles (space charge), this shape was maintained.

Such control over the beam's shape opens the way to more intense and precise beams for neutron sources and future accelerators.