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.
as written by the authors arXiv:2601.13517
Multi-turn charge-exchange injection is the primary method of creating high-intensity hadron beams in circular accelerators, and phase space painting during injection enables tailoring of the accumulated phase space distribution. A technique we call eigenpainting allows injection of particles into a single mode of a coupled ring, providing full four-dimensional control of the phase space distribution. Under ideal conditions, uniform eigenpainting generates a linear-force equilibrium distribution in the transverse plane, with zero volume in four-dimensional transverse phase space, even including space charge. We have implemented eigenpainting for the first time in the Spallation Neutron Source (SNS) Accumulator Ring. Injecting 8.8 $$\mu$$C of 800 MeV beam, we obtain a final ratio of intrinsic transverse emittances of $$\approx$$2.4. We analyze the effect of space charge on the final distribution through comparison of the reconstructed phase space to particle-in-cell simulations.
Space charge is the mutual repulsion of particles in a beam: it's like many identically charged balls trying to fly apart, but magnetic fields hold them together.