An Interaction of Slipping Beams

Successful models of the fraught dynamics of two particle beams in close contact lead to smoother sailing in an area of particle acceleration.

A new method improves the circulating beams in the Recycler Ring (which is located underground, beneath the long, skinny ponds shown here). The ring is a major component of Fermilab’s accelerator chain.

The Science

Accelerators generate beams of subatomic particles for cutting-edge science. The greater a beam’s intensity, the more opportunities there are to study particle interactions. One way to increase the intensity is to merge two beams with a technique called slip-stacking. However, when combining them, the beams’ interaction may cause instability. Alexey Burov modeled these effects and concluded that a special feedback would make the beams much more stable. The required feedback was then designed and implemented by Nathan Eddy and his Fermilab team. The result was a 20 percent increase in proton beam intensity and a reduction in beam loss by a factor of 2.

The Impact

Slip-stacking doubles the intensity of particle beams. However, it makes them more prone to instabilities and particle losses. To suppress these undesirable effects, Alexey Burov’s analysis and resulting feedback mechanism helped pave the way for future particle accelerators that rely on slip-stacking to achieve high intensities.


By stacking two separate, full-to-the-brim beams, researchers maximize the number of particles circulating through the ring. That’s good for creating high-intensity beams, but there’s a trade-off. Accelerator-generated beams are made of bundles of particles called bunches. Accelerating two separate beams that are not only in close proximity but whose constituent bunches are in a constant dance with each other is a highly charged situation. In Fermilab’s Recycler Ring—a major component of the lab’s accelerator chain—each beam comprises about 500 bunches. Such a high number made coupled-bunch interaction a powerful source of collective instabilities. Partly because the distances between the two slip-stacking beams’ bunches are always changing, the beam dynamics is complicated to model. One major effect is that the interacting bunches bump each other off their orbits. Burov mathematically modeled this effect, solved the equation, and suggested an idea of a feedback to compensate for these undesirable departures from the path. In the feedback system, pickup sensors placed inside the circular accelerator measure the offsets of the bunches from their intended orbits. An amplifier receives this information and then sends it to a kicker that gives the straying bunches a kick that sends them on the right path. Soon after the idea of the special feedback was proposed, an accelerator team lead by Nathan Eddy designed and installed the device into the Recycler Ring. As a result, the Recycler beam losses were nearly halved, and the beam intensity increased by 20 percent; the proton beam power achieved 700 kilowatts, one of the goals of Fermilab’s accelerator program.


Alexey Burov

Nathan Eddy


The Department of Energy, Office of Science, High Energy Physics funded this research. The new device was installed at the Fermilab Accelerator Complex.


A. Burov, “Coupled-beam and coupled-bunch instabilities.” Physical Review Accelerators and Beams 21, 114401 (2018). [DOI: 10.1103/PhysRevAccelBeams.21.114401]

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