Particles in a collider aren’t necessarily evenly dispersed along the beam path. Instead they’re often clumped together in bunches with space in between. The series of bunches are sometimes called a bunch train, a pulse, or simply ‘the beam’.
When particles enter a collider, the source sends them out in bunches. In the ILC, for example, each bunch will contain 20 billion electrons, which then pass through accelerating cavities. Each accelerating cavity has a radio-frequency electromagnetic field that gives particles energy. The field changes over time, creating waves. These electromagnetic waves also maintain particle bunches: just as a surfer finds a ‘sweet spot’ on an ocean wave, particles group together around the ‘sweet spot’ of the electromagnetic wave in a cavity. But why bunch particles together at all?
First, having more particles present creates more collisions, and more collisions will provide more information for scientists. Accelerator builders and experimenters work together to determine the ideal number and frequency of beam bunches to facilitate the data taking. “Sometimes experiments want a continuous distribution,” says Fermilab scientist Elvin Harms. “But often detectors need some dead time—time to process the results of a collision and re-arm the detector before the next bunches of particles enter the detector and collide.”
In addition, the oscillating nature of radio-frequency acceleration generally requires that particles are bunched. An accelerating system has neutral and reverse capabilities, so particles must be clumped together so that they are kept away from these regions. Riding the ‘sweet spot’ keeps the bunch intact.
An ILC bunch train will consist of 1,312 bunches. The current bunch design of the ILC foresees bunch trains that are a scant five nanometers high and last one millisecond, with a distance of 369 nanoseconds between each bunch, colliding 14,000 times per second.