beam bunches and the beam pipe

A particle beam is not a smooth, continuous stream like water from a hose. It is more like a string of tightly packed clusters with gaps between them — the clusters are called bunches. Each bunch holds an enormous number of particles squeezed into a tiny packet only centimetres long and thinner than a hair, and the bunches march around the ring evenly spaced, like cars convoying down a highway with set gaps.

There are good reasons for this clumping. A radio-frequency cavity only pushes correctly during part of its oscillation, so particles must be gathered into bunches that arrive at each cavity in time with the field — particles spread out continuously would be pushed the wrong way half the time. Bunching also concentrates the particles so they are dense enough to collide often when two beams cross. All of this happens inside the beam pipe: a smooth tube, often only centimetres across, held at an ultrahigh vacuum emptier than outer space. The vacuum is essential, because a single collision with a stray gas molecule can knock a particle out of the beam; over hours of circulation, even a whisper of leftover air would erode the beam away.

These two humble features quietly shape what a collider can do. The number of bunches and how many particles each carries feed directly into the luminosity, and how tightly the bunches can be squeezed at the crossing point determines the collision rate. The beam pipe must be both an excellent vacuum vessel and transparent enough not to disturb the particles, and at the detectors it is made especially thin so the collision debris can fly out into the instruments with as little disruption as possible.