linear accelerator (linac)

A linear accelerator, or linac, is the simplest shape an accelerator can take: a straight line. Picture a long corridor with a series of doorways, and a runner who gets a push from behind at every doorway. By the end of the corridor the runner is sprinting flat out. A linac does exactly this for charged particles: it lines up many accelerating gaps in a row, and the particle gains a slice of energy at each one as it travels straight from one end to the other.

The clever part is timing. The accelerating gaps are powered by an oscillating radio-frequency field, so each gap is only pushing in the right direction part of the time. The particle must arrive at each gap exactly when the field is set to shove it forward. As the particle speeds up, the spacing of the gaps is arranged to keep it in step, like the rungs of a ladder getting farther apart as you climb faster. Because everything happens once, in a straight pass, the particle never has to be bent around — which avoids a serious problem (energy lost to synchrotron radiation) that plagues circular machines, especially for light particles like electrons.

Linacs are everywhere. Small medical linacs deliver radiation to treat cancer in hospitals worldwide. In particle physics they serve two roles: as the powerful first stage of nearly every big accelerator complex, getting beams up to speed before injecting them into a ring, and occasionally as the main event. The Stanford Linear Accelerator (SLAC) was a 3-kilometre electron linac that produced Nobel-winning discoveries, and a leading proposal for a future electron-positron collider, the ILC, is a giant linac for exactly the no-bending reason.